the global incidence of diabetes mellitus ( dm ) has increased alarmingly in the past ten years , becoming one of the most common chronic diseases . it is estimated that this disorder will affect 552 million people by 2030 ( http://www.idf.org/media-events/press-releases/2011/diabetes-atlas-5th-edition ) . changing lifestyle leading to reduced physical activity and increased obesity has been pointed as the major culprit for this increase . dm is a group of metabolic diseases characterized by hyperglycemia due to defects in insulin secretion by pancreatic -cells , insulin action on target tissues , or both . based on its etiology , dm has been classified into four main groups ; ( 1 ) type 1 dm ( t1 dm ) that results from lack of insulin production due to selective autoimmune destruction of pancreatic -cells ; ( 2 ) type 2 dm ( t2 dm ) caused by a combination of insulin resistance in the main target tissues ( liver , muscle , and fat ) and inadequate compensatory insulin secretion response by -cells ; ( 3 ) other specific type of diabetes which includes the genetic defects of the -cell ; ( 4 ) gestational diabetes mellitus ( gdm ) , characterized by glucose intolerance with onset or first recognition during pregnancy . existing treatments for t1 dm and long - term t2 dm patients are primarily focused on insulin supplementation . however , despite the beneficial effects of insulin therapy on glucose homeostasis , poor patient implementation often leads to diabetic complications such as retinopathy and nephropathy as well as cardiovascular and cerebrovascular diseases . the life - threatening side effects of poor insulin management expose the need for new therapeutic strategies to preserve or replenish the functional -cell mass and consequently maintain glucose homeostasis . in this scope , phase ii clinical trials performed in patients with recent - onset t1 dm using anti - cd3 monoclonal antibodies that suppress the immune system showed significant preservation of -cell function for at least 24 months and to decrease insulin requirements . despite these beneficial effects , this intervention alone does not restore normal glucose control highlighting the need for the development of alternative therapies aimed at preserving or replacing destroyed -cells . in the past 10 years , islet transplantation has gained much attention as a cell replacement therapy for restoring the functional -cell mass . unfortunately , the limited supply of islets from donors can not meet the demand imposed by the ever - growing number of t1 dm patients . furthermore , a major hurdle has been the lack of durability of islet function resulting in only less than 50% of recipients with insulin independence two years after transplantation . in addition to side effects produced by immunosuppression , several posttransplant events , such as instant blood mediated inflammatory reaction ( ibmir ) and cytokine cascade , seriously affect the functionality of transplanted islets [ 58 ] . nonetheless , the promising outcome of islet transplantation has prompted the search for alternative sources of -cells . one such alternative has been differentiation of insulin - producing cells from embryonic and adult stem / progenitor cells . however , despite the high number of differentiation protocols described in the literature , none published to date have generated by exclusive in vitro differentiation sufficient numbers of insulin - producing cells meeting all essential criteria that define a functional -cell . a more recent and provocative alternative to cell replacement therapy for the treatment of dm has been the concept of in vivo regeneration to replenish the loss of -cell mass . basically , the ultimate goal of this strategy is to enhance the regenerative capacity of the pancreas . studies in animal models and in humans have demonstrated that obesity - associated insulin resistance as well as increased insulin requirements during pregnancy is matched by a corresponding stimulation in insulin output through -cell hyperplasia and hypertrophy . studies in the non - obese diabetic ( nod ) mouse , an important animal model of experimental autoimmune diabetes , have confirmed the importance of -cell replication and improved survival under pathophysiological conditions . it was shown that -cell replication increases with the occurrence of islet inflammation during progression of dm . nonetheless , by three months mice develop hyperglycemia as the autoimmune attack counteracts the attempt of -cells regeneration by the organism . of note , in condition in which autoimmunity and islet inflammation are blunted using an anti - cd3 immune therapy , newly formed -cells derived from preexisting ones are sufficient to normalized blood glucose levels . a combinatorial approach in which both regeneration and resistance to destruction are enhanced will need to be targeted in order to successfully regain a functional -cell mass and maintain normoglycemia . a regenerative approach to dm is a matter of life and death . in this context , an array of growth factors and transcription factors have been shown to be involved in these regulatory pathways , providing an attractive perspective for the development of new therapeutic strategies to maintain adequate -cell mass for stabilization or even improvement of glucose metabolism in dm patients . the question is which ones from the ever - growing list of factors will convey , alone or in combination , the best winning chance of survival and growth to -cells ? in this context we have selected 4 candidates , the hepatocyte growth factor ( hgf ) , the growth inhibitory peptide or glucose - dependent insulinotropic polypeptide ( gip ) , the transcription factors pax4 , and the orphan nuclear liver receptor homolog-1 ( lrh-1 ) as rising stars in the field of -cell regeneration . these factors exert several beneficial actions on -cells simultaneously such as conferring -cell protection and enhancing proliferation . in addition , some of them have been shown to improve -cell function under pathophysiological conditions . thus , they are very attractive targets for that combinatorial approach to treat dm , in which -cell mass is not only increased but also protected from death . these four factors could also be utilized in -cell replacement therapies as a part of either pre- or postislet transplantation treatments . this paper highlights recent advances pinpointing to the importance of these candidates in -cell regeneration as well as to provide insight on how at the molecular level they merge towards similar targets involved in cell replication and survival . these may eventually be employed alone or in combination in an effort to increase islet mass , viability , and function in patients with dm . hgf is a heterodimeric protein that binds to the tyrosine kinase receptor c - met . stimulation of c - met by hgf results in the activation of several downstream signal transduction pathways including pi3k / akt , mapk , and pkc . interestingly , hgf was also shown to induce wnt - independent nuclear translocation of -catenin subsequent to met--catenin dissociation in liver cells resulting in cell replication . in this context , the hgf / c - met axis possesses potent mitogenic effect on various tissues such as liver , lung , kidney , spinal cord , heart , and skeletal muscle upon injury [ 15 , 16 ] . hgf and c - met are also expressed in the pancreas , and levels are significantly increased in islets of gestating mice as well as in cytokine - treated islets . the latter suggests that the hgf / c - met signaling pathway may be crucial for islet adaptation in response to metabolic demands [ 17 , 18 ] . consistent with this premise , pancreas - specific ablation of c - met increased sensitivity of islets to cytokine - induced apoptosis in vitro through increased nf-b signaling and nitric oxide production . treatment of c - met null animals with multiple low doses of stz , a -cell specific diabetogenic agent , resulted in increased islet chemokine secretion , higher degree of insulitis , and increased -cell apoptosis as compared to that observed in wild type treated littermates . these inflammation features combined with -cell death led to a reduced -cell mass and higher incidence of hyperglycemia . similarly , pregnant mice lacking c - met in the pancreas displayed blunted -cell replication with a concomitant increase in cell death leading to gdm . this defect was linked to reduced levels of the prolactin receptor as well as in levels of the transcription factor foxm1 , two important players in cell proliferation . a decreased nuclear translocation of stat5 and upregulation of the replication inhibitor p27 were also detected in c - met - ablated islets . these studies clearly identify the hgf / c - met signaling pathway as an important axis activated in conditions of metabolic stress in which islets require increased protection and proliferation . thus , manipulation of this pathway may provide a new therapeutic venue for the treatment of dm . consistent with this premise , adenoviral - mediated expression of hgf in nonhuman primate islets markedly improved function of transplanted islets . indeed , only a marginal mass of islets expressing hgf was required to correct hyperglycemia in nod - scid mice as compared to the number of control islets needed to normalize blood glucose levels in the same animals . although similar effects need to be established for human islets , these results are certainly encouraging and validate the usefulness of hgf for novel regenerative and replacement dm therapies . in addition to its effect on proliferation and survival , hgf in combination with betacellulin was recently shown to induce in vitro transdifferentiation of pancreatic ductal endothelial cells ( pdecs ) into -cells [ 22 , 23 ] . approximately , 30% of hgf - treated pdec cells transdifferentiated into insulin - producing cells . the phenotype of these newly differentiated -cells was very similar to that of mature -cells , expressing near physiological levels of insulin and glut2 . interestingly , transdifferentiation of these pdecs was dependent on the temporal and sequential expression of key endocrine development transcription factors such as pdx-1 , ngn3 , neurod , and pax4 . using specific inhibitors for the different signaling pathways activated in pdecs upon hgf treatment , li et al . suggested that hgf induces pdecs differentiation into insulin - producing cells through the pi3k / akt cascade . the latter will also be valuable to determine the potential mitogenic side effect of hgf on pdecs . thus , the balance between hgf - mediated proliferation and transdifferentiation into -cells should be carefully investigated . taken together , recent findings reinforce the value of hgf as a potential therapeutic agent that can increase islet number , viability , and functionality , three vital attributes for the treatment of dm through regenerative medicine . gip and glucagon - like peptide-1 ( glp-1 ) are two incretin hormones produced by the k and l - cells of the intestine , respectively . these incretins are secreted in response to food intake and potentiate glucose - induced insulin secretion from pancreatic -cells in healthy individuals . nonetheless , both peptides are rapidly degraded by dipeptidyl peptidase iv ( dpp - iv ) . in view of their insulinotropic properties , major efforts have focused on developing either mimetics of incretins , which are resistant to dpp - iv or inhibitors of dpp - iv . historically , gip was rapidly discarded as a therapeutic target due to its impaired insulinotropic effect in t2 dm patients . nonetheless , recent evidence suggests that gip may be a promising target for the preservation and regeneration of a functional -cell mass in dm . indeed , gip was shown to be important for -cell development as well as postnatal islet mass expansion and function . transgenic mice expressing a dominant negative gip receptor specifically in pancreatic -cells displayed an early disturbance in pancreatic islet development with a severe reduction in the -cell mass with a commensurate increase in - , - , and pp - cells . adult transgenic animals had reduced number of islets and -cells as compared to control littermates . intriguingly , islet proliferation and apoptosis were only slightly modified as compared to control mice . consistent with altered islet architecture , insulin secretion was impaired and animals developed early - onset dm . in addition to its profound impact on development , gip was also shown to increase mouse islet -cell survival in response to high glucose and lipid levels ( glucolipotoxicity ) through activation of the pi3k / pkb pathway and downregulation of the proapoptotic factor bax . studies using human islets revealed that gip could partially block cytokine - mediated cell death clearly providing protective properties to this incretin . interestingly , this antiapoptotic effect appeared to be conveyed by gip - mediated increases in islet osteopontin . consistent with these in vitro data , administration of a dpp - iv - resistant analogue of gip to rats blunted the adverse effects of streptozotocin on islet -cell destruction and development of hyperglycemia . furthermore , the gip analogue also preserved -cell mass in zdf rats through decreased apoptosis . in vitro , gip was shown to stimulate proliferation of newborn rat islet -cells , a mechanism potentially involving the activation of cyclin d1 . thus , gip analogues resistant to dpp-4 cleavage may represent a promising new class of therapeutic compounds with properties to enhance as well as to preserve the critical -cell mass required to maintain normoglycemia in dm patients . pax genes encode a family of transcription factors that are key regulators of tissue development and cellular differentiation in embryos acting to promote cell proliferation , migration and survival . in the pancreas , pax4 was shown to be essential for the generation of islet cell progenitors and subsequent - and -cell maturation during embryogenesis [ 32 , 33 ] . although detectable , the expression of the transcription factor in adult islet -cells was found to be low as compared to its embryonic expression [ 34 , 35 ] . in contrast , aberrantly high expression levels of pax4 were detected in human insulinomas as well as lymphomas [ 36 , 37 ] . a distinctive attribute of pax4 is that mutations and polymorphisms in this gene have been associated with both t1 dm and t2 dm in several ethnic populations . taken together , these unique characteristics strongly indicate a vital role of pax4 not only during development but also in survival and/or maintenance of cell mass in mature islets . consistent with the notion that pax4 expression may be important for -cell adaptation , in vitro studies performed in human islets demonstrated that glucose as well as growth factors such as betacellulin , activin a , and glp-1 increased pax4 mrna levels . furthermore , pax4 levels were also found elevated in islets derived from t2 dm patients correlating with hyperglycaemia , indicating a potential adaptation of -cell mass in response to insulin resistance . ectopic expression of mouse pax4 in either human or rat islets as well as in the mouse insulinoma min6 cell line conferred protection against cytokine - mediated cell death and promoted islet cell proliferation [ 34 , 39 ] . interestingly , a diabetes - linked mutant variant r121w identified in the japanese population was less efficient in protecting human islets against cytokines . supporting the role of pax4 in survival / maintenance of -cell mass , repression of pax4 in the rat insulinoma ins-1e cell line and in hematologic cell lines that express high levels of the transcription factor provoked apoptosis [ 36 , 40 ] . more recently , conditional overexpression of pax4 in adult -cells was shown to protect transgenic animals against stz - induced hyperglycemia and isolated islets against cytokines , while animals expressing the mutant r121w variant were susceptible to developing hyperglycemia and -cell death by both treatments . these antiapoptotic effects were shown to be mediated by increased expression of the antiapoptotic gene bcl-2 and downregulation of the nf-b pathway . consistent with a role for pax4 in -cell replication , the cell cycle - dependent kinase , cdk4 was increased in pax4 overexpressing islets and promoted the proliferation of a pdx1-positive subpopulation . together , these studies suggest that pax4 functions as a survival and proliferation gene allowing mature islets to expand in response to physiological cues .   an additional astonishing regenerative property of pax4 that was recently uncovered is its capacity to reprogram -cells to -cells . indeed , using an elegant cre / loxp approach in which the cre recombinase was under the transcriptional control of the glucagon gene promoter , collombat et al . showed that forced expression of pax4 in developing -cells induced a phenotypic switch towards -cells . this was accompanied by an increase in islet size and in the quantity of insulin positive cells with a concomitant decrease in -cell number . remarkably , the conversion and thus decrease in the -cell population resulted in neogenesis of new -cells from duct - associated progenitor cells in adult animals . nonetheless , these new replenished -cells were continuously converted to -cells due to pax4 ectopic expression . regeneration of the -cell mass by aberrant expression of pax4 in -cell was able to transiently rescue hyperglycemia in young animals rendered diabetic by chemical treatment . the mechanism by which pax4 achieves reprogramming of - to -cells is thought to occur through the antagonistic effect of pax4 on arx , a key transcription factor involved in -cell lineage commitment and subsequent mature function . it will be of great interest to determine whether the conditional and selective expression of pax4 in mature -cells using an inducible and reversible system will promote conversion to -cells and whether repression of pax4 will revert these cells back to -cells . in this context , two independent studies have recently provided elegant evidence for the replenishment of the -cell mass through -cells conversion subsequent to severe pancreatic damage . using an animal model in which the diphtheria toxin receptor was expressed specifically in -cells , thorel and colleagues obtained a near total -cell ablation through administering of diphtheria toxin ( dt ) while maintaining all other endocrine cells intact . this approach allowed the investigators to examine the contribution of remaining endocrine cell subtypes to putative -cell regeneration . using an inducible , tamoxifen - dependent cre / lox - based lineage tracing system , the authors concluded that following -cell ablation , regeneration stemmed predominantly from a non--cell source . the irreversible labeling of -cells showed that under approximately 99% -cell loss , bihormonal cells expressing both insulin and glucagon were observed which ultimately generated single - hormone insulin positive cells . it is worth noting that the severity of the damage greatly influenced -to--cell conversion , since this transdifferentiation was not observed under approximately 95% destruction . why such reprogramming mechanisms are not observed by slightly lower amounts of -cell loss remains to be understood . consistent with these results , an independent study also claims to have observed pancreatic -cell neogenesis by direct conversion of mature -cells subsequent to pancreatic duct ligation ( pdl ) coupled to alloxan treatment . in this study , an astonishingly rapid - to -cell transdifferentiation was detected , resulting in the formation of new islets within 2 weeks . nonetheless , treated mice never achieved normoglycemia . the authors proposed that the latter was most likely due to the profound and continuing inflammation and disruption of normal organ homeostasis that occurred following pdl / alloxan treatment . an alternatively explanation which is not addressed could also be that the newly formed insulin - positive cells are nonfunctional -cells . although this study substantiates the work of thorel and colleagues , no genetic lineage - tracing experiments were performed to clearly ascertain that -cells were the predominant source of new -cells . in this context , it is intriguing that although there was a greater rate of -cell replication as compared to -cell replication ( 6% versus 5% at 7 days after treatment ) , the authors excluded these residual -cells as a potential source for regeneration in this particular injury model . thus , whether a bona fide- to -cell reprogramming occurs in the pdl / alloxan model remains to be clearly established , and key factors driving this process have yet to be identified . in this context , determining whether pax4 is implicated in the conversion of mature  to -cells in these in vivo models of -cell regeneration will be of great interest . indeed , a recent study has challenged the role of pax4 on the transdifferentiation of - to -cells , pointing to men1/menin as the main factor driving this process . in this case , it was found that specific ablation of men1 in -cells triggered reprogramming towards -cells with the subsequent development of insulinoma . although men1 could have a role in -cell plasticity , it is currently difficult to discriminate whether this is a specific effect or only related to tumorigenesis . independently , the identification of signaling molecules that trigger reprogramming of -cells may offer new therapeutic tools for the treatment of dm . taken together , pax4 certainly fulfills all requirements as a first - class target candidate for the development of innovative -cell regenerative therapies for the treatment of dm . indeed pax4 ( 1 ) increases -cell survival in response to metabolic stresses ( 2 ) stimulates proliferation , and ( 3 ) promotes conversion of - to -cells . nonetheless , it is important to note that long - term expression of pax4 may not be beneficial for -cells as the cells revert to a progenitor phenotype thereby losing their capacity to secrete insulin in response to glucose . thus , identifying small molecules or factors that transiently activate pax4 in either - or -cells will be instrumental for the development of novel antidiabetic treatment . lrh-1 is a member of the nr5a2 subfamily of nuclear receptors which is predominantly expressed in the liver , , exocrine pancreas [ 47 , 48 ] , intestine , and ovaries . in these tissues , lrh-1 regulates expression of genes involved in cholesterol and bile acid metabolism as well as steroidogenesis . lrh-1 was also found to attenuate the acute phase response ( apr ) that is triggered in response to inflammatory cytokine signaling in the liver thereby preventing cell death . more importantly , lrh-1 was found to partially convey the beneficial effects of estrogens on -cell survival . pharmacological studies using activators as well as inhibitors of estrogen receptors revealed that lrh-1 expression was predominantly induced through activation of estrogen receptor ( er). repression of lrh-1 by rna interference abrogated the protective effect conveyed by estrogen on islets against cytokines . furthermore , previous studies have demonstrated that aberrant expression of lrh-1 is associated with the growth of intestinal tumors as well as with the proliferation of pancreatic and hepatic cell lines [ 49 , 53 ] . however , the latter finding is in disagreement with adenoviral - mediated overexpression of lrh-1 in human islets , which did not increase -cell proliferation . lrh-1 appears to promote cell replication by stimulating expression of cyclind1 and e1 , a process that involves a crosstalk with the -catenin / tcf4 signaling pathway . low levels of -catenin in mature islet may provide an explanation for the lack of lrh-1 effect on cell proliferation . in contrast , lrh-1 was found to confer protection against cytokine- and stz - induced apoptosis . the steroidogenic enzymes cyp11a1 and cyp11b1 that are involved in glucocorticoid biosynthesis were increased in transduced islets . as these steroids are potent immunosuppressors , glucocorticoids in a paracrine manner may mediate the protective effect of lrh-1 . of note , glucocorticoids were shown to be diabetogenic and to hamper islet function in vivo [ 55 , 56 ] . these findings led to their exclusion from immunosuppressive regimens given to patients subsequent to islet transplantation . however , a recent study demonstrated that exogenous glucocorticoids have potent anti - inflammatory properties on human islets . furthermore , although glucocorticoid - treated human islets exhibited a rapid reduction in glucose - induced insulin secretion observed within 24 hours , these islets performed considerably better than control islets in long - term culture . consistent with the beneficial impact of glucocorticoids on islet integrity , a recent study demonstrated that optimal elevation of glucocorticoids in -cells was a compensatory mechanism that prevented high - fat diet - induced -cell failure . thus , in contrast to previous beliefs , low levels of glucocorticoids generated endogenously by factors such as lrh-1 may be advantageous to preserve -cells against metabolic aggressions . given the importance of lrh-1 in regulating several key metabolic pathways including lipid metabolism as well as in modulating cellular homeostasis ( cell proliferation and survival ) , attempts to regulate its activity could be of therapeutic value for the treatment of dm . in this context , although lrh-1 is considered an orphan receptor that possesses constitutive activity , small endogenous phospholipid were shown to bind the ligand pocket domain of the protein and to increase its activity . taking advantage of this knowledge , a recent study demonstrated that administration of the phospholipids dilauroyl phosphatidylcholine ( dlpc ) decreased hepatic steatosis and improved glucose homeostasis in two mouse models of insulin resistance . these results have led to a pilot human clinical study in usa to explore the effects of dlpc in pre - t2 dm patients . in parallel , it will be of interest to determine whether small molecule agonists of lrh-1 recently developed by whitby and colleagues   can protect human islets against stressed - induced cell death and prevent onset of hyperglycemia in animal models of experimental autoimmune diabetes . the anti - inflammatory properties combined with the prosurvival effects of lrh-1 highlight the immense potential of this newcomer and rising star target for the treatment of both t1 dm and t2 dm . dm is now considered one of the most common noncommunicable diseases in the world causing 5% of all deaths per year . therefore , new therapies should aim at restraining -cell death and promote -cell performance in patients in order to improve blood glucose without treatment - derived side effects . currently , a potpourri of growth factors and transcription factors have been highlighted for their ability to preserve and/or increase  cell mass . the hierarchy or specific networks connecting them together and value in treating diabetic patients are yet to be established . most likely these factors will have to be used in combination to optimize viability of -cells as well as to increase the replication capacity . nonetheless , as current published data point to a declining capacity of -cells to replicate with age [ 62 , 63 ] , efforts should focus on characterizing factors capable of preserving and protecting the -cell cell mass under pathophysiological situations . in this context , hgf , gip , pax4 , and lrh-1 in various permutations may hold the key to a successful regenerative therapy applied to dm . indeed , these factors appear to either complement or share common signaling pathways and downstream targets resulting in increase islet viability and performance ( figure 1 ) . for instance , hgf and pax4 both inhibit the nf-b pathway leading to increased -cell survival . similarly , gip and pax4 enhance expression of the antiapoptotic gene bcl-2 . as we have previously demonstrated that glp-1 stimulates pax4 expression in human islets , it will be of interest to determine whether gip also increases pax4 expression resulting in the downstream activation of the bcl-2 gene . consistent with this possibility , both gip - mediated -cell protection and glp-1-conveyed increases in pax4 are relayed by the pi3k signaling pathway [ 27 , 35 ] . hgf combined with small lrh-1 agonists may be sufficient to stimulate human islet -cell expansion . indeed , hgf was shown to provoke the release of -catenin from c - met with its subsequent translocation to the nuclei . accumulation of nuclear -catenin may reach sufficiently high levels to interact with lrh-1 and activate downstream target gene such as cycd1 . in addition , both lrh-1 and hgf have been shown to decrease inflammation , an important mediator of cell death in dm [ 18 , 54 ] . finally , hgf and pax4 have the additional property to promote transdifferentiation of pdecs and -cells into -cells . this could be extremely beneficial as an alternative method to replenish a functional -cell mass due to the limited replication capacity of -cells . it is also interesting to note that -cells appear less susceptible to autoimmune attack with an apparent increase in the number of -cells in type 2 diabetic subjects . in this context , a combined hgf / c - met , pax4 , and gip therapy could be optimal : -cell protection with increased proliferation as well as the generation of new -cells from -cells . nonetheless one important consideration to take into account is that regeneration and tissue responses are very different under various contexts , such as the severity of the injury or the age of disease onset . furthermore , vigilance is of essence also to restrain the potential deregulated cellular growth when using such factors . thus , to be successful , this type of regenerative therapy requires intervention at a threshold point at which -cells are still present and that the pancreas retains some regenerative plasticity . alternatively , these factors could also be useful for optimizing islet transplantation . indeed , increased islet isolation yields and posttransplantation islet performance and survival could be feasible using a combination of hgf , gip , pax4 , and lrh-1 . the latter approach would reduce the number of islets required for transplantation and improve long - term islet function . islet -cell regeneration is a fast moving field in which great advances can be achieved in the next few years with promising potential for the treatment of dm . the next step is to elucidate the molecular mechanisms that intertwine hgf , gip , pax4 , and lrh-1 together in promoting survival and rejuvenation of islet cells . these studies will most likely highlight additional factors that may become novel targets for regenerative therapies .