ENDOTHELIAL FENESTRATIONS IN TUMOR MICROVASCULATURE AS A POTENT PREDICTIVE MARKER OF ANTI-VEGF THERAPY IN CLEAR CELL RENAL CELL CARCINOMA

Abstract

INTRODUCTION AND OBJECTIVE: Vascular endothelial growth factor (VEGF)-targeted therapies show significant anti-tumor effects for advanced clear cell renal cell carcinomas (CC-RCCs) and kinds of surrogate markers that can successfully predict tumor responses, are highly required. Previously, experimental studies using VEGF neutralization in mice tumor model revealed that VEGF-dependent capillaries in tumor vessels were characterized by the existence of fenestrations in endothelium on electron microscopy study. Based on those results, we examined if endothelial fenestrations were associated with VHL status and tumor responses to anti-VEGF therapy in RCC. METHODS: Twenty-four primary CC-RCCs were examined for the status of VHL, microvessel density and existence of endothelial fenestrations. In mice xenograft models, VHL-/786-O RCC subclones stably transfected with plasmid empty (pRC3) or wild type VHL (WT8) were injected subcutaneously in nude mice to determine the correlation between phenotypes of tumor capillaries and their responses to anti-VEGF therapy. RESULTS: The number of endothelial fenestrations was significantly increased in CC-RCC specimens harboring VHL mutation comparing those without. This finding was also reproducible in mice xenograft models in that capillaries established from VHL null pRC3 cells exhibited more abundant endothelial fenestrations compared to those from VHL transfected WT8 cells. Treatment with bevacizumab resulted in the significant decrease in the mean tumor size of pRC3 but not WT8 cells. In bevacizumab sensitive pRC3 xenografts, a significant reduction of the number of endothelial fenestrations and microvessel density was observed after the treatment. In contrast, a xenograft established from WT8/HIF2 P531A cells which overexpressed an activated form of HIF2 constitutively, exhibited no significant increase of the number of endothelial fenestrations nor reduction of tumor size against bevacizumab treatment compared to that from WT8/mock cells. CONCLUSIONS: Our results suggest that sporadic CCRCCs with VHL mutation harbor VEGF-dependent tumor vessels and those capillaries are potent target for anti-VEGF therapy. Therefore, endothelial fenestration could be a useful surrogate marker in predicting susceptibilities of CC-RCCs to that therapy. Our results also indicated that an increase of VEGF dependent capillaries in VHL -/RCC is not merely caused by the subsequent VEGF overproduction followed by HIF accumulation.