Protein kinase Cι promotes nicotine-induced migration and invasion of cancer cells via phosphorylation of μ- and m-calpains

Nicotine is a major component in cigarette smoke that activates the growth-promoting pathways to facilitate the development of lung cancer. However, it is not clear whether nicotine affects cell motility to facilitate tumor metastasis. Here we discovered that nicotine potently induces phosphorylation of both mu- and m-calpains via activation of protein kinase C iota( PKC iota), which is associated with accelerated migration and invasion of human lung cancer cells. Purified PKC iota directly phosphorylates mu- and m-calpains in vitro. Overexpression of PKC iota results in increased phosphorylation of both mu- and m-calpains in vivo. Nicotine also induces activation of c-Src, which is a known PKC iota upstream kinase. Treatment of cells with the alpha(7) nicotinic acetylcholine receptor inhibitor alpha-bungarotoxin can block nicotine-induced calpain phosphorylation with suppression of calpain activity, wound healing, cell migration, and invasion, indicating that nicotine-induced calpain phosphorylation occurs, at least in part, through a signaling pathway involving the upstream alpha(7) nicotinic acetylcholine receptor. Intriguingly, depletion of PKC iota by RNA interference suppresses nicotine-induced calpain phosphorylation, calpain activity, cell migration, and invasion, indicating that PKC iota is a necessary component in nicotine-mediated cell motility signaling. Importantly, nicotine potently induces secretion of mu- and m-calpains from lung cancer cells into culture medium, which may have potential to cleave substrates in the extracellular matrix. These findings reveal a novel role for PKC iota as a nicotine-activated, physiological calpain kinase that directly phosphorylates and activates calpains, leading to enhanced migration and invasion of human lung cancer cells.