Transcriptome analysis revealed that PME-1 suppresses inflammatory signaling, activates PI3K/Akt signaling, and promotes epithelial-mesenchymal transition.

Protein phosphatase 2A (PP2A) is an essential serine/threonine protein phosphatase that belongs to the type2A protein phosphatase family with PP4 and PP6. PP2A functions as a trimeric holoenzyme, and the composition of the trimer is regulated by the methyl-esterification (methylation) of PP2A. Demethylation of PP2A is catalyzed by protein phosphatase methyl-esterase-1 (PME-1). Despite the physiological and pathophysiological importance of PME-1, the impact of changes in PME-1 expression on the transcriptome has not been reported. This study provides transcriptome data to gain a comprehensive understanding of the effects of PME-1 knockout on intracellular signaling of mouse embryonic fibroblasts. Our data showed that PME-1 suppresses inflammatory signaling, activates PI3K/Akt signaling, and promotes epithelial-mesenchymal transition.

PP2A inhibitor SET promotes mTORC1 and Bmi1 signaling through Akt activation and maintains the colony-formation ability of cancer cells.

Protein phosphatase 2A (PP2A) is an essential tumor suppressor, with its activity often hindered in cancer cells by endogenous PP2A inhibitory proteins like SE translocation (SET). SET/PP2A axis plays a pivotal role in the colony-formation ability of cancer cells and the stabilization of c-Myc and E2F1 proteins implicated in this process. However, in osteosarcoma cell line HOS, SET knock-down (KD) suppresses the colony-formation ability without affecting c-Myc and E2F1. This study aimed to unravel the molecular mechanism through which SET enhances the colony-formation ability of HOS cells and determine if it is generalized to other cancer cells. Transcriptome analysis unveiled that SET KD suppressed mTORC1 signaling. SET KD inhibited Akt phosphorylation, an upstream kinase for mTORC1. PP2A inhibitor blocked SET KD-mediated decrease in phosphorylation of Akt and a mTORC1 substrate p70S6K. A constitutively active Akt restored decreased colony-formation ability by SET KD, indicating the SET/PP2A/Akt/mTORC1 axis. Additionally, enrichment analysis highlighted that Bmi-1, a polycomb group protein, is affected by SET KD. SET KD decreased Bmi-1 protein by Akt inhibition but not by mTORC1 inhibition, and exogenous Bmi-1 expression rescued the reduced colony formation by SET KD. Four out of eight cancer cell lines exhibited decreased Bmi-1 by SET KD. Further analysis of these cell lines revealed that Myc activity plays a role in SET KD-mediated Bmi-1 degradation. These findings provide new insights into the molecular mechanism of SET-regulated colony-formation ability, which involved Akt-mediated activation of mTORC1/p70S6K and Bmi-1 signaling.

Involvement of PP2A methylation in the adipogenic differentiation of bone marrow-derived mesenchymal stem cell.

Bone marrow-derived mesenchymal stem cells (BM-MSCs) are multipotent stem cells with ability to self-replicate and differentiate into mesodermal derivatives, such as adipocytes and osteoblasts. BM-MSCs are a critical component of the tumour microenvironment. They support tumour progression by recruiting additional BM-MSCs and by differentiating into myofibroblasts (also called cancer-associated fibroblasts). Protein phosphatase 2A (PP2A) is an essential serine/threonine protein phosphatase that regulates a broad range of cellular signalling. PP2A forms a heterotrimer to dephosphorylate specific substrates. The reversible methylesterification (methylation) of Leu309 in the catalytic subunit of PP2A (PP2Ac) regulates biogenesis of the PP2A holoenzyme. It is unknown whether the methylation of PP2Ac plays a role in BM-MSC differentiation. Our experiments determined that protein levels of PP2A subunits and PP2A methyltransferase (LCMT-1) are significantly altered during differentiation. PP2Ac methylation levels in BM-MSCs decrease over time in response to an adipogenic differentiation stimulus. However, blockage of PP2A demethylation using the PP2A dimethyl-esterase inhibitors enhanced adipocyte differentiation. This suggests that PP2Ac demethylation is involved in adipocyte differentiation resistance. The results of our study provide a greater understanding of the regulation of BM-MSCs differentiation by PP2A holoenzyme.

Distribution of SET/I2PP2A protein in gastrointestinal tissues.

SET (also called I2PP2A and TIF-1) is a multi-functional protein that regulates a variety of cell signaling including nucleosome assembly, histone binding, and tumorigenesis. Elevated SET protein levels are observed in various human tumors, and are correlated with poor prognosis and drug-resistance. We recently reported that SET protein levels in cancer cells were positively correlated with poor prognosis of gastric cancer patients. Using immunohistochemistry, SET protein was observed not only in cancer cells, but also in some interstitial cells. However, the tissue distribution of SET has not been investigated. Here we performed co-immunofluorescent staining to characterize SET protein distribution in gastrointestinal tissues. We found that even though the positive rate is much lower than epithelial cells, SET protein is also expressed in non-epithelial cells, such as monocytes/macrophages, neural cells, myofibroblasts, and smooth muscle cells. Our results indicate an extensive role of SET in a variety of cell types.