Acquired resistance to vemurafenib restrains thyroid cancer stem cell self-renewal by suppressing STAT3 activation

Vemurafenib (PLX4032) is a B-Raf kinase-specific inhibitor that has been approved for treating BRAF-mutated melanoma but is ineffective in treating thyroid cancer. Cancer stem cells (CSCs) play an important role in drug resistance (DR). Our present study aims to determine the status of CSCs in thyroid cancer cells that have acquired adaptive drug resistance to PLX4032. We first established a DR anaplastic thyroid cancer (ATC) cell line by culturing SW1736 cells in the media containing gradually increasing concentrations of PLX4032 (0.1-4 μM) for 6 months. We found that HER3, a member of the ErbB/HER receptor tyrosine kinase (RTK) family, and its downstream MAPK and PI3K pathways are highly activated in DR cells due to increased expression of Yes-associated protein (YAP), a key transcription factor in the Hippo signaling pathway. DR cells can readily proliferate in the presence of PLX4032 (4 μM). However, DR cells express lower levels of the stemness-related genes including Gli1, BMI1, and SOX2, form fewer thyrosphere, and contain fewer aldehyde dehydrogenase (ALDH)-positive cells than parent naïve (PN) cells. DR cells also fail to form tumor xenografts in immunodeficient mice. Mechanistically, constitutive ERK activation in DR cells results in the increased expression of SOCS3 (Suppressor Of Cytokine Signaling 3 expression) and the suppression of STAT3 activation. STAT3 knockout and Ruxolitinib (Rux), a specific inhibitor of Janus kinases (JAK), inhibit the expression of the stemness-related genes. In contrast, STAT3 overexpression increases stemness-related gene expression. Our study suggests that thyroid cancer cells adapted to PLX4032 have limited self-renewal capacity due to impaired STAT3 activation and decreased expression of the stemness-related genes.