We previously reported that the pVHL‐atypical PKC‐JunB pathway contributed to promotion of cell invasiveness and angiogenesis in clear cell renal cell carcinoma (ccRCC), and we detected chemokine (C‐C motif) ligand‐2 (CCL2) as one of downstream effectors of JunB. CCL2 plays a critical role in tumorigenesis in other types of cancer, but its role in ccRCC remains unclear. In this study, we investigated the roles and therapeutic potential of CCL2 in ccRCC. Immunohistochemical analysis of CCL2 expression for ccRCC specimens showed that upregulation of CCL2 expression correlated with clinical stage, overall survival, and macrophage infiltration. For functional analysis of CCL2 in ccRCC cells, we generated subclones of WT8 cells that overexpressed CCL2 and subclones 786‐O cells in which CCL2 expression was knocked down. Although CCL2 expression did not affect cell proliferation in vitro, CCL2 overexpression enhanced and CCL2 knockdown suppressed tumor growth, angiogenesis, and macrophage infiltration in vivo. We then depleted macrophages from tumor xenografts by administration of clodronate liposomes to confirm the role of macrophages in ccRCC. Depletion of macrophages suppressed tumor growth and angiogenesis. To examine the effect of inhibiting CCL2 activity in ccRCC, we administered CCL2 neutralizing antibody to primary RCC xenografts established from patient surgical specimens. Inhibition of CCL2 activity resulted in significant suppression of tumor growth, angiogenesis, and macrophage infiltration. These results suggest that CCL2 is involved in angiogenesis and macrophage infiltration in ccRCC, and that CCL2 could be a potential therapeutic target for ccRCC.

Vascular endothelial growth factor (VEGF) and mammalian target of rapamycin are well-known therapeutic targets for renal cell carcinoma (RCC). Sunitinib is an agent that targets VEGF receptors and is considered to be a standard treatment for metastatic or unresectable clear cell RCC (ccRCC). However, ccRCC eventually develops resistance to sunitinib in most cases, and the mechanisms underlying this resistance are not fully elucidated. In the present study, we established unique primary xenograft models, KURC1 (Kyoto University Renal Cancer 1) and KURC2, from freshly isolated ccRCC specimens. The KURC1 xenograft initially responded to sunitinib treatment, however finally acquired resistance. KURC2 retained sensitivity to sunitinib for over 6 months. Comparing gene expression profiles between the two xenograft models with different sensitivity to sunitinib, we identified interleukin 13 receptor alpha 2 (IL13RA2) as a candidate molecule associated with the acquired sunitinib-resistance in ccRCC. And patients with high IL13RA2 expression in immunohistochemistry in primary ccRCC tumor tends to have sunitinib-resistant metastatic site. Next, we showed that sunitinib-sensitive 786-O cells acquired resistance in vivo when IL13RA2 was overexpressed. Conversely, shRNA-mediated knockdown of IL13RA2 successfully overcame the sunitinib-resistance in Caki-1 cells. Histopathological analyses revealed that IL13RA2 repressed sunitinib-induced apoptosis without increasing tumor vasculature in vivo. To our knowledge, this is a novel mechanism of developing resistance to sunitinib in a certain population of ccRCC, and these results indicate that IL13RA2 could be one of potential target to overcome sunitinib resistance.

Vascular endothelial growth factor (VEGF)‐targeted therapies show significant antitumor effects for advanced clear cell renal cell carcinomas (CC‐RCCs). Previous studies using VEGF inhibitors in mice models revealed that VEGF‐dependent capillaries were characterized by the existence of endothelial fenestrations (EFs). In this study, we revealed that capillaries with abundant EFs did exist, particularly in CC‐RCCs harboring VHL mutation. This finding was recapitulated in mice xenograft models, in which tumors from VHL null cells showed more abundant EFs compared to those from VHL wild‐type cells. Importantly, treatment with bevacizumab resulted in a significant decrease of tumor size established from VHL null cells. Additionally, a significant reduction of EFs and microvessel density was observed in VHL null tumors. Indeed, xenograft from 786‐O/mock (pRC3) cells developed four times more abundant EFs than that from 786‐O/VHL (WT8). However, introduction of the constitutively active form of hypoxia‐inducible factor (HIF)‐2α to WT8 cells failed to either augment the number of EFs or restore the sensitivity to bevacizumab in mice xenograft, irrespective of the equivalent production of VEGF to 786‐O/mock cells. These results indicated that HIF‐2α independent factors also play significant roles in the development of abundant EFs. In fact, several angiogenesis‐related genes including CCL2 were upregulated in 786‐O cells in a HIF‐2α independent manner. Treatment with CCL2 neutralizing antibody caused significant reduction of capillaries with EFs in 786‐O xenograft, indicating that they were also sensitive to CCL2 inhibition as well as VEGF. Collectively, these results strongly indicated that capillaries with distinctive phenotype developed in VHL null CC‐RCCs are potent targets for anti‐angiogenic therapy.

Resistance to the mechanistic target of rapamycin (mTOR) inhibitors, which are a standard treatment for advanced clear cell renal cell carcinoma (ccRCC), eventually develops in most cases. In this study, we established a patient‐derived xenograft (PDX) model which acquired resistance to the mTOR inhibitor temsirolimus, and explored the underlying mechanisms of resistance acquisition. Temsirolimus was administered to PDX model mice, and one cohort of PDX models acquired resistance after repeated passages. PDX tumors were genetically analyzed by whole‐exome sequencing and detected several genetic alterations specific to resistant tumors. Among them, mutations in ANKRD12 and DNMT1 were already identified in the early passage of a resistant PDX model, and we focused on a DNMT1 mutation as a potential candidate for developing the resistant phenotype. While DNMT1 expression in temsirolimus‐resistant tumors was comparable with the control tumors, DNMT enzyme activity was decreased in resistant tumors compared with controls. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR‐associated protein 9‐mediated heterozygous knockdown of DNMT1 in the temsirolimus‐sensitive ccRCC (786‐O) cell line was shown to result in a temsirolimus‐resistant phenotype in vitro and in vivo. Integrated gene profiles using methylation and microarray analyses of PDX tumors suggested a global shift for the hypomethylation status including promotor regions, and showed the upregulation of several molecules that regulate the mTOR pathway in temsirolimus‐resistant tumors. Present study showed the feasibility of PDX model to explore the mechanisms of mTOR resistance acquisition and suggested that genetic alterations, including that of DNMT1, which alter the methylation status in cancer cells, are one of the potential mechanisms of developing resistance to temsirolimus.

The modern strategy including molecular-targeted therapy may improve OS in patients with mRCC and MSKCC intermediate risk features in clinical practice, relative to those with other risk features. However, the prognosis for patients with tumors of histological type other than clear-cell, decreased Hb, elevated LDH, elevated CRP, or metastases at ≥ 3 sites remains poor even in the modern molecular-targeted era. Novel treatment strategies are necessary to improve prognosis in these patients.