the phosphatidylinositol-3-kinase ( pi3k ) signaling pathway is one of the pathways most commonly activated in human cancers . it is a major downstream signaling component of receptor tyrosine kinases ( rtks ) and is critical for the regulation of cell proliferation , growth , differentiation , migration , and survival . thus , it represents one of the most promising targets for cancer prevention and therapy . mounting reports of original studies and reviews have been published , highlighting the paramount importance of this pathway in human cancers . to avoid redundancy with previous publications , in this paper we will focus on summarizing recent progress in pi3k pathway research in head and neck squamous cell carcinoma ( hnscc ) . specifically , this will include considerations of the molecular alterations seen in some components of the pi3k pathway , as well as functional studies of the role of the pi3k pathway in hnscc initiation , invasion , and metastasis studied both in vitro and in vivo , with a particular focus on the use of genetically engineered mouse models ( gemms ) . finally , we will explore the potential of the pi3k pathway as a target for chemoprevention and cancer therapy . hnscc refers to squamous cell carcinomas ( sccs ) arising from the oral cavity , tongue , pharyngeal , and laryngeal regions . as the 6th most common human cancer worldwide , they generate about 600,000 new cases and 350,000 cancer deaths each year [ 4 , 5 ] . hnsccs usually occur at a relatively late age and at a higher frequency in males possessing the well - known etiological factors of tobacco and/or alcohol usage [ 4 , 5 ] . recently , however , the incidence of hnscc is increasing in women of a relatively young age , correlating with human papilloma virus ( hpv ) infection [ 4 , 5 ] . historically , the best known molecular alterations in hnscc were the inactivation of tumor suppressors , such as p16 and p53 , and activation of oncogenes , such as egfr and stat3 [ 5 , 6 ] . we have studied the role of transforming growth factor beta ( tgf ) pathway in hnscc using both human hnscc samples and gemm approaches [ 710 ] . our studies indicate that inactivation of the type ii receptor of tgf ( tgfrii ) and the downstream signal mediator of tgf , smad4 , plays a crucial role in hnscc development and progression [ 8 , 10 ] . perhaps the most comprehensive studies on molecular alterations of hnscc came from two recent publications describing whole exome sequencing on human hnscc samples [ 11 , 12 ] . two major results were generated from these papers : ( 1 ) the discovery of novel molecular alterations of the notch signaling pathway in human hnscc samples and ( 2 ) the validation of the pi3k pathway as one of the major targets for molecular alterations in human hnscc samples , including alterations of the oncogene pik3ca and the tumor suppressor gene pten . these two papers , together with many previous reports , clearly demonstrate the importance of the pi3k pathway in hnscc . furthermore , they suggest its possible involvement in every aspect of hnscc development and progression , including tumor initiation , invasion , recurrence , resistance to therapeutics , and metastasis . there are three classes of pi3ks , with ia pi3k being the type most widely implicated in human cancers . class ia pi3k primarily phosphorylate phosphatidylinositol-4,5,bisphosphate [ pi(4,5)p2 ] in the plasma membrane to generate the second messenger phosphatidylinositol-3,4,5,trisphosphate [ pi(3,4,5)p3 ] . class ia pi3k are most often activated by rtk signaling and indirectly by ras . upon rtk signaling , p85 binds to either phosphotyrosine residues or adaptor molecules . this binding serves both to recruit the p85-p110 heterodimer to the plasma membrane and to relieve the basal inhibition of p110 by p85 . the 3-phosphatase pten ( phosphatase and tensin homologue ) dephosphorylates pi(3,4,5)p3 and catalyses the reverse reaction . pi(3,4,5)p3 binds a subset of pleckstrin homology domain - containing proteins , including 3-phosphoinositide - dependent protein kinase ( pdk1 ) and protein kinase b ( also called akt ) to the plasma membrane . once there , akt is phosphorylated at thr308 by pdk1 and ser473 by the mammalian target of rapamycin ( mtor ) complex 2 ( mtorc2 ) . it is believed that akt is the central signal mediator of the canonical pi3k signaling pathway . however , recent studies also suggest that the link between pi3k and akt can be uncoupled [ 1518 ] and that oncogenic pi3k signaling can be transmitted through an akt - independent pathway , further adding to the complexity of pi3k signal transduction . akt phosphorylates numerous downstream targets that regulate a wide array of cellular processes important in tumor development and progression . one of its major effectors is mtor complex 1 ( mtorc1 ) , which is activated in multiple human cancers and is one of the major targets in the pi3k pathway for chemoprevention and therapy   ( figure 1 ) . in human hnscc , these include gain - of - function mutations and amplifications in pik3ca ( the gene coding p110 , the catalytic subunit of pi3k ) , loss of heterozygosity and inactivating mutations in pten , and overexpression / activation of akt and mtor signaling [ 5 , 6 ] . several reports utilizing the gemm approach , including our own studies , have confirmed the functional importance of these molecular alterations in hnscc development and progression . in the following sub - sections , we will summarize several molecular alterations and functional studies , particularly those using gemms , involving molecular components of the pi3k pathway in hnscc tumorigenesis . pik3ca , the gene coding for the catalytic subunit p110 of pi3k , is one of the most commonly mutated oncogenes in multiple human malignancies ( 3441 mutated samples among a total of 27725 samples , about 12% , according to the catalogue of somatic mutations in cancer ( cosmic ) database ( http://www.sanger.ac.uk/genetics/cgp/cosmic/ ) ) . most of these mutations are clustered in exon 9 and exon 20 , which corresponds to the helical domain mutant e545k , and the kinase domain mutant h1047r , respectively . almost all pik3ca mutations are gain - of - function mutants , further supporting its oncogenic role in human malignancies . in human hnscc samples , the pik3ca mutation rate is about 10%   but is relatively higher ( 20% ) in hnscc arising from a pharyngeal site . in addition to somatic mutations , genomic amplification of pik3ca has also been reported in several human cancers . interestingly , a significantly higher percentage of pik3ca gene amplification was noted in squamous cell carcinoma , compared to adenocarcinoma in lung . in human hnscc tissue samples , over 30% of cases involving pik3ca amplification involve the candidate gene residing in the common amplification region of 3q26.3 in human hnscc samples [ 23 , 24 ] . pik3ca alterations have been associated with cancer recurrence , metastasis [ 26 , 27 ] , and poor prognosis [ 28 , 29 ] in a variety of human cancers . in hnscc , pik3ca alterations correlate with an advanced stage [ 30 , 31 ] , vascular invasion , and lymph node metastasis . interestingly , in breast cancer cell motility and metastatic potential are differentially enhanced depending on whether their mutations are localized at the helical or kinase domain . an overexpression of the helical domain through mutation e545k of pik3ca produces a more severe metastatic phenotype compared to that of the kinase domain mutation h1047r . although the activated forms of pik3ca , generated through either mutation or amplification , are transforming in vitro [ 1 , 13 ] , their oncogenic potential in vivo has only recently been assessed through the gemm approach . while deletion or inactivation of pik3ca significantly impairs oncogenic transformation   and produces a significant resistance to ras - oncogene - induced tumorigenesis , overexpression of pik3ca results in hyperplasia in ovarian surface epithelium   and can predispose mammary glands to neoplastic transformation . moreover , a knock - in pik3ca h1047r mutant is sufficient to induce lung and breast cancer development [ 3840 ] . work being done in our own lab suggests that these trends apply to hnscc as well . using a hnscc inducible transgenic mouse line that we had previously developed , we observe a strong oncogenic role of pik3ca in hnscc at both initiation and progression when it is overexpressed in head and neck epithelium ( l. du et al . although akt is largely regarded as the dominant mediator of oncogenic pi3k signaling , recent studies suggest that the link between pi3k and akt can be uncoupled [ 1518 ] . for example , pdk1 , but not akt , is activated in some breast cancers with pik3ca mutations   ( figure 1 ) . using a mouse model of breast cancer conditionally expressing the pik3ca h1047r mutant , liu et al . have shown that pik3ca - driven mammary tumors occur via both pi3k - pathway - dependent and pi3k - pathway - independent mechanisms , suggesting the complexity of the pi3k - driven oncogenic mechanisms . to this point , sophisticated pik3ca - gemms for a variety of cancer types may prove to be powerful tools in revealing the role of pik3ca in a context- and stage - specific manner . besides the common alterations of the pik3ca gene encoding the catalytic p110 subunit of class ia pi3k , somatic mutations in the pik3r1 gene encoding the regulatory subunit p85 have been detected in multiple human cancers , including endometrium ( 26% ) , colon ( 5% ) , central nervous system ( 4% ) , breast ( 2% ) , pancreatic ( 2% ) , and skin ( 1% ) ( adapted from the cosmic database ) . interestingly , somatic mutations of pik3r1 are fairly common ( 7% , 3/41 ) in human hnscc samples , with two missense mutations and one in - frame insertion . the functional consequence of these mutants seems oncogenic as the mutants weaken an inhibitory interaction while retaining a stabilizing interaction between p85 and p110 , resulting in an activation of pi3k signaling . however , p85 has also been shown to positively regulate pten , and reduced expression of p85 correlates with decreased pten expression . furthermore , deletion of pik3r1 in mouse liver resulted in aggressive hepatocellular carcinomas with pulmonary metastasis , suggesting a tumor suppressor role . thus , further characterization of pik3r1 mutants using both in vitro and in vivo approaches is warranted to reveal its role in hnscc tumorigenesis . similar to the oncogenic role of the p110 catalytic subunit , the other isoforms of the catalytic subunit , p110 , p110 , and p110 , have been shown to be oncogenic in experimental settings although there are no reports of molecular alterations of these isoform subunits in human cancer samples . p110 has been studied the most among these isoform subunits , and most of the results came from the p110-gemm approaches . using a pten - gemm for prostate cancer , jia et al . showed that ablation of p110 , but not p110 , impeded prostate tumorigenesis . on the other hand , overexpression in a constitutively furthermore , knock - in of a catalytically inactive form of p110 blocked tumor development in an erbb2-gemm for breast cancer . future research , using both human samples and gemm approaches and aiming to assess the roles of these isoform subunits in hnscc , will surely produce interesting results . pten acts as a negative regulator for the pi3k signaling by dephosphorylating pi(3,4,5)p3 and is the second most commonly mutated tumor suppressor in human cancers . it is estimated that the overall frequency of pten mutations in sporadic human cancers is about 12% ( 2044 mutated samples among a total of 17452 samples according to the cosmic database ) , with endometrium cancer displaying the highest frequency among those considered ( 38% , 690 mutated samples among a total of 1837 samples in the cosmic database ) . somatic mutation of pten in sccs of the head and neck is about 3% ( 22 mutated samples among a total of 745 samples in the cosmic database ) but is higher in sccs of skin ( 14% , 92 mutated samples among a total of 658 samples in the cosmic database ) . compared to the relatively less common somatic mutation rate , loss of pten expression was more common ( ~30% ) in human hnsccs . loss of heterozygosity at chromosome 10q near pten was detected in over 70% of the pten - mutated hnsccs , suggesting an inactivation of a typical tumor suppressor . promoter hypermethylation of pten has also been detected in multiple pten expression - lacking human cancers [ 51 , 52 ] . nonetheless , loss of pten expression has been correlated with tumor prognosis and incorporated into the grading system used for human hnscc patients [ 49 , 54 ] . posttranscriptional regulation of pten by the developmental transcription factor grhl-3 has been shown to correlate with pten loss in sccs in both the skin and the head and neck . another potential mechanism for pten loss is through posttranscriptional regulation by recently discovered mrnas , namely mir-21 , mir-26a , and mir-106b-25 , all of which have been identified as pten - targeting mrnas [ 5658 ] . we have shown recently that mir-9 level is positively correlated with pten level in human hnscc cell lines . for example , the ubiquitin ligase nedd4 - 1 has been shown to negatively correlate with pten level . whether nedd4 - 1 overexpression accounts for a subset of pten loss in human hnscc samples requires further investigation ( figure 1 ) . other molecules closely related to pten have also been found to be altered in multiple human cancers . pip3 rac exchanger 2a ( p - rex2a ) has been implicated as a pten - interacting protein and antagonizes pten in human cancers . similar to the phosphatase activity of pten in the pi3k / akt signaling pathway , inositol polyphosphate 4-phosphatase type ii ( inpp4b ) is able to suppress the pi3k / akt signaling pathway and behaves as a tumor suppressor in at least breast and ovarian cancers . lastly , the pten pseudogene ptenp1 has been shown to regulate pten level and acts as a tumor suppressor in human cancers   ( figure 1 ) . one of the major consequences of pten alteration is the activation of its main downstream targets akt and mtor , which are oncogenic in hnscc tumorigenesis and are attractive targets for cancer therapies . for example , the jnk signaling pathway has been found to be a functional target of pten and is significantly associated with pten loss . protein synthesis by the rna - dependent protein kinase ( pkr ) and the subunit of eukaryotic translation initiation factor 2 ( eif2 ) phosphorylation pathway is also required for tumor suppression by pten . these results generate potential therapeutic targets to act alongside targeting of the canonical pi3k signaling pathway . given the potent tumor suppressor role of pten in multiple human cancers , gemms possessing tissue - specific deletion of pten have been created to better understand pten in tumorigenesis . the most striking pten - gemm for human cancer is the prostate pten deletion model , in which mice with a single deletion of pten in prostate cells developed metastatic prostate cancer . pten deletion also resulted in spontaneous tumor development in other organs , such as breast , lung , bladder , and skin , with a wide range of tumor onset pattern and penetrance . furthermore , pten deletion increases susceptibility of mice to the induction of lung cancer by the tobacco carcinogen nnk [ 4-(methylnitrosamino)-1-(3-pyridyl)-1 - 1-butanone ] suggesting a role for pten in tobacco - induced tumor initiation . additionally , pten deletion enhances tumor development and progression in the presence of additional molecular changes , such as ras and p53 [ 68 , 69 ] . using head - and - neck - specific gemms , we deleted pten specifically in the head and neck region and observed both premalignant lesions and tumor development . this head - and - neck - specific pten - gemm can be utilized as a model for testing chemoprevention and therapeutic approaches targeting pi3k pathway ( j. p. shen et al . the serine / threonine kinase akt is the central mediator of the canonical pi3k pathway and mediates multiple cellular processes , including cell survival , proliferation , angiogenesis , metabolism , and protein translation through numerous downstream signaling proteins . for this particular paper , we will focus on the following topics : ( 1 ) molecular alterations of akt in human cancers , ( 2 ) isoform - specific role of akt in human cancers , ( 3 ) tobacco carcinogen - induced akt activation , ( 4 ) in vivo role of akt in human cancers , and ( 5 ) regulation of akt by interacting proteins .   given its central node position in the canonical pi3k pathway , this subsequent akt activation , together with molecular alterations of akt itself , represents one of the most frequent molecular changes in human cancers and provides rationale for targeting akt as a therapeutic approach . all three isoforms of akt , that is akt1 , akt2 , and akt3 , have been reported to be altered in various human cancers [ 70 , 71 ] . somatic mutation occurs most frequently in akt1 and almost exclusively manifests as the e17k mutation [ 72 , 73 ] . this akt1 e17k somatic mutation was detected in about 5% of breast cancers and 3% of both thyroid and urinary cancers although the akt1 e17k mutation has not been identified so far in human sccs of head and neck according to a single report , this mutation has been found in human sccs of lung . mutations of akt2 have been sporadically reported in various human cancers , but none of these mutations occur at the corresponding position of the e17k in akt1 . of particular note , somatic mutation of akt2 mutations of akt3 at both e17k and other sites were reported in melanoma   and endometrial carcinoma . unfortunately , as of yet there are no studies examining akt2 and akt3 mutations in human hnscc samples . it is also worth noting that , although several studies have shown the oncogenic properties of these akt mutations in vitro , a confirmation of their functional significance requires further investigation and demonstration of validity in vivo . in addition to somatic mutation , overexpression of akt isoforms , particularly akt2 , has been reported in multiple human cancers . also , overexpression of akt2 at the messenger level has been shown in breast and colon cancers and seems to correlate with cancer migration , invasion , and metastasis [ 78 , 79 ] . interestingly , akt2 has been shown to be transcriptionally regulated by the master regulator of epithelial - mesenchymal transition ( emt ) , twist , and is associated with tumor progression and metastasis [ 78 , 79 ] . overexpression of akt1 and akt3 has only been shown in human gastric cancer   and melanoma , respectively . in human hnscc samples the overexpression of akt2 , but not akt1 or akt3 , has been reported in one study . pan - akt activation through phosphorylation of ser437 and thr308 is fairly common in multiple human cancers [ 19 , 70 ] . evaluation of these phosphorylation sites yields prognostic value in human lung cancer   and predicts chemotherapeutic benefit in breast cancer . persistent akt activation is also common in human hnscc samples , and occurs as early as the premalignancy stage , including dysplasia and carcinoma in situ , suggesting that akt activation is an early event in human hnscc tumorigenesis [ 6 , 84 , 85 ] . however , reports have also shown akt activation to correlate with a poor clinical outcome in human hnscc patients [ 86 , 87 ] . the role of akt activation in human hnscc development and progression is still in need of further investigation . although akt1 , 2 , and 3 share high sequence homology , clinical studies suggest the existence of isoform - specific roles of akt in multiple human cancers [ 19 , 70 ] . this is further validated by experimental studies using both in vitro and in vivo approaches . while akt1 and akt2 play a similar role in regulating cell survival and proliferation , they behave distinctly in their regulation of cell migration and emt [ 88 , 89 ] . for example , akt1 knockdown induces cell migration and emt in breast cancer cell lines , while akt2 knockdown suppresses these behaviors . this is further exemplified in breast cancer mouse models : while overexpression of akt1 accelerates erbb-2 mediated mammary tumorigenesis and suppresses tumor invasion , overexpression of akt2 markedly increases the incidence of pulmonary metastases in breast cancer . these data suggest that akt1 acts as a metastasis suppressor , while akt2 as a metastasis promoter , further warranting the need to use isoform - specific akt inhibitors in clinical management of cancer patients . distinct downstream targets of each akt isoform might mediate this separate signaling transduction and be responsible for the distinct behavior of akt isoforms in human breast cancer progression and metastasis . a recent report showing regulation of mrna-200 , which plays a critical role in cell migration and emt , by the ratio of akt1 to akt2   suggests another potential mechanism for the isoform - specific roles of akt . yet , there are no human hnscc studies that address these questions though they are needed to guide future clinical trials on hnscc patients using akt isoform - specific inhibitors . though tobacco exposure is one of most important etiological factors in hnscc tumorigenesis , its underlying molecular mechanisms remain poorly understood . in addition to the formation of dna adducts , tobacco carcinogens , such as nnk , activate several signal transduction pathways , including akt , in both normal and cancer cells in the lung . we have shown that , in both hnscc tumors and the adjacent mucosa , akt is activated at a higher frequency in hnscc patients who are smokers compared to those who are nonsmokers . also , adding physiologically relevant concentrations of nnk to normal head and neck epithelial cells and hnscc cell lines will rapidly and constitutively activate akt in a dose - dependent and time - dependent manner . finally , we demonstrated that nnk exposure to mouse head and neck epithelium results in epithelial hyperproliferation and reduced apoptosis , which is correlated with akt activation . these studies suggest that akt activation plays a pivotal role in mediating tobacco - induced hnscc carcinogenesis and that it may be an effective target for chemoprevention . the precise functional consequence of akt activation can not be assessed without in vivo studies . this is particularly critical for the evaluation of the isoform - specific role of akt in context- and stage - specific manners . current in vivo models of akt activation overwhelmingly confirm its oncogenic role at differing levels of potency in various cancer types . however , at least in breast cancer models , the distinct roles of different akt isoforms in cancer progression and metastasis have been observed as described previously . mice constitutively overexpressing an activated akt driven by a keratin 5 promoter developed both skin and head and neck tumors and an increased sensitivity to skin carcinogenesis [ 96 , 97 ] . the oral lesions seen in these specimens were mostly epithelial dysplasia , their malignant transition hampered by the induction of premature senescence . this suggests that akt activation is an early event but not sufficient by itself to hnscc tumorigenesis . this is further supported by the introduction of p53 loss , which synergizes with akt activation to develop metastatic hnscc . numerous proteins similar to pten have been identified as regulators of akt activation and stability . ph domain leucine - rich repeat protein phosphatase ( phlpp ) has been shown to attenuate akt signaling through the regulation of distinct akt isoforms . deletion or loss of expression of phlpp has been reported in a significant fraction of colon and prostate cancers [ 99 , 100 ] . fk506-binding protein 51 ( fkbp51 ) has been shown to act as a scaffolding protein for akt and phlpp , as well as promote the dephosphorylation of akt . posttranslational modifications of akt through ubiquitinating proteins such as ttc3 have also been reported ; however , this protein role in oncogenic akt activation has not been studied . lastly , the p53 target gene ph domain - only protein ( phlda3 ) has been found to compete with the ph domain of akt for binding to membrane lipids , thereby inhibiting akt translocation to the cellular membrane and , thus , its activation . consistent with its function , loss of the phlda3 genomic locus is frequently observed in primary human lung cancer samples . however , such studies have not yet been reported in hnscc cases ( figure 1 ) . although it is widely accepted that akt activation acts as the primary oncogenic mediator in canonical pi3k signaling , as has been stated previously in this paper , recent studies suggest that the link between pi3k and akt can be uncoupled [ 1518 ] . for example , akt signaling is diminished in human breast cancer cell lines and clinical samples harboring pik3ca mutation . in lieu of akt , these cells make use of a signaling pathway involving the pi3k effector pdk1 and its downstream substrate sgk3 . increased gene copy numbers of pdk1 have been found in 21% of breast cancer samples , and total pdk1 mrna and protein have been observed to be overexpressed in the majority of human breast cancer . overexpression of pdk1 promotes invasion and activation of matrix metalloproteinase , while downregulation of pdk1 inhibits migration and experimental metastases of human breast cancer cells . introduction of a hypomorphic mutation of pdk1 in a pten cancer mouse model suppresses tumorigenesis , confirming the oncogenic role of pdk1 as one of the downstream effectors of the pi3k / pten signaling , and suggesting pdk1 as a promising anticancer target . however , the role of pdk1 and its correlation with the canonical pi3k / pten / akt pathway in hnscc development and progression has not been assessed yet .   among the numerous molecules that could act as downstream effectors of the pi3k / akt pathway , mtor is of particular interest . mtor assembles into at least two distinct complexes , that is , mtorc1 and mtorc2 . mtorc2 contains rictor , sin1 and mlst8/gbl and acts upstream of akt to phosphorylate the ser473 of akt . in contrast , mtorc1 contains raptor , pras40 , and mlst8/gbl and acts as a major downstream target of akt . mtorc1 regulates cell growth by controlling key eukaryotic translational regulators , including p70-s6 kinase , and the eukaryotic translational initiation factor , 4e binding protein 1 ( 4e - bp1 ) . in addition , the growth factor receptor - bound protein 10 ( grb10 ) has been recently identified as an mtorc1 substrate . the mtorc1-mediated phosphorylation stabilizes grb10 , leading to the inhibition of the pi3k and erk - mapk pathways . interestingly , grb10 is frequently downregulated in various human cancers , with the loss of grb10 and pten being mutually exclusive . this indicates grb1 as being a tumor suppressor , potentially regulated by mtorc1   ( figure 1 ) . in addition to directly activating mtorc1 , akt also phosphorylates tuberous sclerosis complex ( tsc ) 1 and tsc2 , releasing their inhibition of the ras - like small g protein , rheb , which in turn activates mtorc1 . during conditions of low nutrient availability , mtor signaling is normally inhibited by amp activated protein kinase ( ampk ) , which is activated by its upstream serine / threonine kinase lkb1 ( figure 1 ) . interestingly , these negative regulators of mtor , that is , tsc1 , tsc2 , and lkb1 , are tumor suppressors , and germline mutations of tsc1/2 or lkb1 cause hamartomas and predisposition to multiple malignancies in humans . the critical connection of mtor to the pi3k / akt pathway has led to the prediction that the targeting of mtor may be useful in cancer therapy . indeed , using the mtor inhibitor , rapamycin , yields promising results for multiple human cancers .   in human hnscc , activation of mtor / p70-s6/4e - bp1 pathway is a frequent event in clinical specimens and cell lines . loh of tsc1/2 and dna methylation of tsc2 have been reported in human hnscc samples , and overexpression of tsc2 inhibits cell growth both in vitro and in vivo . a somatic mutation of lkb1 , which leads to the loss of growth inhibition , has been found in a human hnscc patient . decreased nuclear lkb1 levels consistent with the effects of anti - mtor therapy in other cancers , inhibition of mtor by rapamycin displays a potent antitumor effect in hnscc in vitro , in oral carcinogenesis model , and in a hnscc - gemm . targeting mtor has also been shown to be a possible adjuvant therapy for microscopic residual disease in human hnscc patients . however , studies exploring the frequencies of these molecular alterations and their mechanisms in hnscc tumorigenesis are still lacking . activation of pi3k / akt / mtor pathway has been illustrated as an early event in multiple human cancers , suggesting that targeting the pi3k / akt / mtor pathway may have chemopreventive value . this is further exemplified by tobacco exposure activation of akt / mtor in multiple tobacco - related malignancies , including hnscc [ 95 , 119 ] . the tobacco - associated nnk and the dna adduct - forming agent 4-nitroquinoline-1-oxide ( 4nqo ) activate akt / mtor as early as premalignancy stage [ 95 , 119 , 120 ] . inhibition of this pathway by either deguelin , a natural compound belonging to the rotenoid family , or metformin , an antidiabetes medicine , has been shown to possess chemopreventive effects on a tobacco carcinogen - induced lung tumorigenesis model . in addition , mtor inhibition by rapamycin has been shown to prevent early onset of hnscc tumorigenesis in both the 4nqo - induced hnscc mouse model   and a hnscc - gemm . additionally , resveratrol , a phytoalexin enriched in red grapes , strawberries , and peanuts , has been found to be a potent chemoprevention agent for many cancers . one of the major mechanisms of its chemopreventive effect is through inhibition of the pi3k / akt / mtor pathway . it will be interesting to study its chemopreventive effect on both the 4-nqo - induced hnscc mouse model and the hnscc - gemms . personalized cancer therapies with selective molecular targets have emerged as a novel class of anticancer agents , with demonstrated clinical efficacy and less toxicity than conventional therapies . in this situation , the pi3k / akt / mtor pathway has been extensively studied in almost all human malignancies including hnscc and in both experimental and clinical settings . multiple drugs have been designed to target this pathway , making it the most  druggable  pathway for targeted therapies of human cancers . given the scope of this paper , we will only comment on a few aspects . for a more detailed explanation of progress in pi3k targeted therapy on hnscc , please refer to any of the several excellent reviews available on the topic [ 3 , 5 , 126 , 127 ] . ( 1 ) identification of biomarkers for the personalized cancer therapy targeting the pi3k pathwaysince the concept of personalized cancer therapy is based on the identification of a subset of patients whose tumors carry specific molecular alterations , biomarker identification is critical for predicting the effectiveness of targeted therapy . for example , both pik3ca mutations and nuclear phosphorylation of akt are shown as biomarkers for the effectiveness of pi3k inhibitors for human cancer patients [ 128 , 129 ] . in addition , human cancer patients harboring pik3ca mutations are sensitive to targeted therapy using the mtor inhibitor everolimus , while human cancer patients carrying kras mutations are resistant to the treatment . this is further confirmed in a pik3ca - gemm and kras - gemm for lung cancer . while nvp - bez235 , a dual pan - pi3k and mtor inhibitor , led to marked tumor regression in the pik3ca - gemm , it did not affect tumor growth significantly when treating the kras - gemm unless it was used in combination with a mek inhibitor . finally , a recent report of the screening of over three hundred nonredundant pi3k - pathway - relevant phosphopeptides identified pras40 , a molecule involved in protein phosphorylation , as a biomarker correlated with pi3k pathway activation and akt inhibitor sensitivity . since the concept of personalized cancer therapy is based on the identification of a subset of patients whose tumors carry specific molecular alterations , biomarker identification is critical for predicting the effectiveness of targeted therapy . for example , both pik3ca mutations and nuclear phosphorylation of akt are shown as biomarkers for the effectiveness of pi3k inhibitors for human cancer patients [ 128 , 129 ] . in addition , human cancer patients harboring pik3ca mutations are sensitive to targeted therapy using the mtor inhibitor everolimus , while human cancer patients carrying kras mutations are resistant to the treatment . this is further confirmed in a pik3ca - gemm and kras - gemm for lung cancer . while nvp - bez235 , a dual pan - pi3k and mtor inhibitor , led to marked tumor regression in the pik3ca - gemm , it did not affect tumor growth significantly when treating the kras - gemm unless it was used in combination with a mek inhibitor . finally , a recent report of the screening of over three hundred nonredundant pi3k - pathway - relevant phosphopeptides identified pras40 , a molecule involved in protein phosphorylation , as a biomarker correlated with pi3k pathway activation and akt inhibitor sensitivity . ( 2 ) activation of pi3k / akt / mtor pathway as a resistance mechanism to targeted therapy or radiotherapyegfr targeted therapy is the first fda - approved protocol for treating human hnscc patients . activation of the canonical pi3k / akt / mtor pathway seems to be associated with resistance to egfr inhibitor in multiple human cancers . however , a recent study showed that an egfr - activating mutation resistant to targeted therapy activates the mtorc2-nf-b signaling pathway in an akt - independent manner in glioblastoma patients . further studies are necessary to investigate the role of both canonical and noncanonical pi3k pathways in resistance to egfr therapy in human hnscc patients . however , activation of the pi3k / akt / mtor pathway is implicated in all major mechanisms of radioresistance , including intrinsic radioresistance , tumor cell proliferation , and hypoxia . thus , blocking the pi3k / akt / mtor pathway has great potential to enhance the effectiveness of radiotherapy for hnscc patients . egfr targeted therapy is the first fda - approved protocol for treating human hnscc patients . activation of the canonical pi3k / akt / mtor pathway seems to be associated with resistance to egfr inhibitor in multiple human cancers . however , a recent study showed that an egfr - activating mutation resistant to targeted therapy activates the mtorc2-nf-b signaling pathway in an akt - independent manner in glioblastoma patients . further studies are necessary to investigate the role of both canonical and noncanonical pi3k pathways in resistance to egfr therapy in human hnscc patients . however , activation of the pi3k / akt / mtor pathway is implicated in all major mechanisms of radioresistance , including intrinsic radioresistance , tumor cell proliferation , and hypoxia . thus , blocking the pi3k / akt / mtor pathway has great potential to enhance the effectiveness of radiotherapy for hnscc patients . ( 3 ) synergistic effect of combination with other receptor tyrosine kinase targeted therapiesrecent evidence of multiple feedback loops and interactions with other signaling pathways highlights the complexity of pi3k signaling . using an inducible pik3ca - gemm for breast cancer , liu et al . identified c - myc elevation as a potential mechanism by which tumors develop resistance to pi3k - targeted therapies . moreover , inhibition of akt induces activation of upstream rtk signaling pathways , such as her3 , and mtor inhibition causes activation of akt signaling   or mapk pathway . these studies suggest that combination therapies of pi3k - targeted therapy together with targeting c - myc , her3 , or mapk pathway , may be more effective for the treatment of certain human cancer patients . recent evidence of multiple feedback loops and interactions with other signaling pathways highlights the complexity of pi3k signaling . using an inducible pik3ca - gemm for breast cancer , liu et al . identified c - myc elevation as a potential mechanism by which tumors develop resistance to pi3k - targeted therapies . moreover , inhibition of akt induces activation of upstream rtk signaling pathways , such as her3 , and mtor inhibition causes activation of akt signaling   or mapk pathway . these studies suggest that combination therapies of pi3k - targeted therapy together with targeting c - myc , her3 , or mapk pathway , may be more effective for the treatment of certain human cancer patients . mounting evidence clearly shows both the paramount importance of the pi3k pathway in the tumorigenesis of many human malignancies including hnscc , and the promising results of targeting this pathway for treatment of human cancer patients . however , there are still many questions that need to be answered . compared to the extensive studies on pik3ca , there are few studies on the other subunits of class ia pi3ks and their interactions with pik3ca and pten . studies of classes ii and iii of pi3ks in human cancers are generally lacking . although several proteins interacting with pten or akt have been shown to play a role in tumorigenesis , more studies must be undertaken to discover novel molecules modulating the pi3k pathway , and assess their roles in tumorigenesis . in addition to the relatively linear canonical pi3k / akt pathway , more and more noncanonical pathways are expected to be identified . furthermore , the newly discovered mrnas described in this paper add yet another layer of complexity to our understanding of the molecular regulation of the pi3k signaling pathway . integrative mapping of molecular alterations in human cancers , particularly in hnscc samples , is highly demanding . utilization of multiple molecular approaches , especially gemms of the pi3k signaling pathway , will help us to better understand the complexity of this pathway in human cancers , as well as in context and stage - specific manners . ultimately , these studies will yield identifiable biomarkers for improved clinical diagnosis and prognosis , contributing to strategies of therapy and prevention that will allow for the better management of human cancers and better outcomes for human patients .