The Protein Phosphatase PPM1G Destabilizes HIF-1α Expression

PPM1G regulates hepatic ischemia/reperfusion injury through STING-mediated inflammatory pathways in macrophages

BACKGROUND

Ischemia/reperfusion injury (IRI) is generally unavoidable following liver transplantation. Here, we investigated the role of protein phosphatase, Mg2+ /Mn2+ dependent 1G (PPM1G) in hepatic IRI.

METHODS

Hepatic IRI was mimicked by employing a hypoxia/reperfusion (H/R) model in RAW 264.7 cells and a 70% warm ischemia model in C57BL/6 mice, respectively. In vitro, expression changes of tumor necrosis factor-α and interleukin were detected by quantitative real-time polymerase chain reaction (qRT-PCR), western blot analysis, and enzyme-linked immunosorbent assay. The protein expressions of PPM1G and the stimulator of interferon genes (STING) pathway components were analyzed by western blot. Interaction between PPM1G and STING was verified by coimmunoprecipitation (CO-IP). Immunofluorescence was applied for detection of p-IRF3. Flow cytometry, qRT-PCR and western blot were utilized to analyze markers of macrophage polarization. In vivo, histological analyses of mice liver were carried out by TUNEL and H&E staining. Changes in serum aminotransferases were also detected.

RESULTS

Following H/R intervention, a steady decline in PPM1G along with an increase in inflammatory cytokines in vitro was observed. Addition of plasmid with PPM1G sequence limited the release of inflammatory cytokines and downregulated phosphorylation of STING. CO-IP validated the interaction between PPM1G and STING. Furthermore, inhibition of PPM1G with lentivirus enhanced phosphorylation of STING and its downstream components; meanwhile, p65, p38, and Jnk were also surged to phosphorylation. Expression of INOS and CD86 was surged, while CD206, Arg-1, and IL-10 were inhibited. In vivo, PPM1G inhibition further promoted liver damage, hepatocyte apoptosis, and transaminases release. Selective inhibition of STING with C-176 partially reversed the activation of STING pathway and inflammatory cytokines in vitro. M1 markers were also suppressed by C-176. In vivo, C-176 rescued liver damage and transaminase release caused by PPM1G inhibition.

CONCLUSION

PPM1G suppresses hepatic IRI and macrophage M1 phenotype by repressing STING-mediated inflammatory pathways.

Oxygen-regulated post-translation modifications as master signalling pathway in cells

Oxygen is essential for viability in mammalian organisms. However, cells are often exposed to changes in oxygen availability, due to either increased demand or reduced oxygen supply, herein called hypoxia. To be able to survive and/or adapt to hypoxia, cells activate a variety of signalling cascades resulting in changes to chromatin, gene expression, metabolism and viability. Cellular signalling is often mediated via post-translational modifications (PTMs), and this is no different in response to hypoxia. Many enzymes require oxygen for their activity and oxygen can directly influence several PTMS. Here, we review the direct impact of changes in oxygen availability on PTMs such as proline, asparagine, histidine and lysine hydroxylation, lysine and arginine methylation and cysteine dioxygenation, with a focus on mammalian systems. In addition, indirect hypoxia-dependent effects on phosphorylation, ubiquitination and sumoylation will also be discussed. Direct and indirect oxygen-regulated changes to PTMs are coordinated to achieve the cell's ultimate response to hypoxia. However, specific oxygen sensitivity and the functional relevance of some of the identified PTMs still require significant research.

Pyruvate dehydrogenase phosphatase 1 (PDP1) stimulates HIF activity by supporting histone acetylation under hypoxia

Cancer cells, when exposed to the hypoxic tumour microenvironment, respond by activating hypoxia-inducible factors (HIFs). HIF-1 mediates extensive metabolic re-programming, and expression of HIF-1α, its oxygen-regulated subunit, is associated with poor prognosis in cancer. Here we analyse the role of pyruvate dehydrogenase phosphatase 1 (PDP1) in the regulation of HIF-1 activity. PDP1 is a key hormone-regulated metabolic enzyme that dephosphorylates and activates pyruvate dehydrogenase (PDH), thereby stimulating the conversion of pyruvate into acetyl-CoA. Silencing of PDP1 down-regulated HIF transcriptional activity and the expression of HIF-dependent genes, including that of PDK1, the kinase that phosphorylates and inactivates PDH, opposing the effects of PDP1. Inversely, PDP1 stimulation enhanced HIF activity under hypoxia. Alteration of PDP1 levels or activity did not have an effect on HIF-1α protein levels, nuclear accumulation or interaction with its partners ARNT and NPM1. However, depletion of PDP-1 decreased histone H3 acetylation of HIF-1 target gene promoters and inhibited binding of HIF-1 to the respective hypoxia-response elements (HREs) under hypoxia. Furthermore, the decrease of HIF transcriptional activity upon PDP1 depletion could be reversed by treating the cells with acetate, as an exogenous source of acetyl-CoA, or the histone deacetylase (HDAC) inhibitor trichostatin A. These data suggest that the PDP1/PDH/HIF-1/PDK1 axis is part of a homeostatic loop which, under hypoxia, preserves cellular acetyl-CoA production to a level sufficient to sustain chromatin acetylation and transcription of hypoxia-inducible genes.

Clinical Value of PPM1G Gene in Survival Prognosis and Immune Infiltration of Hepatocellular Carcinoma

OBJECTIVE

Liver cancer is one of the most common malignancies, but its prognosis is still poor. Exploring potential biomarkers is an important direction of tumor research. We intend to use bioinformatics methods to explore potential biomarkers related to survival and prognosis of HCC.

METHODS

The mRNA and protein expressions of PPM1G in liver cancer were analyzed by HPA, TIMER, and UALCAN databases, and the effects of PPM1G on the prognosis of liver cancer patients were explored by the GEPIA database. We also explored the correlation between PPM1G expression and liver cancer immune infiltration through the TIMER database and further explored the potential protein interaction network of PPM1G through the STRING database.

RESULTS

The mRNA and protein expression of PPM1G gene in hepatocellular carcinoma tissues was lower than that in normal adjacent tissues. Liver cancer patients with high expression of PPM1G have a better prognosis than those with low expression of PPM1G. The expression of PPM1G is positively or negatively correlated with different immune cells of liver cancer, such as CD4+ T lymphocytes, CD8+ T lymphocytes, B cells, macrophages, and neutrophils.

CONCLUSION

The liver cancer patients with high expression of PPM1G have a good prognosis, and PPM1G gene may be a potential immunotherapy target and prognostic marker of liver cancer.