BRAFV600E mutation influences hypoxia-inducible factor-1α expression levels in papillary thyroid cancer

Abstract

Hypoxia-inducible factor-1α is found frequently overexpressed in solid tumors cells, exerting an important role in angiogenesis, glucose metabolism, cell proliferation, survival and invasion. In thyroid carcinomas, hypoxia-inducible factor-1α expression was found increased in differentiated, poorly differentiated, medullary and anaplastic variants. Hypoxia represents the principal stimulus responsible for hypoxia-inducible factor-1α induction. Other nonhypoxic stimuli increase hypoxia-inducible factor-1α synthesis through the activation of phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways in a cell-type-specific manner. We have previously shown the role of BRAFV600E mutation in papillary thyroid cancer cells as a factor that facilitates tumor cell growth and progression. In this study, we tested the hypothesis that BRAFV600E mutation influences hypoxia-inducible factor-1α expression in papillary thyroid carcinoma cells. We analyzed 27 papillary thyroid carcinomas, 13 of which presented BRAFV600E mutation. In tumor tissues, immunoreactivity for hypoxia-inducible factor-1α was detected in the majority of analyzed BRAFV600E mutated cases. Transcriptional analyses revealed elevated hypoxia-inducible factor-1α levels with significant differences between wild-type and mutated group. A BRAF wild-type papillary thyroid carcinoma cell line and a BRAFV600E mutated papillary thyroid carcinoma cell line were selected to study the effects of BRAF mutation on hypoxia-inducible factor-1α expression in vitro. Knockdown of mutant BRAFV600E or both the wild type and the BRAFV600E by RNA interference induced a significant reduction of hypoxia-inducible factor-1α expression at mRNA and protein levels. Pharmacological inhibition of BRAF significantly reduces hypoxia-inducible factor-1α expression levels in papillary thyroid carcinoma cell line harboring BRAFV600E mutation. Our results suggest that hypoxia-inducible factor-1α is expressed in papillary thyroid carcinomas and is regulated not only by hypoxia but also by BRAFV600E-mediated signaling pathway.