ERK activation and cell growth require CaM kinases in MCF-7 breast cancer cells

Previous studies on MCF-7 breast cancer cells have shown that the G-protein coupled receptor (GPCR) agonist carbachol increases intracellular calcium levels and the activation of extracellular signal-regulated kinase (ERK). Calcium and calmodulin regulate the calcium/calmodulin-dependent kinase (CaM kinase) family of proteins that have been proposed to regulate ERK and gene transcription. Our results suggest that both estrogen (E2) and carbachol treatment of MCF-7 breast cancer cells trigger phosphorylation of ERK1/2 and the transcription factor Elk-1. Carbachol and estrogen triggered nearly a four- to sixfold increase in MCF-7 cell proliferation by 96 h, respectively. Carbachol-stimulated ERK activation and cell growth was completely blocked by the Muscarinic M₃-subtype GPCR inhibitor, 4-DAMP, and siRNA against the M₃-subtype GPCR. Interestingly, blockade of CaM KK with the selective inhibitor STO-609 prevented carbachol activation CaM KI, ERK, Elk-1, and cell growth. Consistent with these observations, knockdown of CaM KKα and CaM KIγ with shRNA-containing plasmids blocked ERK activation by carbachol. In addition, Elk-1 phosphorylation and luciferase activity in response to carbachol treatment was also dependent upon CaM kinases and was inhibited by U0126, STO-609, and siRNA knockdown of CaM kinases and ERK2. Finally, blockade of either CaM KK (with STO-609) or ERK (with U0126) activities resulted in the inhibition of carbachol- and estrogen-mediated cyclin D1 expression and MCF-7 cell growth. Taken together, our results suggest that carbachol treatment of MCF-7 cells activates CaM KI, ERK, the transcription factor Elk-1, cyclin D1, and cell growth through CaM KK.