Suppression of cancer cell migration and invasion by protein phosphatase 2A through dephosphorylation of μ- and m-calpains

Abstract

The μ- and m-calpains are major members of the calpain family that play an essential role in regulating cell motility. We have recently discovered that nicotine-activated protein kinase C ι enhances calpain phosphorylation in association with enhanced calpain activity and accelerated migration and invasion of human lung cancer cells. Here we found that specific disruption of protein phosphatase 2A (PP2A) activity by expression of SV40 small tumor antigen up-regulates phosphorylation of μ-and m-calpains whereas C2-ceramide, a potent PP2A activator, reduces nicotine-induced calpain phosphorylation, suggesting that PP2A may function as a physiological calpain phosphatase. PP2A co-localizes and interacts with μ- and m-calpains. Purified, active PP2A directly dephosphorylates μ- and m-calpains in vitro. Overexpression of the PP2A catalytic subunit (PP2A/C) suppresses nicotine-stimulated phosphorylation of μ- and m-calpains, which is associated with inhibition of calpain activity, wound healing, cell migration, and invasion. By contrast, depletion of PP2A/C by RNA interference enhances calpain phosphorylation, calpain activity, cell migration, and invasion. Importantly, C2-ceramide-induced suppression of calpain phosphorylation results in decreased secretion of μ- and m-calpains from lung cancer cells into culture medium, which may have potential clinic relevance in controlling metastasis of lung cancer. These findings reveal a novel role for PP2A as a physiological calpain phosphatase that not only directly dephosphorylates but also inactivates μ- and m-calpains, leading to suppression of migration and invasion of human lung cancer cells. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.