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The enzyme PIMT methylates abnormal aspartyl residues in proteins. U-87 MG cells are commonly used to study the most frequent brain tumor, glioblastoma. Previously, we reported that PIMT isoform I possessed oncogenic features when overexpressed in U-87 MG and U-251 MG glioma cells. Higher levels of wild-type PIMT stimulated migration and invasion in both glioma cell lines. Conversely, PIMT silencing reduced these migratory abilities of both cell lines. These results indicate that PIMT could play a critical role in glioblastoma growth. Here, we investigated for the first time, molecular mechanisms involving PIMT in the regulation of epithelial to mesenchymal transition (EMT) upon TGF-β1 treatments. Gene array analyses indicated that EMT genes but not PIMT gene were regulated in U-87 MG cells treated with TGF-β1. Importantly, PIMT silencing by siRNA inhibited in vitro migration in U-87 MG cells induced by TGF-β1. In contrast, overexpressed wild-type PIMT and TGF-β1 had additive effects on cell migration. When PIMT was inhibited by siRNA, this prevented Slug induction by TGF-β1, while Snail stimulation by TGF-β1 was increased. Indeed, overexpression of wild-type PIMT led to the opposite effects on Slug and Snail expression dependent on TGF-β1. These data highlighted the importance of PIMT in the EMT response dependent on TGF-β1 in U-87 MG glioma cells by an antagonist regulation in the expression of transcription factors Slug and Snail, which are critical players in EMT.

The protein L-isoaspartyl (D-aspartyl) methyltransferase (PIMT) recognizes abnormal L-isoaspartyl and D-aspartyl residues in proteins. Among examined tissues, PIMT shows the highest level in the brain. The U-87 MG cell line is a commonly used cellular model to study the most frequent brain tumor, glioblastoma. Previously, we reported that PIMT amount increased when U-87 MG cells were detached from the extracellular matrix. Recently, we also showed that PIMT possessed pro-angiogenic properties. Together, these PIMT features led us to postulate that PIMT could play a critical role in glioblastoma growth. Here, we investigate PIMT role in U-87 MG cell viability, adhesion, migration, invasion, and colony formation and in the reorganization of the actin and tubulin cytoskeleton. PIMT inhibition by siRNA significantly reduced in vitro cell migration and invasion in various assays, including wound-healing assay, Boyden chambers coated with gelatin and Matrigel invasion assay. Conversely, in stably transfected U-87 MG cells overexpressing wild-type PIMT, cell migration, invasive capacity and colony formation significantly increased. However, in stably transfected cells with the gene encoding for mutated PIMT(D83V), despite of its overexpression, migration and invasion remained similar to those observed in control cells. In all these conditions, cell viability was unaffected. Importantly, overexpressed wild-type PIMT and mutated PIMT(D83V) have opposite effects on the organization of microtubules and actin cytoskeleton and thus on morphology of U-87 cells. These data highlighted the importance of PIMT level and its catalytic activity in migration and invasion of U-87 glioma cells and its possible contribution in cancer invasion during glioma growth.

The protein L-isoaspartyl (D-aspartyl) methyltransferase (PIMT) methylates proteins carrying altered aspartyl residues in their structure. PIMT is postulated to limit the accumulation of these damaged proteins with abnormal aspartyl residues. However, little is known about the role of PIMT in tumor growth and almost nothing about its involvement in angiogenic processes. We previously reported that PIMT was up-regulated when endothelial cells were detached from extracellular matrix, leading us to postulate that PIMT could play a critical role during angiogenic steps, since the contacts between endothelial cells and the extracellular matrix are intensively regulated during this process. Here, we demonstrated that PIMT down-regulation by siRNA in human umbilical vein endothelial cells (HUVECs) inhibited both cell migration and tube formation in vitro when stimulated by vascular endothelial growth factor (VEGF). Conversely, overexpression of wild-type PIMT promoted HUVEC migration in the presence of VEGF, while this response was prevented in cells transfected with the inactive mutant PIMT(D83V). Similar results were obtained with the two forms of PIMT regarding their capacity to regulate the action of VEGF during the formation of capillary-like structures in vitro. Together, these data highlight the importance of the catalytic activity of PIMT to mediate VEGF effects during endothelial cell migration and tube formation in angiogenesis. Furthermore, these results identify a new function for PIMT as an enzyme involved in pro-angiogenic processes.