Constitutive activation of the Rearranged during Transfection (RET) proto-oncogene leads to the development of
MEN2A medullary thyroid cancer (MTC). The relatively clear genotype/phenotype relationship seen with RET mutations and the development of
MEN2A is unusual in the fact that a single gene activity can drive the progression towards metastatic disease. Despite knowing the oncogene responsible for
MEN2A, MTC, like most
tumors of neural crest origin, remains largely resistant to
chemotherapy. Constitutive activation of RET in a SK-N-MC cell line model reduces cell sensitivity to
chemotherapy. In an attempt to identify components of the machinery responsible for the observed RET induced chemoresistance, we performed a proteomic screen of
histones and associated
proteins in cells with a constitutively active RET signaling pathway. The proteomic approach identified
DNA-
PKcs,
a DNA damage response
protein, as a target of the RET signaling pathway. Active
DNA-
PKcs, which is phosphorylated at site
serine 2056 and localized to
chromatin, was elevated within our model. Treatment with the RET inhibitor RPI-1 significantly reduced s2056 phosphorylation in RET cells as well as in a human
medullary thyroid cancer cell line. Additionally, inhibition of
DNA-
PKcs activity diminished the chemoresistance observed in both cell lines. Importantly, we show that activated
DNA-
PKcs is elevated in medullary thyroid
tumor samples and that expression correlates with expression of RET in thyroid
tumors. These results highlight one mechanism by which RET signaling likely primes cells for rapid response to DNA damage and suggests
DNA-
PKcs as an additional target in MTC.