Fibronectin-induced proliferation in thyroid cells is mediated by alphavbeta3 integrin through Ras/Raf-1/MEK/ERK and calcium/CaMKII signals.

We recently demonstrated in an immortalized thyroid cell line that integrin stimulation by fibronectin (FN) simultaneously activates two signaling pathways: Ras/Raf/MAPK kinase (Mek)/Erk and calcium (Ca 2 )/calcium calmodulin-dependent kinase II (CaMKII). Both signals are necessary to stimulate Erk phosphorylation because CaMKII modulates Ras-induced Raf-1 activity. In this study we present evidence that extends these findings to normal human thyroid cells in primary cul- ture, demonstrating its biological significance in a more phys- iological cell model. In normal thyroid cells, immobilized FN- induced activation of p21Ras and Erk phosphorylation. This pathway was responsible for FN-induced cell proliferation. Concurrent increase of intracellular Ca 2 concentration and CaMKII activation was observed. Both induction of p21Ras activity and increase of intracellular Ca 2 concentration were mediated by FN binding to v3 integrin. Inhibition of the Ca 2 /CaMKII signal pathway by calmodulin or CaMKII inhib- itors completely abolished the FN-induced Erk phosphoryla- tion. Binding to FN induced Raf-1 and CaMKII to form a pro- tein complex, indicating that intersection between Ras/Raf/ Mek/Erk and Ca 2 /CaMKII signaling pathways occurred at Raf-1 level. Interruption of the Ca 2 /CaMKII signal pathway arrested cell proliferation induced by FN. We also analyzed thyroid tumor cell lines that displayed concomitant aberrant integrin expression and signal transduction. These data con- firm that integrin activation by FN in normal thyroid cells generates Ras/Raf/Mek/Erk and Ca 2 /CaMKII signaling path- ways and that both are necessary to stimulate cell prolifera- tion, whereas in thyroid tumors integrin signaling is altered. (J Clin Endocrinol Metab 90: 2865–2873, 2005)