Inhibition diminishes cell viability via PKC alpha (PKCa) in thyroid cancer cells

Abstract

Thyroid carcinoma (TC) is the most common endocrine neoplasia.Its incidence has increased in the last 40 years worldwide. It comprises a group of tumors of different lineage and biological behavior.About half of TC are driven by an acquired activating mutation inthe BRAF oncogene. While targeted therapies have improved outcomes in melanoma patients, most TC patients become resistant orrecur suggesting that new or additive non-cross-reactive therapiesare needed. We have previously shown that PKCa mediates TSHand thyroid hormones proliferative effects in TC. Recent evidenceindicates that together PKCa overexpression and BRAF mutationshould contribute to tumorigenesis and resistance to anticancertherapies. We found that by inhibiting BRAF expression with RNAi inanaplastic TC cells with BRAF mutation, PKCa expression decreases as well, suggesting that the latter is found downstream of BRAF.Furthermore, a decrease in the expression of the cell proliferationmarker PCNA was observed in BRAF-depleted cells by westernblot analysis. Also, TC cells were sensitive to increasing doses ofthe BRAF inhibitor widely used in the clinic vemurafenib/PLX4032in a dose-dependent manner (p<0.0001) by Cell Titer Blue (CTB)assay. To begin to study the combined inhibition of PKC and BRAF,CTB assays were performed with increasing doses of vemurafenibin presence or absence of the PKC inhibitor GF109203X at selective concentrations in follicular TC cells carrying BRAF mutation. Weconfirmed the dose-dependency of vemurafenib and found that thecombination leads to a significant decrease in cell viability (p<0.5).Our results establish that the effective dual PKCa and BRAF blockade can significantly drive tumor proliferation inhibition. The results obtained could provide new therapeutic targets and alternatives tothe treatments currently used for this disease. Despite its increasingincidence and mortality in many cases, TC constitutes a very poorlystudied area in our country.