Regulation of protein L-isoaspartyl methyltransferase by cell-matrix interactions: involvement of integrin alphavbeta3, PI 3-kinase, and the proteasome.
The enzyme L-isoaspartyl methyltransferase (PIMT) is known to repair damaged proteins that have accumulated abnormal aspartyl residues during cell aging. However, little is known about the mechanisms involved in the regulation of PIMT expression. Here we report that PIMT expression in bovine aortic endothelial cells is regulated by cell detachment and readhesion to a substratum. During cell detachment, the PIMT level was rapidly and strongly increased and correlated with a stimulation of protein synthesis. Aside from endothelial cells, PIMT levels were also regulated by cell adhesion in various cancer cell lines. The upregulation of PIMT expression could be prevented by an anti-alphavbeta3 antibody (LM609) or by a cyclic RGD peptide (XJ735) specific to integrin alphavbeta3, indicating that this integrin was likely involved in PIMT regulation. Moreover, we found that PIMT expression returned to the basal level when cells were replated on a substratum after detachment, though downregulation of PIMT expression could be partly prevented by the PI3K inhibitors LY294002 and wortmannin, as well as by the proteasome inhibitors MG-132, lactacystin, and beta-lactone. These findings support the assumption that the PIMT level was downregulated by proteasomal degradation, involving the PI3K pathway, during cell attachment. This study reports new insights on the molecular mechanisms responsible for the regulation of PIMT expression in cells. The regulation of PIMT level upon cell-substratum contact suggests a potential role for PIMT in biological processes such as wound healing, cell migration, and tumor metastasis dissemination.