Multifaceted Functions of Prokineticins in Reproductive Cancers and Proposed Associated Therapies

Institut National de la Santé et de la Recherche Médicale, Unité 1036, Grenoble, France University Grenoble-Alpes, 38000, Grenoble, France Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA), Biosciences and Biotechnology Institute of Grenoble, Grenoble, France Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5249, Laboratoire de Chimie et Biologie des Métaux, Grenoble, France Georgetown University Medical Center, Pre-Clinical Science Building, GD01/office number, 3900 Reservoir Rd NW, Washington DC 20007 University of Lyon 1, University Hospital Lyon Sud, Department of Gynecological Surgery and Oncology, France French Reference Center for Gestational Trophoblastic Diseases, University Hospital Lyon Sud, 165, Chemin du Grand Revoyet, 69495 Pierre Bénite, France University Hospital of Grenoble, Department of Obstetrics and Gynaecology, and Laboratoire d’Aide à la ProcréationCECOS, La Tronche, France


Prokineticin Family
In mammals, the prokineticin family is composed of two members, the prokineticin 1 (PROK1) also called EG-VEGF (Endocrine Gland-derived Vascular Endothelial Growth Factor) and the prokineticin 2 (PROK2) also called Bv8 [1,2]. EG-VEGF is transcribed from the prok1 gene composed of three exons located on chromosome 1 in 1p21 [3,4]. The gene that transcribes PROK2 is located on the chromosome 3 at 3p13.
Prokineticins and their receptors are expressed in various tissues such as ovary, testis, adrenal gland, placenta, uterus, brain, digestive tract and bone marrow [5][6][7]. In many organs prokineticin's expression has been shown to be dynamic throughout physiological processes such as, circadian rhythm [8] menstrual cycle [9] and throughout pregnancy [10,11]. In these systems, prokineticins have been shown to be regulated by local effectors and/or endocrine hormones. In the ovaries PROK1 is regulated by -hCG (human Chorionic Gonadotropin), estrogen and FSH [12][13][14]. In the placenta PROK1 has been shown to be regulated by hypoxia, hCG and PPAR [10,15,16].
They belong to the GPCR family, with seven transmembrane domains that interact with heterotrimeric G proteins. PROKR1 and PROKR2 were initially called GPR73A and B, then receptors of prokineticin 1 and prokineticin 2, respectively. They share 85% homology in their amino acid sequence, but differ in their N-terminal parts [1,2,17].

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Strong similarities were found in their transmembrane domains, suggesting similarities in their mechanisms of activation [17]. In human, the prokr1 and prokr2 genes include two exons and are located on two different chromosomes. The prokr2 gene is located on chromosome 2 at 2q14 and prokr1 gene is located on chromosome 20 at 20p13 [1]. The two prokineticins act as powerful ligands for both PROKR1 and PROKR2 receptors. Interestingly, differential and tissue-specific expressions were reported for the two receptors. PROKR2 is abundantly expressed in the brain and testes, while PROKR1 is mainly expressed in peripheral tissues such as the spleen, prostate, pancreas, heart and blood cells. In the reproductive system, high levels of both receptors have been reported in the testis, ovary, endometrium and placenta [6,9,12,18,19].
Several studies have been carried out to determine the affinity of the two ligands to their receptors. It clearly appears that PROK2 exhibits higher affinity for PROKRs   Table 1: Reports an updated list of the different type of tumors that were investigated in relation to the potential involvement of the prokineticin family in their growth, progression and/or aggressiveness.

Prokineticins and Diseases
As exemplified through this listing, the prokineticin system appears to be deregulated in many types of cancers.
However the involvement of the ligands and/or their receptors is tightly dependent on the type of cancer, on its severity and its localization within the affected tissue.
Because most of the examined cancers in relation to the prokineticin system emanate from descriptive studies involving cohorts enrolled at advanced stages of cancer, it J Cancer Sci Clin Ther 2020; 4 (4): 408-423 DOI: 10.26502/jcsct.5079082

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is difficult to conclude on whether the deregulation of PROK system is a cause or consequence of the cancer.
Nevertheless, most of the data confirmed the deregulation of prokineticins at these stages, suggesting their tight association with cancer's aggravation. In these cases, their deregulations should rather be considered as consequences of the disease.
Because most advanced and far-reaching studies on prokineticins and cancer have been investigated in the reproductive system, the following sections will report the updated investigations on prokineticins in relation to prostate, testis, ovarian and placental cancers. These subsections will report the associated deregulated member, its role in the development and /or progression of the related cancer and will propose associated therapies, when applicable.

Prokineticins in the prostate and testicular cancers
PROKs and their receptors are expressed in the testis and the prostate. In the prostate, PROK1 is localized in the glandular epithelial cells and PROK2 as well, as the two receptors are expressed in the prostate epithelial cells [47].
In the testis, PROK1 is predominantly expressed in  [53]. Moreover, implication of prokineticins has also been investigated in ovarian cancer.
Ovarian cancer is the 5 th cause of cancer deaths in women and it has the highest rate of death among gynecological cancers [54]. Malignant tumors that originate from the epithelium account for more than 90% of ovarian cancers.
Among these is the high grade serous ovarian cancer; it accounts for nearly 70% of all ovarian cancers and is considered as the most threatening one [55]. Despite substantial survival benefits, chemotherapies are highly toxic and numerous patients develop resistance within few months after the first treatment. Therefore, there is still a need for efficient and less harmful treatment for this cancer [55].
Despite the fact that PROK1 is no longer detectable after menopause in the ovaries, some studies reported PROK1 positive cells at the tumors' periphery [56]. Two other studies reported significant correlations between PROK1 increased expression and the aggressiveness of the cancer, suggesting its potential consideration as a therapeutic target for this cancer [41,46]. On the contrary, Zhang et al. 2003 demonstrated that PROK1 mRNA levels decreased in the later stages of ovarian tumor development compared to premenopausal ovaries or early stages carcinomas [49].
Furthermore, in the same study, it was reported that PROK1 was secreted by the tumor microenvironment and especially by tumor-infiltrating T cells [49]. Importantly, the observation that PROK1 was mainly expressed by immune infiltrating cells in the ovary has been substantiated by other studies demonstrating that prokineticins could not only be expressed by tumor cells but also by immune cells in the tumor microenvironment [57]. Altogether, these findings strongly suggest that the balance between the angiogenic and the pro-inflammatory functions of the prokineticins in a given cancer may grandly contribute to the development and or progression of that cancer. While the role of the PROKs in the angiogenic process that accompany the early stages of ovarian cancer is strongly proposed, their expression by the local immune cells suggests that these proteins could be considered as specific targets to attenuate inflammatory associated reactions in the ovarian cancer.

Prokineticins in the placental cancer
Since the first ontology study on prokineticins, published

Antagonisation of the Prokineticin System as a Therapeutic Opportunity for Reproductive Cancers
The prokineticin system is equally involved in angiogenic and inflammatory processes, which are two key routes that drive cancer development and progression. In relation to angiogenesis, PROKs and especially PROK1 inhibition has been proposed as combinatory option of therapies that target the most angiogenic factor, the VEGF. This was very attempting, because the effects of both factors in regards to the angiogenic processes were somewhat complementary.
Beyond this important aspect, patients treated with anti-VEGF therapies show strong refractoriness [63], suggesting that exploration of new anti-angiogenic therapies that target less ubiquitous and more tissue-specific factors such as prokineticins might lead to better results. This can be applicable for reproductive cancers that affect the ovary and the placenta.
In regards to the prokineticin system involvement in inflammation, growing literature is addressing their local role in regulating immune responses. To date, these studies mainly reported the involvement of PROK2 in these functions [64]. In the case of ovarian cancer, PROK2 was shown to be expressed by components of the immune system, especially the myeloid cells and macrophages [49,63]. These cells have been reported to represent a crucial cell subpopulation responsible for the refractoriness to anti-VEGF treatment [49,63]. These data strongly suggest that therapies targeting not only PROK1 for its pro-angiogenic effects, but also PROK2 for its mediated refractoriness responses can advance the treatment of cancers that overexpress these proteins, such as the ovarian cancer.  significantly decreased tumor growth in poorly and highly vascularized cancers [49,63]. These effects were similar to those obtained with anti-VEGF treatments and the combination of both treatments was sometimes even more efficient [49,63]. Through the inhibition of the prokineticin signaling, the tumor vasculature is no longer expanding; it acquires necrotic properties and exhibits a decrease in the production and secretion of pro-inflammatory cytokines.
Altogether these findings, strongly propose that therapies targeting prokineticins will not only be efficient to attenuate their pro-angiogenic activities but also to decrease their exacerbation of inflammatory responses within the tumor.

Conclusion and Perspectives
To date, the proposed therapies that target

Conflicts of Interest
The authors declare no potential conflicts of interest.