α3β1 integrin regulates MMP-9 mRNA stability in immortalized keratinocytes:: a novel mechanism of integrin-mediated MMP gene expression

Matrix metalloproteinases facilitate cell migration and tumor invasion through their ability to proteolyse the extracellular matrix. The laminin-binding integrin alpha 3 beta 1 is expressed at high levels in squamous cell carcinomas and in normal keratinocytes during cutaneous wound healing. We showed previously that alpha 3 beta 1 is required for MMP-9/gelatinase B secretion in immortalized mouse keratinocytes (MK cells) and that this regulation was acquired as part of the immortalized phenotype, suggesting a possible role for alpha 3 beta 1 during malignant conversion. In the current study, we identify a novel mechanism whereby alpha 3 beta 1 regulates the induction of MMP-9 expression that occurs in response to activation of a MAPK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway. Inhibition of MEK/ERK signaling in wild-type MK cells with a pharmacological inhibitor, U0126, showed that ERK activation was necessary for high levels of endogenous MMP-9 gene expression and activity of a transfected MMP-9 promoter. Furthermore, activation of MEK/ERK signaling in these cells with an oncogenic mutant of Ras, RasV12, increased both endogenous MMP-9 gene expression and MMP-9 promoter activity. Experiments with alpha 3 beta 1-deficient MK cells revealed that alpha 3 beta 1 was required for both baseline levels and RasV12-induced levels of MMP-9 mRNA expression. However, alpha 3 beta 1 was not required for RasV12-mediated activation of ERK or for ERK-dependent MMP-9 promoter activity. Direct comparison of mRNA turnover in the wild type and alpha 3-null MK cells identified a requirement for alpha 3 beta 1 in stabilization of MMP-9 mRNA transcripts. These results identify a novel function for integrins in promoting mRNA stability as a mechanism to potentiate MAPK-mediated gene expression. They also suggest a role for alpha 3 beta 1 in maintaining high levels of MMP-9 mRNA expression in response to oncogenic activation of MEK/ERK signaling pathways.