Interferon-Inducible Protein 9 (CXCL11)-Induced Cell Motility in Keratinocytes Requires Calcium Flux-Dependent Activation of μ-Calpain

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Abstract
Keratinocyte migration is critical to reepithelialization during wound repair. The motility response is promoted by growth factors, cytokines, and cytokines produced in the wound bed, including those that activate the epidermal growth factor (EGF) receptor. The Alu-Leu-Arg-negative CXC chemokine interferon-inducible protein 9 (IP-9; also known as CXCL11, I-TAC, beta-R1, and H-174) is produced by keratinocytes in response to injury. As keratinocytes also express the receptor, CXCR3, this prompted us to examine the role and molecular mechanism by which IP-9 regulates keratinocyte motility. Unexpectedly, as CXCR3 liganding blocks growth factor-induced motility in fibroblasts, IP-9 alone promoted motility in undifferentiated keratinocytes (37 ± 6% of the level of the highly motogenic EGF) as determined in a two-dimensional in vitro wound healing assay. IP-9 even enhanced EGF-induced motility in undifferentiated keratinocytes (116 ± 5%; P< 0.05 compared to EGF alone), suggesting two separate mechanisms of action. IP-9-increased motility and -decreased adhesiveness required the intracellular protease calpain. The increases in both motility and calpain activity by IP-9 were blocked by pharmacological and molecular inhibition of phospholipase C-β3 and chelation of calcium, which prevented an intracellular calcium flux. Molecular downregulation or RNA interference-mediated depletion of μ-calpain (calpain 1) but not M-calpain (calpain 2) blocked IP-9-induced calpain activation and motility. In accord with elimination of IP-9-induced de-adhesion, RNA interference-mediated depletion of calpain 1 but not calpain 2 prevented cleavage of the focal adhesion component focal adhesion kinase and disassembly of vinculin aggregates. In comparison, EGF-induced motility of the same undifferentiated keratinocytes requires the previously described extracellular signal-regulated kinase to the M-calpain pathway. These data demonstrate that while both EGF- and IP-9-induced motility in keratinocytes requires calpain activity, the isoform of calpain triggered depends on the nature of the receptor for the particular ligand. Interestingly, physiological nonapoptotic calcium fluxes were capable of activating μ-calpain, implying that the calcium requirement of μ-calpain for activation is attained during cell signaling. This is also the first demonstration of differential activation of the two ubiquitous calpain isoforms in the same cell by different signals.