CXCR4, CXCR7 and PBRM1 are responsible for everolimus and cabozantinib resistance in human renal cancer cells

The mTOR inhibitor everolimus (RAD001), previously used in first-line treatment of metastatic renal cancer (mRCC), is currently reserved for the following lines of therapy. However, many patients eventually develop resistance to RAD001. To shed new light on the mechanism of RAD001 resistance, A498 cells resistant to 1-5-10 µM of RAD001 were developed (A498-RAD 1-5-10). A498-RAD-resistant cells overexpressed the chromatin remodeling factor PBRM1 and mTOR and downregulated the chemokine receptors CXCR4 and CXCR7. To reverse RAD001 resistance, PBRM1 knockdown was conducted in A498-RAD10 cells. PBRM1 knockdown partially restored sensitivity to RAD001 while inducing CXCR7 but not CXCR4 expression. In A498-RAD10 cells, the CXCR7 transcriptional repressor YY1 is overexpressed and bound to the CXCR7 promoter. As CXCR4 was robustly downregulated in A498-RAD10, and the PBRM1 knockdown only partially restored RAD001 sensitivity, CXCR4 was transfected into A498-RAD10 (A498-RAD10-CXCR4). CXCR4 completely restored RAD001 sensitivity in resistant cells while reducing PBRM1 expression, implying negative feedback. Interestingly, A498-RAD10 cells were cross-resistant to cabozantinib, the tyrosine kinase inhibitor used in first-line treatment of mRCC with nivolumab. Cabozantinib resistance of A498-RAD10 cells was reversed by PBRM1 knockdown or CXCR4 re-expression, mimicking the RAD001 resistance. A498-CABO4-resistant cells were developed and showed PBRM1 overexpression and downregulated CXCR4 and CXCR7. In silico data supported a context‑dependent role of PBRM1 in ccRCC patients. To the best of our knowledge, this is the first description of a mechanism of RAD001 and cabozantinib resistance through PBRM1 overexpression and CXCR7/CXCR4 downregulation and suggest new therapeutic perspective for cabozantinib-resistant patients.