Downregulation of RING ligase praja2 boosts receptor tyrosine kinases in kidney cancer
Clear cell renal cell carcinoma (ccRCC) is the most common variant of kidney cancer in the adult population. Late diagnosis, resistance to therapeutics and recurrence of metastatic lesions account for the highest mortality rate among kidney cancer patients. Identifying novel biomarkers for early cancer detection and the mechanisms underlying ccRCC growth and progression will provide clues to treat this aggressive malignant tumor. Here, we report that the RING ligase praja2 is a novel component of the endocytic system that supports clathrin-mediated receptor endocytosis. At molecular level, we identify the adaptor protein AP2 as a binding partner and substrate of praja2.  Functionally, we demonstrate that praja2 is required for AP2-mediated receptor endocytosis and clearance. Downregulation of praja2 in RCC cells and tissues is associated with a marked upregulation of membrane receptors, as EGFR, VEGFR and TfR. A negative feedback loop links EGF signaling to proteolysis of praja2 and sustains downstream mitogenic and proliferative pathways. Restoring praja2 expression in RCC cells remarkably decreases EGFR levels, rewires cancer cell metabolism and inhibits RCC growth and metastatic diffusion. In praja2 knockout mice, upregulation of RTKs levels associates with profound histopathological renal alterations. Our findings identify praja2 as a component of the endocytic pathway that supports receptor endocytosis and clearance. Downregulation of praja2 in RCC cells, thus, sustains RTK signaling and promotes kidney cancer growth and diffusion.