Calcium/Calmodulin-Dependent Protein Kinase II and Its Endogenous Inhibitor α in Medullary Thyroid Cancer.

Purpose: Calcium/calmodulin-dependent kinase II (CaMKII) is involved in the regulation of cell proliferation. Its endogenous inhibitor (hCaKIINa) is expressed in some cell types. We determined the role of CaMKII in RET-stimulated proliferation and hCaMKIINa in medullary thyroid carcinoma (MTC). Experimental Design: We analyzed the role of RET mutants on CaMKII activation in NIH3T3 and in MTC cell lines, and determined the effect of CaMKII inhibition on RET/ERK pathway and cell proliferation. Then the expression of hCaKIINa mRNA was determined by real-time PCR in primary MTC and it was correlated with some clinicopathologic parameters. Results: RETC634Y and RETM918T mutants expressed in NIH3T3 cells induced CaMKII activation. CaMKII was activated in unstimulated MTC cells carrying the same RET mutants and it was inhibited by RET inhibition. Inhibition of CaMKII in these cells induced a reduction of Raf-1, MEK, and ERK phosphorylation, cyclin D expression, and cell proliferation. hCaKIINa mRNA expression in primary MTC was very variable and did not correlate with gender and age at diagnosis. Serum calcitonin, (R2 1/4 0.032; P 1/4 0.017), tumor volume (P 1/4 0.0079), lymph node metastasis (P 1/4 0.033), and staging (P 1/4 0.0652) were negatively correlated with the hCaKIINa mRNA expression. Conclusions: CaMKII is activated by RET mutants and is activated at baseline in MTC cells where it mediates the oncogenic pathway leading to cell proliferation. The mRNA expression of its endogenous inhibitor hCaKIINa inversely correlates with the severity of MTC. CaMKII might represent a new target for MTCtherapy and hCaKIINais a marker of disease extension