Pericytes Elicit Resistance to Vemurafenib and Sorafenib Therapy in Thyroid Carcinoma via the TSP-1/TGFβ1 Axis

Purpose: The BRAF V600E oncogene modulates the papillary thyroid carcinoma (PTC) microenvironment, in which pericytes are critical regulators of tyrosine-kinase (TK)-dependent signaling pathways. Although BRAF V600E and TK inhibitors are available, their efficacy as bimodal therapeutic agents in BRAF V600E -PTC is still unknown. Experimental Design: We assessed the effects of vemurafenib (BRAF V600E inhibitor) and sorafenib (TKI) as single agents or in combination in BRAF WT/V600E -PTC and BRAF WT/WT cells using cell-autonomous, pericyte coculture, and an orthotopic mouse model. We also used BRAF WT/V600E -PTC and BRAF WT/WT -PTC clinical samples to identify differentially expressed genes fundamental to tumor microenvironment. Results: Combined therapy blocks tumor cell proliferation, increases cell death, and decreases motility via BRAF V600E inhibition in thyroid tumor cells in vitro . Vemurafenib produces cytostatic effects in orthotopic tumors, whereas combined therapy (likely reflecting sorafenib activity) generates biological fluctuations with tumor inhibition alternating with tumor growth. We demonstrate that pericytes secrete TSP-1 and TGFβ1, and induce the rebound of pERK1/2, pAKT and pSMAD3 levels to overcome the inhibitory effects of the targeted therapy in PTC cells. This leads to increased BRAF V600E -PTC cell survival and cell death refractoriness. We find that BRAF WT/V600E -PTC clinical samples are enriched in pericytes, and TSP1 and TGFβ1 expression evoke gene-regulatory networks and pathways (TGFβ signaling, metastasis, tumor growth, tumor microenvironment/ECM remodeling functions, inflammation, VEGF ligand–VEGF receptor interactions, immune modulation, etc.) in the microenvironment essential for BRAF WT/V600E -PTC cell survival. Critically, antagonism of the TSP-1/TGFβ1 axis reduces tumor cell growth and overcomes drug resistance. Conclusions: Pericytes shield BRAF V600E -PTC cells from targeted therapy via TSP-1 and TGFβ1, suggesting this axis as a new therapeutic target for overcoming resistance to BRAF V600E and TK inhibitors. Clin Cancer Res; 1–20. ©2018 AACR.