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

Abstract

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 μ- and m-calpains via activation of protein kinase Cι(PKCι), which is associated with accelerated migration and invasion of human lung cancer cells. Purified PKCι directly phosphorylates μ- and m-calpains in vitro. Overexpression of PKCι results in increased phosphorylation of both μ- and m-calpains in vivo. Nicotine also induces activation of c-Src, which is a known PKCι upstream kinase. Treatment of cells with the α7 nicotinic acetylcholine receptor inhibitor α-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 α7 nicotinic acetylcholine receptor. Intriguingly, depletion of PKCι by RNA interference suppresses nicotine-induced calpain phosphorylation, calpain activity, cell migration, and invasion, indicating that PKCι is a necessary component in nicotine-mediated cell motility signaling. Importantly, nicotine potently induces secretion of μ- 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 PKCι as a nicotine-activated, physiological calpain kinase that directly phosphorylates and activates calpains, leading to enhanced migration and invasion of human lung cancer cells. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.