G-protein coupled chemoattractant receptors and cancer

2. INTRODUCTION

Chemoattractant receptors are a superfamily of G-protein coupled seven transmembrane cell surface receptors (GPCRs). This superfamily includes GPCRs for classical chemoattractants such as formyl peptides produced by Gram⁻ bacteria and host cell mitochondria, platelet activating factor (PAF), activated complement component 5 (C5a) and leukotriene B4 (LTB4), as well as receptors for new class of host peptides termed chemokines (1). Chemokines are categorized into four major subfamilies (CXC, CC, CX3C, and C), based on the arrangement of their conserved N-terminal cysteine residues in the mature proteins. There are approximately 50 chemokines and at least 18 chemokine GPCRs. Some classical and chemokine GPCRs are notorious for their ligand promiscuity, while many chemoattractants interact with more than a single receptor. These properties of the chemoattractant and their GPCRs were thought to represent their functional redundancy. However in vivo studies, in particular, in experiments with gene deleted mice, indicate that each chemoattractant and GPCR may uniquely contribute to one or more specific host responses. Chemoattractant GPCRs have been shown to be essential for leukocyte accumulation at the sites of inflammation, infection and tissue injury. Some GPCRs are involved in development, homeostasis and host cell infection by HIV (2, 3).

Experimental and clinical studies also indicate that chemoattractant GPCRs play a crucial but often not fully appreciated role in cancer progression. Indeed, malignant tumor cells can often “hijack” the normal physiological functions of chemoattractant GPCRs and by sensing ligands produced in microenvironment millieu to proliferate, to evade immune surveilance, to invade surrounding tissues and to disseminate. In addition, chemoattractant GPCRs expressed on mononuclear phagocytes have been shown to be major mediators of cell infiltration in tumor parenchyma which often promote, rather than limit the tumor growth (4, 5). Furthermore, some chemoattractant GPCRs are expressed by vascular endothelial cells which by responding to chemoattractants produced by tumor and stromal cells, lay foundations to angiogenesis and vasculogenesis in tumor (6) (Figure 1, Table 1).

3. CHEMOATTRACTANT GPCRS AND TUMOR GROWTH

Some chemokines that use GPCRs are directly involved in the transformation, survival and growth of tumor cells (Figure 1). For instance, in CT26 colon carcinoma cells, although the expression of CXCR5, which recognizes the ligand CXCL13, was low in vitro, this receptor was up-regulated when tumor cells were transplanted in mice, suggesting adaptation of tumor cells in tumorigenic microenvironment enables the cells to express more receptors to sense growth signals for their advantage and CXCR5 is crucial for tumor cell growth in the liver (7). Tumorigenic viruses such as Kaposi's sarcoma-associated herpes virus-8 (HHV-8) encode a CXCR2-like GPCR, which interacts with several chemokines, including CXCL1 and IL-8 (CXCL8). Transgenic expression of HHV8-encoded receptor results in the development of angioproliferative lesions resembling Kaposi's sarcoma in mice (8), indicating that chronic and excessive signaling through the HHV8-GPCR promotes oncogenic cellular transformation. US28, a chemokine GPCR encoded by human cytomegalovirus (HCMV), binds a broad spectrum of chemokines and shows constitutive activation of pathways linked to cell proliferation. Expression of US28 in tumor cells results in a proangiogenic and transformed phenotype by up-regulating the expression of vascular endothelial growth factor (VEGF). U28 also accelerates cell cycle progression and growth. US28-expressing tumor cells grow more rapidly when transplanted into nude mice. In glioblastoma cells infected with the newly isolated clinical HCMV strain Titan, US28 was shown to be involved in the HCMV-induced angiogenic phenotype. Hence, the constitutively active chemokine GPCR US28 might act as a viral oncogene to enhance HCMV-associated tumor progression (9).

One of the distinct characteristics of chemoattractant GPCRs in tumor cells is that some of them can respond to growth signals present in the tumor microenvironment. For instance, highly malignant human glioblastoma cells express formyl peptide receptor (FPR), which was originally identified as a GPCR mediating myeloid cell chemotaxis and superoxide release in response to bacterial N-formylated peptides (10) . In human glioblastoma cells, FPR was found to mediate cell proliferation in sub-optima culture conditions after ligand stimulation. This was associated with FPR agonist induced rapid phosphorylation of p38 and ERK1/2-MAPKinases and activation of the anti-apoptosis protein Bcl2. Interestingly, necrotic tumor cells were able to release FPR agonist activity in support of the notion that mitochondrial formylated peptides isolated from ruptured cells are FPR ligands (10, 11). Thus, FPR in glioblastoma cells may play an important role in promoting tumor growth by interacting in an autocrine or paracrine manner with locally produced stimulants (11). In addition, in human glioblastoma cells, activated FPR through a src kinase pathway induces epidermal growth factor receptor (EGFR) phosphorylation, which mediates part of glioblastoma growth promoting activity of FPR (12).

Similar to FPR in glioblastoma cells, the chemokine GPCR CXCR2 is expressed in several established melanoma cell lines, which concomitantly produce the CXCR2 ligands CXCL1 and CXCL8 (13-16). Blocking either the activity of CXCL1 and CXCL8 or their receptor CXCR2 inhibits melanoma cell growth (14, 17). The CXCR2 may also be involved in the growth of head and neck cancer (18), non-small cell lung carcinoma NSCLC (19), ovarian cancer (20), prostate cancer (21) and small-cell lung carcinoma (SCLC) (22). Chemokine GPCR CXCR4 is expressed in at least 23 hematopoietic and solid cancers (23-36). CXCR4 by interacting with agonist SDF-1 alpha (CXCL12) enhances the survival of some glioma (30) and melanoma (37) cells in suboptimal culture conditions, and may mediate tumor growth or metastasis in vivo. Since CXCL12 is produced by cells at the primary tumor sites or in the organs where tumor cells disseminate, the elevated expression of GPCR for this chemoattractant by tumor cells greatly favors the tumor progression. For example, subcutaneous tumors in mice formed by human prostate cancer cells overexpressing CXCR4 were two- to threefold larger in volume and weight (35). And neutralizing either CXCR4 or CXCL12 in vivo with specific antibodies inhibited tumor growth (35). Some chemoattractant GPCRs, such as CXCR1 (16, 21, 22, 38), CXCR5 (7, 34), CXCR7 (39), CCR1 (40-42), CCR3 (43, 44), CCR5 (42, 45) and the receptor for the platelet activating factor (PAFR) (46, 47), are also found to be expressed in various tumor types and are implicated in tumor growth (Table 1). Thus, some chemoattractants and their GPCRs, may be considered as potential targets for directly inhibiting tumor cell proliferation. However, it should be noted that tumor growth is controlled by multiple factors and ideal therapeutic results may not be achieved by targeting a single chemoattractant GPCR.

4. CHEMOATTRACTANT GPCRS AND TUMOR ANGIOGENESIS/ANGIOSTASIS

During their expansion, tumor cells use several strategies to satisfy their increasing needs for nutrients and oxygen, including modification of their microenvironment and switch of gene-expression programs to produce angiogenic factors. Chemoattractant GPCRs regulate tumor angiogenesis through two approaches: Those expressed on vascular endothelial cells (EC) mediate EC recruitment and proliferation to extend the new vasculature by responding to tumor or stromal cell produced agonists such as chemokines CXCL8 and CXCL12; In a second approach, tumor cells express aberrant levels of certain GPCRs, which upon activation, promote the release of angiogenic factors recruiting and activating vascular EC. For instance, in human glioblastoma cells, activation of the GPCR FPR enhances tumor cell release of VEGF. This is evidenced by an in vitro matrigel experimental system in which conditioned media from FPR-agonist stimulated glioblastoma cells support the formation of tubular structures by human umbilical cord EC and loss of the activity by addition of anti-VEGF antibody (11). This provides strong evidence for the capacity of activated FPR to stimulate the release of VEGF by tumor cells.

Another GPCR, the receptor for the platelet activating factor (PAF), may also play a role in mediating angiogenesis of transplanted tumor. For instance, two structurally different PAF receptor antagonists, WEB2170 and CV3988, significantly reduced the formation of new blood vessels in tumors formed by subcutaneous implantation of MDA-MB231 breast cancer cells in SCID mice. These results implicate the potential of PAF released by breast cancer cells to contribute to tumor development by stimulating the angiogenic response (47). PAF triggers angiogenesis both by stimulating the recruitment of EC and by inducing the expression by tumor and stromal cells of several angiogenic factors and chemokines including basic and acidic fibroblast growth factor, placental growth factor, VEGF and its specific receptor flk-1, HGF and KC (48).

Additionally, some cancer cells utilize CC and CXC chemokines such as CCL2, CCL5 and CXCL8 to recruit leukocytes in the tumor stroma, and these immune cells, which may comprise a large proportion of the tumor mass, release VEGF and other angiogenic factors to recruit EC and promote the growth of blood vessel cells (49).

A large number of pro-angiogenic CXC chemokines possess a Glu-Leu-Arg (ELR) motif preceding the first cysteine, and activate a GPCR CXCR2 expressed on EC (50, 51). Among pro-angiogenic ELR⁺ chemokines, CXCL8 has received special attention due to its involvement in multiple stages of tumor angiogenesis. On the one hand, EC when stimulated by VEGF release CXCL8 therefore prolonging and amplifying the effect of VEGF through activation of CXCR2 that further enhances EC motility and proliferation (52). On the other hand, CXCL8, as a potent neutrophil chemoattractant and activator, also increases the release of VEGF by neutrophils (51, 53). Recent identification of a candidate tumor-suppressor gene ING4 provides strong evidence for the role of CXCL8 in tumor angiogenesis (54, 55). ING4 binds the p65 (RelA) subunit of NFkappaB and retains it in the cytosol (55), therefore limiting the expression of NFkappaB-regulated genes, including CXCL8 by tumor cells. Tumor cells that do not express ING4 express increased level of CXCL8, and knockdown of CXCL8 in ING4-deficient cells results in decreased angiogenesis in transplanted tumor and the rate of tumor growth (55). Blocking NFkappaB or the release of CXCL8 by tumor cells each prevents angiogenesis and the growth of transplanted tumor (50). Other chemokine GPCRs implicated in angiogenesis in cancer, include CXCR1 in prostate cancer (21) and CCR1, 5 in multiple myeloma (24, 56).

Some chemokine GPCRs are also used by the host as a check on abnormal angiogenesis in tumors (50, 51). Some non-ELR CXC chemokines such as CXCL4, 9, 10 and 11 are anti-angiogenic (angiostatic). CXCL9, 10 and 11 share a GPCR CXCR3. Clarification of the role of CXCR3 in mediating angiostatic activity comes from the discovery that CXCR3 exists as two alternative splice forms, CXCR3A and CXCR3B (57). While CXCR3A mediates CXCR3 ligand-dependent chemotactic activity of mononuclear cells, CXCR3B mediates the angiostatic activity of CXCL4, CXCL9, CXCL10 and CXCL11 on human microvascular endothelial cells (58). CXCL4 (PF-4) was the first chemokine described to inhibit neovascularisation (59). CXCR3B was identified to act as a functional receptor for CXCL4 (50, 58, 60). Specific antibodies to CXCR3B immunolocalize this GPCR to EC within neoplastic tissues. Nevertheless, it remains controversial as to whether in vivo these CXCR3 ligands use CXCR3 on EC to mediate their angiostatic effects. Tumor cells expressing wild-type CXCL10 showed remarkable reduction in tumor-associated angiogenesis. However, mutation of CXCL10 resulting in partial loss of receptor binding did not significantly alter its angiostatic ability. By contrast, expression of a CXCL10 mutant that failed to bind CXCR3, also failed to inhibit tumor angiogenesis implying the involvement of CXCR3 (57). In a model of human NSCLC in SCID mice, the expression (or intratumor reconstitution) of CXCL10 has been shown to inhibit angiogenesis, whereas neutralizing antibodies against CXCL10 resulted in enhanced tumor-derived angiogenic activity (61). CXCL11 was also found to inhibit tumor angiogenesis in a CXCR3-dependent manner (62). However, not all ELR⁻ chemokines are anti-angiogenic, for instance, CXCL12, an ELR⁻ chemokine which stimulates the GPCR CXCR4 expressed on endothelial cells, is pro-angiogenic rather than angiostatic (35, 63).

It has been proposed that angiogenesis is regulated by a complex balancing process between opposing angiogenic and angiostatic factors (64). In fact, the relative quantity of angiogenic and angiostatic chemoattractants in tumor microenvironment may determine the outcome of neovascularization. Many tumor cells appear to have evolved to acquire the ability to create an environment in favor of pro-angiogenesis (Figure 1). For instance, in contrast to normal lung tissue, human NSCLC tumor specimens contain higher levels of angiogenic ELR⁺ CXC chemokines as compared with the angiostatic ELR⁻ CXC chemokines that use CXCR3 (61, 65). Overexpression of the ELR⁻ CXC chemokine CXCL9 resulted in the inhibition of NSCLC growth and metastasis in association with a decrease in tumor-derived vessel density (66). The level of CXCL9 in human specimens of NSCLC was not significantly different from that found in normal lung tissue (66), therefore the increased expression of ELR⁺ CXC chemokines such as CXCL8 (67, 68) and epithelial-neutrophil activating peptide ENA-78 (CXCL5) (65) in these tumors is not counter-balanced by a concomitant increase in CXCL9. Thus, a physiological balance between pro- and anti-angiogenic chemokines in the normal lung is toppled in the presence of tumor cells.

5. CHEMOATTRACTANT GPCRS AND TUMOR METASTASIS

Tumor metastasis causes a significant reduction in the quality of life and long-term survival of cancer patients, therefore is one of the most serious challenges to successful cancer treatment (69). In order for a tumor cell to metastasize, it must acquire a motile phenotype, being able to detach from the primary tumor mass, trafficking in the lymphatic and blood vessel network, sticking on to and extravasating the vessel wall and growing in draining lymphonodes or a distant organ. Many cancers metastasize to specific organs with an incidence much greater than would be expected from the circulatory pattern between the primary tumor site and the secondary organs. A considerable number of tumor cells express chemoattractant GPCRs (Table 1) and respond to their specific agonists in vitro by chemotaxis. Results obtained in in vivo experiments also indicate that certain chemokines can serve as tissue-specific attractants for tumor cells, promoting tumor cell adhesion to the vascular wall and migration to a particular site (2). Very often this so-called organ-specific metastasis is caused by the aberrant expression of chemokine GPCRs in cancer cells concomitant with the release of chemokine ligands from the secondary organs by stromal cells, infiltrating leukocytes or even by residing cancer cells in an autocrine fashion (51). Whether the most important initial effect of chemokines in the context of tumor is to promote the survival or the spread of cancer cells is still a subject of debate (51). However, it is clear that tumor cells ultimately gain, and so are selected for, the ability to utilize certain chemokines and their GPCRs to metastasize to regional and distant organs (Figure 1).

The concept that a particular pair of chemokine and GPCR may promote organ-specific tumor metastasis was first experimentally proposed in a mouse lymphoma model (70) in which a kidney-specific metastasizing variant tumor cell line showed potent chemotactic responses to a chemokine MCP-1(CCL2) produced by kidney mesangial cells. Although this concept of chemokine-induced organ specific tumor cell metastasis initially encountered skepticism, later studies (26) provided supportive evidence showing that the chemokine GPCR, CXCR4, is more highly expressed in mammary carcinoma tissues and its ligand CXCL12 is expressed in a variety of stromal tissues including lymph nodes, bone marrow and lung, where mammary carcinoma cells preferentially metastasize (26). CXCR4 is commonly found in human and murine cancer cells (51, 71), and its involvement in tumor metastasis has been documented in a variety of tumors, including SCLC, NSLC, pancreatic cancer, glioma (30), neuroblastoma, melanoma and in cancers of colon (72), thyroid (73), prostate (35), ovary and kidney (25), as well as in hematopoietic malignancies, including multiple myeloma, acute leukaemia and chronic lymphocytic leukemia (51, 69, 71, 74).

In a breast cancer model, inhibition of CXCR4 by blocking antibodies, small-molecule inhibitors and knockdown of CXCR4 with small interference RNA prevented the metastatic spread of cancer cells to target tissues such as regional lymph nodes and lung in immunocompromised mice (26). The success in experimental treatment brought hope to use CXCR4 blockers as a complementary therapy for human breast cancer in which CXCR4 is highly expressed (75). The reason for higher level expression of CXCR4 in breast cancer cells was partially explained by the aberrant activity of ERBB2 (also known as HER2) oncogenic receptor tyrosine kinase, which occurs in ~30% of breast cancers and limits the degradation of CXCR4 and predicts poor prognosis. Therefore, deregulation of receptor tyrosine kinase might promote the acquisition by a breast cancer cell of a metastatic phenotype by maintaining cell-surface expression of CXCR4. Studies of renal cell carcinoma, particularly those that harbor mutations in the von Hippel–Lindau tumor-suppressor gene (VHL), revealed that the protein product of VHL induces the rapid degradation of hypoxia-inducible factor-1 alpha (HIF1alpha), a transcription factor that orchestrates the response to tissue hypoxia (76). Renal cell carcinomas with VHL mutations have much higher levels of CXCR4 and a poor prognosis suggesting that CXCR4 expression is under the control by HIF1 alpha. CXCL12 expression is also regulated by HIFlalpha and its release from hypoxic areas contributes to the repair of ischemic tissue injuries by recruiting circulating or resident stem cells (74, 77). Therefore, tumors that harbour VHL mutations express simultaneously high levels of CXCL12 and CXCR4, thereby establishing an autocrine loop promoting renal cell cancer growth and survival (78). The activation of HIFlalpha by the hypoxic conditions often found in the tumor microenvironment also stimulates the expression of CXCR4 in cancer cells (79). Ultimately, this enables tumor cells to escape from areas of low oxygen by migrating towards a gradient of CXCL12, which is released in local draining lymph nodes, and then to spread to other CXCL12-expressing organs. This may explain why CXCR4–CXCL12 axis may play as a key role in metastasis of solid tumors (79). However, it may be too simplistic to envision that the mere ability of CXCL12 to induce tumor cell chemotaxis explains why this chemokine mediates tissue-specific metastasis, because CXCL12 is expressed in a wide variety of tissues and can perform functions other than inducing tumor cell chemotaxis, such as promotion of tumor cell growth/survival and cytokine secretion (51, 80).

In addition to CXCR4, other chemokine GPCRs such as CXCR1 (16), CXCR2 (15, 16, 18, 22), CXCR5 (7), CXCR6 (23), CXCR7 (23), CCR1 (41), CCR4 (81, 82), CCR6 (23, 83-91), CCR7 (23, 92-100), CCR10 (23, 26, 101), CCR11 (102) and CX3CR1(103) are also implicated in mediating the metastasis of cancer cells (Table 1). Quantitative real-time RT-PCR revealed substantial expression of CCR7, CCR9 and CXCR6 as well as CXCR4 mRNA in nasopharyngeal carcinoma (NPC) cell lines (23). Of these, however, only CCR7, CCR6 and CXCR4 were functional. Negative immunoreactivity for CCR7, CCR6 and CXCR4 was demonstrated in almost all nasopharyngeal (NP) specimens from patients with primary NPC and in those with regional metastatic NPC (23). However, expression of two or three of these GPCRs was demonstrated in NPC specimens from patients with liver metastasis (23). Therefore, increased expression of certain chemokine GPCRs may be an indication of tumor cell transformation to metastatic phenotype. CXCR4 expression was found to be at high levels in NPC with poor differentiation and in metastatic tumor cells in regional lymph (104). In experimental models, NPC cell lines with functional CXCR4 were more prone to metastasis to lymph nodes (104). In a mouse model, expression of functional CCR7 enhances the metastasis of B16 murine melanoma cells to regional lymph nodes and the metastasis is blocked by antibodies against CCL21 (93, 100), the CCR7 ligand expressed by both venule endothelial cells and lymphatic endothelial cells in lymph nodes (105). Intravenous administration of CCR7 expressing B16 or control B16 cells yielded comparable numbers of tumor nodules in the lung without forming visible metastasis in lymph nodes, indicating that CCR7 expression specifically enhances the metastasis of B16 melanoma cells through a lymph-mediated, but not a blood-borne, cascade (93). CCR7-dependent lymphatic dissemination of melanoma cells is reminiscent of the physiological trafficking of dendritic cells (DC) into draining lymph nodes (106), which is mediated by interaction of CCR7 on DC with CCR7 chemokine ligands in lymph nodes. Thus tumor cells adopted normal leukocyte homing tactics for their distant metastasis. In support of this model system, CCR7-positive gastric carcinoma cells exhibit a higher incidence of lymph node metastasis, and patients with CCR7-positive tumors have a significantly poorer prognosis than those with CCR7-negative tumors (94). Similar observations have been obtained in esophageal carcinoma patients (95). Other chemokine GPCRs implicated in tumor cell spreading include CCR10 expressed in melanoma cells (26), with the ligand, CCL27, produced constitutively by keratinocytes in the skin, a common site of melanoma dissemination. CCR4 expressed in adult T-cell leukemia may promote malignant cell invasion of the skin, where one of the CCR4 ligands, CCL17, is expressed (107). CD30⁺ cutaneous lymphomas express CCR3, which by responding to its ligand CCL11, may guide tumor cells to form skin lesions (43).

It should be noted that the pattern of chemokine GPCR expression may vary considerably between primary tumor cells and cell lines. For instance, CXCR6 was detected in primary melanoma and metastases, but not in melanoma cell lines and congenital nevi considered as a precancerous lesion in the skin. CXCR4 and CCR1 were the only 2 chemokine GPCRs that were consistently expressed in melanocytes, melanoma cell lines, and primary as well as metastatic melanoma. In addition, CCR1 was found to be increased significantly over melanoma progression. Overall, studies revealed restricted and differential pattern of chemokine GPCR expression in primary and metastatic melanoma tissue distinct from established tumor cell lines. Thus, it is essential to thoroughly examine primary tumors for chemoattractant GPCR expression, rather than to deduce hypothesis from experiments obtained only with cell lines.

Another relevant question is whether and how the metastasis of tumor cells is controlled over long distances by gradients of a single chemokine. It is more likely that multiple host and tumor factors including chemokine GPCRs participate in the detachment of tumor cells from the primary sites, their circulation in the lymphatic and blood vasculature, adhesion to vascular wall, and extravasation into lymph nodes or distant organs where they proliferate to form secondary tumor foci. In this context, chemokines may be involved in several stages of tumor metastasis, namely adhesion of metastasizing tumor cells to the vascular wall and their extravasation into the tissue. However, certain chemokines such as CXCL12 and CX3CL1 has been also shown to support the survival of tumor cells, to promote their invasiveness and angiogenesis, thus may contribute to the expansion of primary tumor as well as the establishment of growing metastatic lesions (35, 108, 109).

6. THERAPEUTIC IMPLICATIONS OF CHEMOATTRACTANT GPCRS IN CANCER

Given that chemokines may play important roles in multiple steps of tumor progression and metastasis, these molecules and their GPCRs have been regarded as potential targets for treatment. Monoclonal antibodies and small molecules against chemokine GPCRs have been useful for inhibiting the growth or spread of malignant tumor cells in experimental animal models. For instance, in addition to inhibition of the lymph node metastasis of human breast carcinoma cells in SCID mice by anti-CXCR4 antibody (26), pretreatment of non-Hodgkin lymphoma cells with anti-CXCR4 antibody also reduced the proliferation of the tumor cells in immunodeficient mice (110). However, whether the inhibitory effects on tumors observed with these antibodies are caused only by the inhibition of tumor cell chemotaxis or growth remains unclear, because antibody-bound tumor cells may be subject to Fc-mediated trapping by innate immune cells in the liver or lung followed by lysis.

Expectations are growing with small molecule GPCRs inhibitors that are less likely to elicit side host responses. A specific antagonist of CXCR4, AMD3100, which is also a potent blocker of CXCR4 mediated cell entry of human immunodeficiency virus, was shown to inhibit the growth of gliomas in the brain by inducing apoptotic tumor cell death (111). In addition, a bisphosphonate YM529 inhibits CXCR4-mediated bone invasion of prostate cancer in an animal model (112). However, because the CXCL12-CXCR4 system plays an important role in a variety of homeostatic processes, such as the translocation of hematopoietic stem cells as well as the development of cardiovasculature and the neural system, clinical applications of CXCR4 inhibitors must be designed and monitored with great caution (51).

One should bear in mind that given the complex nature of carcinogenesis and tumor metastasis formation let alone the heterogeneity of cancers in different organs and even in a single tumor, it is unlikely that blocking a GPCR on tumor or vascular cells will become a magic 'cure' for cancer. More realistic thinking is to further sort out the precise role of these GPCRs in every step of carcinogenic processes and combine the use of anti-GPCR or anti-agonist agents with other therapeutic regimens to reduce tumor burden and to render tumor cells more dormant therefore clinically more manageable.

7. PERSPECTIVES

Accumulating evidence suggests that chemoattractant GPCRs play important roles in mediating the progression and metastasis of malignant tumors. However, in the long history of the studies of tumor biology, the introduction of chemoattractant GPCR research is only at a relatively young stage. More in-depth molecular epidemiologic and genetic surveys are obviously required to draw a more intact pattern of chemoattractant GPCR expression in tumor cells and their in vivo function. The association of such GPCRs with tumor stem cells is far from clear and more rigorous investigation may reveal the expression of some of the GPCRs in determining the motile phenotype of tumor stem cells. It is interesting to note that single nucleotide polymorphisms (SNPs) of CCR5, CCR2, CCL5, SDF1 (CXCL12) and CXCR6 have been linked to the predisposition or protection of individuals in certain tumors (113-116). Whether these SNPs may just represent an overall alteration in homeostasis of the individuals or drastically affect the expression and function of a particular ligand or receptor that may be directly involved in malignant transformation and progression require further investigation. Nevertheless, available literature has provided a clear venue for consideration of chemoattractant GPCRs as targets for future research and development of novel therapeutics for cancer.