Renal cell carcinoma (RCC) is the seventh most common malignant condition among men and twelfth among women, representing 2% to 3% of all cancers.¹ Thirty to 40% of affected patients present with stage III or stage IV disease. It has an estimated incidence of 57,760 per year, which has increased 2% to 3% per year with no significant decrease in mortality rates.² Median survival of patients with metastatic disease is merely 13 months.¹ Studies have established that tumor and stroma interact through a variety of cytokines, chemokines, and growth factors.³ Recent evidence suggests chemokines may facilitate tumor growth, survival, and metastatic potential of various cancers including RCC. Chemokines have a potential to be utilized as tumor markers and novel targets of antiangiogenic therapy. Investigating the role of various chemokines in the development and metastasis of cancer has become a major focus of contemporary research. We examined the relevant literature and present a review of selected chemokines and their roles in renal cell cancers.

Methods

Peer-reviewed published manuscripts in MEDLINE and PubMed databases were examined. Subject headings included chemokines, RCC, renal cancer, CXCR7, CXCR4, CXC chemokine receptor, and tumor markers. We provide an update on current literature as it pertains to the role of chemokines in RCC.

Chemokines and Chemokine Receptors

Chemokines, or chemotactic cytokines, are small, secreted proteins (8-1 kD) that function in leukocyte recruitment into inflammatory sites and to secondary lymphoid organs.⁴ The biologic effects of chemokines are mediated through specific G-protein coupled seven-transmembrane span receptors. They are primarily expressed on leukocytes, though also on nonlymphoid cells, including endothelial and epithelial cells.^5,6 CXC chemokine receptor type 4 (CXCR4) and CXC chemokines receptor type 7 (CXCR7) are receptors for stromal derived factor-1 (SDF-1) ligand. They are highly conserved genetic sequences that regulate cell trafficking, cell survival, and growth. More than 50 chemokines and over 20 different chemokine receptors have been identified.^6,7 They play a variety of roles in embryogenesis, angiogenesis, hematopoiesis, atherosclerosis, tumor growth, and metastasis, and function as co-receptors for several human immunodeficiency virus strains. 

In addition to their role in the immune system, chemokines are critical in tumor progression. CXCR4 is expressed by tumor cells, and its interaction with ligand SDF-1α often guides the site of tumor metastasis. CXCR4 is one of the most common chemokine receptors and has been discovered to be overexpressed in more than 23 different human cancers.⁸ Moreover, its expression correlates with recurrent, high-grade tumors, bone/lung metastasis, and poor rate of cancer-specific survival in clear cell renal carcinoma.^9,10 Evidence suggests that the increased expression is driven by hypoxia-­inducible factor (HIF)-1 and vascular endothelial growth factor (VEGF). HIF-1 responds to oxygen concentration changes in tissues. It is hypothesized that within hypoxic regions of expanding tumors, chemokine receptors might be upregulated to facilitate escape from primary tumor site.¹¹ 

Burns and colleagues¹² reported that CXCR7 facilitates angiogenesis, and its blockade inhibits tumor growth in mouse models. Maishi and colleagues¹³ compared gene expression profiles of tumor endothelial cells with normal endothelial cells. The expression of CXCR7 was investigated using real-time polymerase chain reaction (PCR). The results revealed up to 20 times higher expression level of CXCR7 in cultured tumor cells than in cultured normal cells.¹³ 

The role of interleukin 8 (IL-8) and natural killer (NK) cells in RCC is also becoming more evident. IL-8 is noted to be higher in metastatic RCC.^14,15 It regulates the expression of chemokine receptors such as CXCR7 and facilitates tumor growth in prostate cancer.¹⁶ Studies show that elevated levels of IL-8 support androgen-independent growth in prostate cancer, and it is associated with biochemical recurrence.¹⁷ Cózar and colleagues¹⁸ characterized the immune lymphocyte infiltrate in RCC at the tumor invasive margin. The study noted significantly higher proportions of NK cell in the tumor infiltrating lymphocytes than in peripheral blood lymphocytes. When compared for tumor stage, patients with metastasis exhibited fewer NK cells and higher proportion of CCR4/CD41 cells in peripheral blood lymphocytes.¹⁸

CXCR3 appears to play a protective role through promoting cellular immunity. It is expressed by tumor-infiltrating lymphocytes. In a study by Klatte and ­colleagues,¹⁹ tumor specimens of 154 patients with localized clear cell renal carcinoma were evaluated for CXCR3 expression. Results showed that 96% of the tumor specimens stained positive for CXCR3. Patients with low CXCR3 expression (< 30%) had a significantly worse prognosis than patients with high CXCR3 expression associated with a 5-year disease-free survival rate of 57% versus 82% (P = .009). Multivariate analysis showed CXCR3 was an independent predictor for disease-free survival following nephrectomy for localized disease.¹⁹

Distinguishing Renal Cell Carcinoma and Benign Renal Tumors

Chemokine receptors CXCR4, CXCR7, SDF-1 ligand, and IL-8 may have a role in molecular subtyping of RCC. A recent prospective study by Gahan and colleagues²⁰ shows promising results in predicting metastatic RCC as well as differentiating between RCC and oncocytoma. Using reverse transcriptase-PCR techniques of mRNA, 86 tumor tissue samples matched with 80 healthy kidney samples were analyzed. The transcript levels of IL-8, CXCR4, and CXCR7 were 10- to 20-fold higher in clear cell RCC tissue than oncocytoma specimen (P < .01). In addition, median IL-8 and SDF1-γ transcript levels were significantly higher in chromophobe subtype than in oncocytoma (median 16.5 and 50 vs 0.83 and 1.4, respectively; P < .05). Combination of markers CXCR7 plus SDF1-γ and CXCR7 plus IL-8 independently predicted metastasis with approximately 80% accuracy, suggesting their potential use in risk stratification.²⁰

Role of Chemokines in Metastasis 

The process of recruitment of immune cells is well understood. Tumors are infiltrated by inflammatory cells including neutrophils, macrophages, and lymphocytes. Neutrophils are recruited to tumor sites by a gradient of chemokines. The cells roll along endothelium, bind to chemokines, attach firmly to endothelial cell layer, and migrate to the underlying tissue. Chemokine production by tumor cells, stromal cells, and tumor-associated leukocytes can promote or inhibit tumor development. For example, ELR1CXC chemokines attract CXCR2-expressing pro-tumoral neutrophils. In contrast, ELR–CXC chemokines attract activated CXCR3 T lymphocytes and NK cells, which exert an antitumoral cytotoxic activity.²¹ Chemokines also affect the process of angiogenesis, as discussed further.

Emerging evidence suggests downregulation of neutrophil chemokines CXCL5 and IL-8 may be the rate-limiting step prior to metastasis. Using RCC cell lines with varying lung metastatic potential in mice, López-Lago and colleagues²² demonstrated an inverse correlation between metastatic activity and level of chemokines. Results indicate that neutrophils recruited to the lung by tumor secrete chemokines, thereby developing an antimetastatic barrier. In addition, neutrophils display a higher cytotoxic activity against poorly metastatic cells as compared with highly metastatic cells. During loss of expression of specific chemokines, RCC cells are able to overcome the metastatic barrier of blood neutrophils.²² Controlling these critical steps may help further the treatment options in metastatic RCC.

Role of Chemokines in Sunitinib Resistance

Cancer cells depend on angiogenesis to survive and proliferate. Research efforts in tumor angiogenesis pathways have led to the development of new treatment options in metastatic RCC. Drugs such as sunitinib inhibit tyrosine kinase receptor in the vascular endothelial growth factor and platelet-derived growth factor pathways. Despite their initial effectiveness, a majority of patients become resistant after 1 year and exhibit disease progression.²³ To understand the underlying mechanism, Huang and colleagues¹⁵ established sunitinib-resistant xenograft models. The investigators discovered higher density of microvessels in sunitinib-resistant tumors indicating angiogenesis. An increase in secretion of IL-8 was also observed. Moreover, coadministration of IL-8 neutralizing antibodies resulted in resensitization of sunitinib.¹⁵ These findings present novel treatment prospects in reversing sunitinib resistance in efforts to prolong survival.

Role of Chemokines in Other Cancers

CXCR7 plays a crucial role in a variety of pathologies. It has been detected in biopsies and cell lines of breast, lung, cervix, and prostate cancers. Wang and colleagues²⁴ noted high level of expression of CXCR7 in prostate cancer. Prostate cancer cells over-expressing CXCR7 grew larger and better vascularized tumors. 

Evidence suggests that the receptor regulates cell proliferation through enhanced cell survival, adhesion, and chemotaxis. Darash-Yahana and colleagues²⁵ demonstrated subcutaneous xenografts of prostate cancer PC3 cells that overexpressed CXCR4 showed greater blood vessel density, functionality, and invasiveness of tumors into surrounding tissues. Moreover, the addition of antibodies targeting CXCR4 inhibited CXCR4-dependent tumor growth and vascularization.²⁵

CXCR7 also seems to increase expression of proangiopoietic ­factors such as IL-8 and VEGF, tumor invasiveness factors such as CD44, cadherin-11, IL-8, VEGF, and tumor growth factor-β (TGF-β).²⁴ In a multivariate analysis high expression levels of CXCR4 (P =.0061), CXCR7 (P =.0194) and CXCR4/CXCR7 (P = .0235) combination were independent prognostic factors in predicting poor prognosis in renal cancers.²⁶ 

Conclusions

Modulating the immune system to identify, suppress cancer cell growth, and prevent metastasis may provide an additional therapeutic approach in the armamentarium against RCC. Recent discoveries have offered meaningful insights into the interplay between tumor and immune system. Further research is needed to develop specific therapies that exploit these molecular mechanisms in an effort to improve ­survival. 

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