Pericytes elicit resistance to vemurafenib and sorafenib therapy in thyroid carcinoma via the TSP-1/TGF{beta}1 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 co-culture, 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 blocked tumor cell proliferation, increased cell death, and decreased 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 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.

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