Protein L-isoaspartyl methyltransferase regulates p53 activity.

Protein methylation plays important roles in most, if not all, cellular processes. Lysine and arginine methyltransferases are known to regulate the function of histones and non-histone proteins through the methylation of specific sites. However, the role of the carboxyl-methyltransferase protein L-isoaspartyl methyltransferase (PIMT) in the regulation of protein functions is relatively less understood. Here we show that PIMT negatively regulates the tumour suppressor protein p53 by reducing p53 protein levels, thereby suppressing the p53-mediated transcription of target genes. In addition, PIMT depletion upregulates the proapoptotic and checkpoint activation functions of p53. Moreover, PIMT destabilizes p53 by enhancing the p53-HDM2 interaction. These PIMT effects on p53 stability and activity are attributed to the PIMT-mediated methylation of p53 at isoaspartate residues 29 and 30. Our study provides new insight into the molecular mechanisms by which PIMT suppresses the p53 activity through carboxyl methylation, and suggests a therapeutic target for cancers.

Requirement of protein l-isoaspartyl O-methyltransferase for transcriptional activation of trefoil factor 1 (TFF1) gene by estrogen receptor alpha.

Lysine- and arginine-specific methyltransferases have been shown to act as either direct or secondary transcriptional co-activator of the estrogen receptor (ERα). However, little is known about the role of protein l-isoaspartyl O-methyltransferase (PIMT) on transcriptional regulation. Here, we show that PIMT acts as a co-activator for ERα-mediated transcription. Activation of the estrogen response element (ERE) promoter by ß-estradiol (E(2)) was suppressed by knockdown of PIMT, and enhanced by overexpression of wild-type PIMT. However, the ERE promoter activity was resistant to E(2) stimulation in cells overexpressing a catalytically inactive PIMT mutant, G88A. Consistent with these results, the expression of the endogenous ERα response gene trefoil factor 1 (TFF1) by E(2) was completely abrogated by PIMT depletion and decreased to approximately 50% when PIMT mutant G88A was expressed. In addition, over-expression of PIMT significantly increased the levels of TFF1 mRNA in the presence or absence of E(2). Interestingly, PIMT interacted with ERα and was distributed to the cytosol and the nucleus. The chromatin immunoprecipitation analysis revealed that PIMT was recruited to the promoter of TFF1 gene together with ERα in an E(2)-dependent manner, which was accompanied by uploading of RNA polymerase II on the promoter. Taken together, the results suggest that PIMT may act as a co-activator in ERα-mediated transcription through its recruitment to the promoter via interacting with ERα.

Destabilization of TNF-α mRNA by Rapamycin.

Stimulation of mast cells through the high affinity IgE receptor (FcεRI) induces degranulation, lipid mediator release, and cytokine secretion leading to allergic reactions. Although various signaling pathways have been characterized to be involved in the FcεRI-mediated responses, little is known about the precious mechanism for the expression of tumor necrosis factor-α (TNF-α) in mast cells. Here, we report that rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR), reduces the expression of TNF-α in rat basophilic leukemia (RBL-2H3) cells. IgE or specific antigen stimulation of RBL-2H3 cells increases the expression of TNF-α and activates various signaling molecules including S6K1, Akt and p38 MAPK. Rapamycin specifically inhibits antigen-induced TNF-α mRNA level, while other kinase inhibitors have no effect on TNF-α mRNA level. These data indicate that mTOR signaling pathway is the main regulation mechanism for antigen-induced TNF-α expression. TNF-α mRNA stability analysis using reporter construct containing TNF-α adenylate/uridylate-rich elements (AREs) shows that rapamycin destabilizes TNF-α mRNA via regulating the AU-rich element of TNF-α mRNA. The antigen-induced activation of S6K1 is inhibited by specific kinase inhibitors including mTOR, PI3K, PKC and Ca(2+)chelator inhibitor, while TNF-α mRNA level is reduced only by rapamycin treatment. These data suggest that the effects of rapamycin on the expression of TNF-α mRNA are not mediated by S6K1 but regulated by mTOR. Taken together, our results reveal that mTOR signaling pathway is a novel regulation mechanism for antigen-induced TNF-α expression in RBL-2H3 cells.