G protein-coupled receptors as promising cancer targets

G protein-coupled receptors (GPCRs) regulate an array of fundamental biological processes, such as growth, metabolism and homeostasis. Specifically, GPCRs are involved in cancer initiation and progression. However, compared with the involvement of the epidermal growth factor receptor in cancer, that of GPCRs have been largely ignored. Recent findings have implicated many GPCRs in tumorigenesis, tumor progres- sion, invasion and metastasis. Moreover, GPCRs contribute to the establishment and maintenance of a microenvironment which is permissive for tumor formation and growth, including effects upon sur- rounding blood vessels, signaling molecules and the extracellular matrix. Thus, GPCRs are considered to be among the most useful drug targets against many solid cancers. Development of selective ligands targeting GPCRs may provide novel and effective treatment strategies against cancer and some antican- cer compounds are now in clinical trials. Here, we focus on tumor related GPCRs, such as G protein- coupled receptor 30, the lysophosphatidic acid receptor, angiotensin receptors 1 and 2, the sphingosine 1-phosphate receptors and gastrin releasing peptide receptor. We also summarize their tissue distribu- tions, activation and roles in tumorigenesis and discuss the potential use of GPCR agonists and antagonists in cancer therapy.

Introduction

GPCRs regulate many biological functions by coupling to heterotrimeric guanine nucleotide-binding proteins (G proteins) [1,2]. Heterotrimeric G proteins are composed of 3 subunits, Gα, Gβ, and Gγ, which bind the guanine nucleotide GDP in their basal state. Once activated by ligand binding, GTP displaces GDP, leading to the heterotrimeric protein dissociation into a βγ dimer and the GTP- bound α monomer [2]. Gα subunits have been classified into 4 families: Gαs, Gαi/o, Gαq/11, and Gα12/13. Gα-GTP and Gβγ subunit complexes mediate a variety of downstream signaling cascades (Fig. 1) [1,3,4]. Through G proteins, GPCRs regulate nearly all phys- iological functions, while as a consequence GPCR dysregulation is involved in numerous human diseases and disorders such as type II diabetes [5,6], Alzheimer’s disease [7,8], hypertension [9,10] and heart failure [11,12]. Thus, GPCRs are crucial targets for many cur- rently prescribed drugs [13]. Surprisingly, however, only a limited amount of effort has so far been put into research investigating the roles of GPCRs in cancer, particularly when compared to that carried out on the epidermal growth factor receptor (EGFR) in this context.

Recent data have indicated that many GPCRs and their ligands are involved in cancer initiation and progression, including aberrant cell proliferation, invasion, metastasis, migration, adhesion and angio- genesis [14]. Therefore, GPCRs are considered to be one of the most useful therapeutic targets for treating cancer and the targeting of GPCR-mediated cell signaling has emerged as an important strat- egy for cancer drug-discovery research. In this review, we summarize recent development regarding the involvement of GPCRs in cancers. We also focus on the clinical application of GPCR ligands and signaling-pathway transactivation initiated by cross-talk and co- regulation between GPCRs and other receptors. This review is intended to provide a broad overview of the roles of GPCRs in cancer and aid in the identification of suitable targets for cancer therapy.

Aberrant expression and activation of GPCRs in cancer

Cancer cells express GPCRs in an aberrant manner, including those cancers which derive from the lung, prostate, colon, pancreas and mesenchymal cells (from tumor microenvironments) and those GPCRs, which stimulate cell proliferation [15,16], migration [17], in- vasiveness and angiogenesis [18–20]. For instance, melanocortin-1 receptor polymorphisms are related to an increased risk of skin cancer [21]. High-level production of ligands such as lysophosphatidic acid (LPA), D-erythro-sphingosine-1-phosphate (S1P) and chemokines can induce aberrant GPCR activation, which is frequently associ- ated with cell transformation, proliferation, angiogenesis, metastasis and drug resistance [22,23]. Moreover, in LNCap and PC3 prostate cancer cells, angiotensin II (Ang-II) and bradykinin (BK) receptors are overexpressed and mediate cell growth through coupling to Gαq and Gα13 signaling (Fig. 1) [24–27]. Activation of the PI3K-Akt- mTOR pathway also plays a central role in cancer cell growth, survival, migration and metabolism (Fig. 1) [28,29]. Currently, in vitro and in vivo data indicate that Ang-II exhibits the potential to enhance androgen receptor (AR) expression in prostate cancer cells through the angiotensin-II type-1 receptor (AT1R) [25]. Ang-II and BK are known to accelerate DNA synthesis in PANC-1 pancreatic cancer cells [30]. In addition, the orphan G protein-coupled receptor GPR56 can couple to Gα12/13 and induce Rho-dependent signaling path- ways, which in turn can promote neural progenitor cell migration, demonstrating the importance of G protein signaling in the devel- opment of the central nervous system (Fig. 1) [31]. The human histamine receptor H1 (HRH1) is expressed in a wide variety of cancers including those of the bladder, brain, blood, head and neck, lung, ovaries and skin [32]. The HRH1 mainly couples to Gαq/11, inducing PLCβ activation and subsequent release of the second mes- sengers inositol trisphosphate (IP3) and diacylglycerol followed by PKC activation and Ca2+ release (Fig. 1) [33–36]. Among the GPCR family members, the gonadotropin-releasing hormone (GnRH) re- ceptor has been reported to be overexpressed in various tumor cells such as melanoma, prostate and endometrial carcinomas, leiomyomas, breast cancer, choriocarcinoma, epithelial and stromal ovarian tumors and, as such, the GnRH receptor is considered a well- established target for treating cancer in clinical practice [37–39].

In uterine leiomyosarcoma, as well as ovarian and endometrial car- cinomas, the GnRH receptor can couple to the Gαi protein and decrease intracellular cAMP levels, which results in down-regulated gene transcription and antiproliferative effects in tumor cells (Fig. 1) [39]. The G protein-coupled estrogen receptor (GPR30, also known as GPER) is thought to mediate the multifaceted actions of estro- gens in different tissues, including cancer cells. In patients with endometrial or ovarian cancer, GPR30 overexpression is associ- ated with lower survival rates and moreover, GPR30 overexpression is also involved in an elevated risk of developing metastases in pa- tients with breast cancer (Table 1) [40–42]. Therefore, there is an urgent need to demonstrate what overexpression of which GPCRs occurs in tumorigenesis and to identify their associated down- stream signaling mechanisms.

GPCR mutations in cancer

Both activating and inactivating mutations in GPCR genes have been clearly shown to cause several human diseases, including ma- lignancy (Table 2). To date, clinical studies combined with in vitro functional-expression studies have identified over 600 inactivat- ing mutations and almost 100 activating mutations in GPCRs, which are implicated in more than 30 different human diseases. Mutant GPCRs may contain missense (one amino acid substituted for another) and nonsense mutations (inappropriately located stop codons) [43].

Initial work has identified a pair of conserved extracellular cys- teine residues linking the first and second extracellular loops (ECL1and ECL2) via a disulfide bond in most family A and family B Clearly, further dissection of the mechanisms by which muta- tions alter GPCR functions will guide new therapeutic interventions and more effective drug-discovery research (Table 2).

Key individual GPCRs involved in cancers

GPCRs and downstream signaling pathways regulate an array of biological processes and cellular functions, playing vital roles in the progression of various cancers. Herein, we summarize the roles of 5 particular GRCRs in human cancers and discuss the therapeutic implications of treatment with appropriate agonist and antago- nist ligands in malignancy.

GPR30

Under normal circumstances GPR30 has been shown to mediate the various biological responses to estrogens. However, the overexpression of GPR30 has been reported in various malig- nances, such as those of the breast [55], endometrial [56], ovarian [57], thyroid [58] lung and prostate (Table 1) [59]. Moreover, GPR30 is overexpressed in invasive breast cancer and may decrease sur- vival rates in patients with endometrial or ovarian cancer (Table 1) [41,42,60,61]. Overwhelming evidence has revealed that GPR30 modulates the growth of hormonally responsive cancers includ- ing endometrial, ovarian, and breast cancer [62–64]. Therefore, GPR30 may play an important role in regulating estrogen responsiveness in the progression of estrogen receptor (ER)-related cancers. Hence it is possible that GPR30 blockade therapy may prove to be a useful strategy for ER-related cancer treatment.

GPR30-mediated signaling pathways related to cancer

In response to estrogen stimulation, GPR30 induces both rapid signaling and transcriptional events [60]. GPR30 couples to Gαs and Gαi/o and is associated with cell survival, migration, adhesion and Ca2+ mobilization. GPR30 mediates Gαs activation leading in turn to adenylyl cyclase activation, inducing mobilization of intracellu- lar Ca2+ stores, as well as the activation of mitogen-activated protein kinase (MAPKs) and phosphoinositide 3-kinase (PI3K) [65]. GPCR30 also induces rapid, non-genomic estrogenic actions through the Gαi/o protein, with the resulting downstream signal activation leading to the release of heparin-bound EGF (HB-EGF) and subsequent matrix metalloproteinase dependent transactivation of EGFRs [66,67].

GPR30 cross-talk with other signaling pathways

GPR30 activation leads to the release of HB-EGF which then trig- gers activation of the EGFR and MAPK transduction pathways [68]. Prossnitz et al. [69] reported that GPR30/EGFR/ERK1/2 signals, me- diated by GPR30 induced growth arrest of ER-positive breast cancer cells, using the GPR30 agonist G-1. During this process, activation of the MAP kinase ERK1/2 by GPR30 was mediated via EGFR transactivation. Cross-talk among the GPR30-EGFR signaling path- ways may play a vital role in cancer drug resistance, especially in receptor targeted therapies.

Clinical applications of GPR30 regulators

GPR30 is implicated in breast cancer metastasis and may serve as a novel target for endocrine therapy. Wei et al. [70] demon- strated that GPR30 activation by the agonist G-1 inhibited the growth of ER-positive breast cancer cells in vitro. In vivo data showed that G-1 treatment significantly suppressed the growth of SkBr3 xeno- graft tumors and increased the survival rate. Their results strongly suggested that GPR30 is a potentially important target and sug- gested that G-1 may be a viable drug candidate for ER-positive breast cancer therapy. The discovery of multiple signal pathways medi- ated by GPR30 makes it an even more promising target and lays the foundation for future development of GPR30-based therapies for ER-positive breast cancer treatment.

Clinical studies have also provided evidence that 4-hydroxytamoxifen and ICI 182,780 can bind to GPR30 and acti- vate its downstream signaling pathways, which mediate the expression levels of target genes and promote cell growth in nu- merous types of cancer cells [65,71–74]. These drugs are widely used clinically as cancer therapeutics, they can also be used to demon- strate the potential effects of activated GPR30, in vitro [75]. Future work should focus on detecting GPR30 expression levels, its cellu- lar and tissue distribution, applications of GPR30 agonists or antagonists and the application of this to the development of new approaches for treating carcinomas [41,42,76].

Lysophosphatidic acid receptor (LPAR)

Lysophosphatidic acid (LPA) has been proposed as a potent inducer of cancer progression at multiple levels [77]. It can bind to several cell-surface GPCRs with high affinity and indeed, there are at least 6 GPCRs currently identified as LPA receptors, which are named LPA1–6 [78,79].LPA1 is involved in a diverse range of cellular processes such as motility and metastasis. It is overexpressed in human breast cancer cells and mediates cell transformation (Table 1) [80]. LPA2 activa- tion has been shown to increase cell migration, cell survival and metastasis [77,81,82]. LPA has been shown to promote the inva- sive activity of colon and ovarian cancers via LPA2, concurrently with PI3K-Akt and ERK1/2 signaling pathway activation (Fig. 1) [83]. Limited studies of the involvement of LPA3 in tumorigenesis have been conducted, although it has been suggested that LPA3 regu- lates the chemotaxis of immature dendritic cells and pain levels [77,80–82]. LPA has been shown to be a ubiquitous inducer of tumor cell proliferation, for instance, LPA1 activation modulates the pro- liferation of human DLD1 colon cancer cells [84]. LPA2 and LPA3 mediate the proliferation of HCT116 or LS173T cells [85] respec- tively, suggestive of cell-type dependent difference in the abilities of LPA receptors to induce human colon cancer cell proliferation (Table 1). These data explain why the LPA1–3 receptors are con- sidered as high-quality therapeutic targets for drug development studies in breast cancer.

LPAR-mediated signaling pathways

The binding of LPA to its receptors (which are coupled to at least 3 subtypes of G proteins (Gαq/11, Gαi and Gα12/13)) results in the activation of multiple downstream signaling cascades. Through Gαq and Gαi, LPA receptors can stimulate a variety of signaling path- ways, including the PLC-β/PKC/Ca2+ pathway, the Ras-Raf-1-MAPK pathway and the PI3K-Akt pathway, while inhibiting the adenylyl cyclase-AMP pathway [86,87]. In addition, the effects of LPA- mediated stress fiber formation and focal adhesion assembly are induced by Gα12/13-dependent activation of RhoA (Fig. 1) [86,87].

Cross-talk between the LPA receptor and other signaling pathways

The C-terminal of LPA2 contains a unique sequence which binds to class I PDZ domains and so the effects of LPA2 on cellular sig- naling are mainly modulated by interactions with PDZ-containing proteins, resulting in interaction with the leukemia-associated Rho guanine nucleotide exchange factor (Rho GEF), and PDZ-Rho GEF (Fig. 1) [82]. Lee et al. [88] showed that the membrane-associated guanylate kinase inverted orientation-3 increased the interaction of LPA2 with Gα12 and decreased the tumorigenic capacity by at- tenuating the activities of nuclear factor-κB and c-Jun N-terminal kinase.

Clinical applications of LPA receptor regulators

Clinical research combined with in vitro functional expression studies has suggested that LPA and its receptors have an impor- tant role in this topic and in response to medicinal chemistry studies, LPAR antagonists have been developed. In 2001, LPA1/3 competi- tive antagonists based upon, isoxazole and thiazole, were first described, the most active of which were Ki16425 and Ki16198. These 2 compounds suppressed pancreatic cancer invasion and me- tastasis to the liver, lung, and brain in a mouse model [83,89]. The LPAR antagonists BMS-986020 and SAR-100842 have entered clin- ical trials for treating idiopathic pulmonary fibrosis and systemic sclerosis, respectively [90]. In addition, a series of LPA1 antago- nists, cyanopyrazoles, were shown to have a potential role in controlling inflammatory disorders [91–93]. Another of the isoxazole and thiazole compounds, AM152, also known as BNS-986020, entered a phase-II clinical trial in 2015 [91,94]. Subsequently, a small- molecule LPA agonist, named Rx100, has been shown to effectively reduce radiation-induced lethality in mice when administered orally or subcutaneously (1 h before or 3 h after radiation exposure) [95]. However, no further progress has been reported to date.

Angiotensin-II receptor

The An-II peptide is the primary effector ligand of the rennin- angiotensin system and is a major mediator of blood pressure and cardiovascular homeostasis. The type-I (AT1R) and type-II recep- tors (AT2R) represent the 2 main subtypes of Ang-II receptors [96]. In vitro results have shown that the AT1R is overexpressed in breast carcinoma cells, pancreatic adenocarcinoma cells and hepatocarcinoma cells, while in vivo results have indicated that AT1R is overexpressed in ER-positive breast cancers [97], glioblastomas [98], pancreatic ductal cancers [99], squamous cell carcinomas of the skin [96] and gastric cancers [100]. In addition, AT2R expres- sion has been linked to a worsened prognosis with astrocytomas (Table 1) [101]. These findings suggest a role for these receptors on carcinogenesis and neoangiogenesis.

ATR-mediated signaling pathways related to cancer

Ang-II-induced AT1R activation can couple to Gαq/11 and stim- ulate phospholipases A2, C, and D [102,103]. AT1R activation was also found to promote IP3/Ca2+ signaling [104], as well as the ac- tivation of protein kinase C isoforms [105], MAPKs, several tyrosine kinases (Pyk2, Src, Tyk2 and FAK) and the NF-κB pathway (Fig. 1) [102–106]. The AT1R also signals via Gαi/o and Gα12/13 and acti- vates G protein-independent signaling pathways, including β-arrestin-mediated MAPK activation and Janus kinase (JAK)/ signal transducer and activator of transcription (STAT) signals [107–110]. Hence, it maybe that these major intracellular signal- ing pathways could be involved in the potential effects of these receptors in cancer cell proliferation and angiogenesis.

Notably, AT1R transactivates the EGFR in prostate and breast cancer cells, leading to ERK activation, as well as STAT3 and PKC phosphorylation [111,112]. Moreover, AT1R stimulates the expres- sion of vascular endothelial growth factor receptor 2 (VEGFR2) and angiopoietin-2 in endothelial cells [113–115]. In microvascular en- dothelial cells, the AT1R subtype also shows anti-apoptotic effects by inhibiting the PI3K-Akt pathway, resulting in up-regulated survivin expression and suppressed caspase-3 activity (Fig. 1) [116]. In con- trast, AT2R inhibits VEGFR2-induced Akt phosphorylation and endothelial nitrous oxide synthase, leading to suppressed human endothelial cell migration and tube formation [101]. In addition, AT2R inhibits cell proliferation by trans-inactivating and inhibiting EGFR autophosphorylation [117,118]. AT2R also directly interacts with ErbB3, a member of the EGFR family [119]. Recently, a novel family of AT2R-interacting proteins was identified as inhibitors of EGF- induced pancreatic cancer cell proliferation [120,121].These data suggest that AT1R and AT2R exert different effects on cancer cell proliferation and angiogenesis.

Cross-talk between the angiotensin receptor and other signaling pathways

The Ang-II-AT1R pathway is frequently associated with tumor growth and tumor-associated angiogenesis, in vivo. When acti- vated, the AT1R is able to stimulate the EGFR and its downstream signaling pathways (primarily MAPK activation) [122–124]. Recent data have shown that an AT1R antagonist reduced the expression of VEGFa, in vitro [100]. For instance, in ovarian cancer cell lines, Ang-II promoted cell invasion and VEGFa secretion via AT1R. In lung cancer cells, the increased expression of VEGFa and VEGFR2 was mediated by AT1R [125]. These findings suggested that AT1 recep- tor signaling blockade might be a potential and effective strategy for treating cancer.

Clinical applications of angiotensin receptor regulators

Recently, increasing interest has arisen regarding the potential role of angiotensin converting enzyme inhibitors (ACEis) in anti- cancer research [126]. Studies on the use of ACEis in experimental animal models have provided evidence for a protective effect of these drugs against tumor development. Administration of Captopril, an ACEis used widely in clinical settings as an antihypertensive med- ication, resulted in significantly reduced tumor growth, angiogenesis, and tumor sizes in xenograft models, while increasing mouse sur- vival [127–130]. In addition, Candesartan, an AT1R blocker, completely inhibited angiogenesis-related gene expression (includ- ing that of VEGF and hypoxia inducible transcription factor 2α HIF- 2α) and strongly attenuated tumor sizes, vascularization and lung metastases [131–133]. Moreover, L-158,809, a selective AT1R an- tagonist, inhibited the growth of the human pancreatic cancer cell line, Capan-2, in a dose-dependent manner [134]. A significant re- duction in rat C6 glioma cell growth and the production of several growth factors (for example, VEGF) was also observed in response to Losartan (an AT1R antagonist) treatment, both in vitro and in vivo [135]. Further studies are needed to evaluate the potential of AT1R blockade as a possible novel endocrine-targeted therapy. As AT1R blockers have been used for antihypertensive therapy without serious side effects, we suggest that they also might be safe, effective and novel therapeutics for treating cancer.

Gastrin releasing peptide receptor (GRP-R)

Increasing evidence has suggested that Gastrin releasing peptide (GRP), also known as the mammalian bombesin, is involved in the growth of several neoplasms. Overexpression of GRP and its recep- tors have been observed in various cancer cells and tissues, including lung [136], prostate [137], breast [138], stomach [139], colorectal [140], gastric [141], head and neck cancer [139], pancreatic [142], colon cancers [143] and neuroblastomas [139] (Table 1). The pres- ence of high-affinity GRPR in human malignancies has prompted the development of reagents for diagnosis, radiotherapy and che- motherapy [144,145].

GRP receptors-mediated signaling pathways related to cancer

It is well documented that increased GRPR expression corre- lates with neuroblastoma tumor aggression [146,147]. Signaling pathways associated with GRPR activation in neuroblastoma cell cycle progression and angiogenesis have been the subject of extensive studies. Qiao et al. [148] have reported that GRPR inhibition de- creased the expression of key regulators of protein synthesis and cell metabolism by suppressing the PI3K/Akt/mTOR pathway, which is commonly associated with the stimulation of aerobic glycolysis in cancer cells. A GRPR blocker reversed the aggressive phenotype of the human neuroblastoma cell line BE(2)-C by decreasing cell proliferation, inhibiting DNA synthesis, and inducing cell cycle arrest at the G2/M phase in vitro [149]. In addition, GRPR silencing sig- nificantly suppressed neuroblastoma tumorigenicity by preventing colony formation in vitro and decreased xenograft growth and liver metastasis in vivo [139,149]. Most notably, GRPR transactivated the focal adhesion kinase in SK-N-SH cells and BE(2)-C cells, leading to the activation of downstream regulators of neuroblastoma tumori- genicity [150]. Therefore, GRP/GRPR signaling may be involved in regulating multiple steps of tumorigenesis.

Cross-talk between GRP receptors and other signaling pathways

Activation of GRPR promoted the transcriptional upregulation of EGFR and downstream MAPK pathway phosphorylation, which induced head and neck cancer cell invasion and proliferation [151,152]. In addition, autocrine GRP/GRPR activation also directly activates with Src-dependent cleavage of EGFR pre-ligands [153] and subsequently mediates EGFR phosphorylation and MAPK pathway activation [154]. These data suggest inhibition of GRPR may have an effect upon EGFR-related pathways and that this GRPR inhibi- tion can block EGFR pathways by interfering with intracellular EGFR- activated mediators.

Clinical applications of GRP receptor antagonists

A murine monoclonal antibody (2A11) antibody against GRP has been used as a potent anticancer drug in clinical practice, indeed, it has been reported to reduce the incidence of lung cancer in pa- tients in phase I clinical trials [155]. In addition, it has been suggested that injection of a novel DNA vaccine targeting GRP may inhibit murine melanoma growth in vivo [156]. More recently, a study in- volving a small-molecule inhibitor of GRP provided evidence that the compound 77427 reduced tumor cell growth in vitro and an- giogenesis in vivo [157]. The experimental use of synthetic doxorubicin–bombesin conjugates [158] and camptothecin– bombesin conjugates [159] has revealed a protective effect of these agents against tumor development. These results have indicated that GRPR-specific inhibitors have beneficial effects in terms of reduc- ing both tumor cell growth and angiogenesis, implying their potential use as a clinical tool in the management of tumor growth.

S1P receptor

In most pathophysiological situations and in some experimen- tal tumor models, S1P mediates its biological effects via activation of a specific family of 5 GPCRs termed S1P1–S1P5 [160,161]. S1P re- ceptors are expressed on various cell types including neurons, cardiomyocytes, and endothelial cells (Table 1) [162]. The S1P1–3-Rs are expressed in almost all organs, while S1P4-R is predominantly expressed on lymphoid and hematopoietic tissues [163]. Expres- sion of S1P5-R has been detected in the white matter of the central nervous system and spleen [164] (Table 1). Therefore, it is clear that the expression of S1PRs is tissue-specific.

S1P receptors-mediated signaling pathways related to cancer

Activation of the S1P receptor (S1P1) causes coupling to the Gαi subtype, leading to the up-regulation of multiple intracellular sig- naling pathways, such as the Ras-ERK [165], PI3K-Akt-Rac [166], PLC [167] and Rho pathways [168] (Fig. 1). S1P1 also directly interacts with activated STAT3 [169], increasing tumor growth and metas- tasis in colitis-associated cancer [170]. In addition, S1P1 also displays a tumorigenic effect by promoting STAT3 activation [170] and driving tumor growth [171] and metastasis [172]. Of note, S1P1 has been reported to be associated with ER-positive breast cancer tissues via enhanced ERK-pathway activation and reduced apoptosis.

Cross-talk between S1P receptors and other signaling pathways

Recent findings have demonstrated that cross talk between STAT3 and S1P-SphK-S1PR pathways may play an essential role in inflammation-induced tumorigenesis [173] and tumor progres- sion in the intestine [174] (Fig. 1). STAT3 may elevate activation of the S1P-SphK-S1PR axis, which, in a reciprocal manner, facilitates the maintenance of STAT3 activation through a positive-feedback loop in epithelial cells [170]. Collectively, these data provide a basis for developing novel therapeutic strategies of sphingolipid-centric therapeutics and anti-inflammatory drugs against colorectal cancer, especially those cancers associated with inflammation.

Clinical applications of S1PR agonists

S1P-R agonists have been used as potent chemotherapeutic agents for treating carcinomas. For example, FTY720, the S1P1,3–5-R wide specificity agonist fingolimod [175], has been used to treat cancers such as breast [176], glioblastoma [177], prostate [178], lung [179], ovarian [164] and hematopoietic malignancies [164]. However, FTY720 is now contraindicated in patients with compromised cardiac function. Cardiovascular liabilities persist with more selective S1P1-R agonists that have entered clinical trials, including PF-04629991 [180], Ponesimod [181], CS-077 [182] and BAF312 [183]. Com-
pared with the S1P1-R, the therapeutic application of S1P2–5-R agonists remains poorly explored thus far.

Indeed, few studies have shown S1P1-R agonists to be promis- ing for clinical applications since 2010. Therefore, further preclinical and clinical investigations are required to elucidate whether the new S1P1-R agonists do indeed show an improvement over the safety profile of FTY720 and also to develop new drug paradigms for use in transplantation, chronic inflammation, cancer or autoimmune settings.

Conclusions

Cells communicate with the external environment through cell membrane receptors of which the three main classes are: GPCRs, receptor tyrosine kinases (RTKs), and ion channel receptors. Among these 3 classes of receptors, the role of RTKs in cancer is the best documented. EGFR is overexpressed in various epithelial tumors and it is notably associated with the development of non-small-cell lung cancer [184]. In tumor cells, the aberrant activation of EGFR due to activating mutations can lead to dysregulated downstream cel- lular signaling pathways, tumor growth [185], tumor progression and shortened patient survival [186]. The insulin like growth factor-1 (IGF-1) receptor, a heterotetrameric RTK [187], is considered a major player in the pathophysiology of human cancer [188]. IGF-1 induced receptor tyrosine phosphorylation [189] and PI3K/Akt signaling ac- tivation in previous studies [190]. Consequently, IGF-1R is a rational target for cancer therapy in most cancers, and several anti-IGF-1R drugs have been developed and approved for advancement to clin- ical trials [191]. Thus, EGFR and IGF-1R are both attractive targets for anti-cancer agents, and indeed, several drugs including mono- clonal antibodies and small molecule tyrosine kinase inhibitors, including Sorafenib, Gefitinib and Erlotinib, have all been used in clinical oncology studies.

Ion channel receptors are also involved in various tumors. The P2X7 receptor, an ATP-gated plasma membrane ion channel, is overexpressed in various cancer cells and tissues [192–195]. More- over, stimulation of P2X7 by ATP can activate a proliferative pathway in various carcinoma cells [196]. In addition, transient receptor po- tential (TRP) channels are thought to be associated with changes in intracellular Ca2+ concentrations [197] and the alteration of pro- liferative pathways [198], thereby elevating or suppressing apoptosis in affected cells [199]. The TRP channel is overexpressed in several carcinoma types [200–203]. For example, TRPM8 is now a novel mo- lecular target in androgen-regulated LNCap prostate cancer cells [204]. TRPV4 appears to be a critical regulator of tumor angiogen- esis and tumor vessel maturation [205]. These findings will direct attention toward their crucial roles in tumorigenesis and promote the development of novel strategies for treating ion channel- related cancers.

Thus, as mentioned above, RTKs and ion channels are also prom- ising targets for cancer therapy in their own right, just as it appears that crosstalk between GPCRs and RTKs and ion channels plays a crucial role in tumorigenesis. Mounting evidence suggests that GPCR and EGFR/ErbB overexpression often contributes to cancer growth [206]. Cross-talk between these pathways at the receptor level con- tributes to head and neck squamous cell carcinoma via triggering EGFR/ErbB signaling by a GPCR ligand [207,208]. In addition, nu- merous anti-IGF-1R drugs have been developed and are currently being evaluated in clinical trials. Cross-talk between the IGF-1R and GPCR signaling systems is an important element in the bio- logical responses elicited by these signaling systems [209]. RTKs display marked stimulatory effects in cancers and when com- bined with crosstalk from GPCRs, a compelling case is made for considering combined treatments that target both protein classes. Identification of the transduction network maps connecting several GPCR-dependent signals with other transduction pathways will fa- cilitate deeper investigations concerning the biological potential of these receptors. Cross-talk among signaling pathways may ad- ditionally play an important role in cancer drug resistance, especially in receptor targeted therapies. We hope that the description of po- tential cross-talk among signaling pathways may provide novel ideas for investigation of drug resistance and encourage the discovery of compensatory pathways for conquering resistance in targeted therapy.

In conclusion, although GPCRs represent the most important family of drug targets, their roles as cancer targets remain under- exploited to the extent that only a few anticancer compounds that interfere with GPCR-mediated signaling are currently used in clin- ical practice. Mounting evidence suggests that GPCRs can potentially serve as effective targets for cancer therapy. Continuing efforts to fully understand the biological functions and associated mecha- nisms of the numerous GPCRs will likely facilitate the development of new targets and innovative pharmacological strategies for treat- ing cancer patients. Indeed, by directly targeting GPCRs or more selectively targeting specific downstream signaling components, many viable approaches for developing novel therapeutic strate- gies against cancer potentially exist. Many GPCR agonists have been identified as potent cellular growth factors for a variety of cell types. This review highlights the emerging information on the primary action of selected GPCRs in cancer progression. These findings offer a broad overview of the biological activity elicited by GPCRs in tu- morigenesis and contribute to the investigation of novel pharmacological approaches for cancer patients.