Glutaminolysis Epigenetically Regulates Antiapoptotic Gene Expression in Idiopathic Pulmonary Fibrosis Fibroblasts

Fibrotic responses involve multiple cellular processes, including epigenetic changes. Epigenetic changes are sensitive to alterations in the tissue microenvironment such as the flux of tricarboxylic acid (TCA) cycle metabolites. TCA metabolites directly regulate epigenetic states, in part by regulating histone modification-related enzymes. Glutaminolysis is a critical metabolic process by which glutamine is converted to glutamate by glutaminase and then to α-ketoglutarate (α-KG), a TCA cycle metabolite. Idiopathic pulmonary fibrosis (IPF) is a disease characterized by aberrant metabolism, including enhanced glutaminolysis. IPF fibroblasts are apoptosis resistant. In this study, we explored the relationship between glutaminolysis and the resistance to apoptosis of IPF fibroblasts. Inhibition of glutaminolysis decreased expression of XIAP and survivin, members of the inhibitor of apoptosis protein (IAP) family. α-KG is a cofactor for JMJD3 histone demethylase, which targets H3K27me3. In the absence of glutamine, JMJD3 activity in fibroblasts is significantly decreased, whereas H3K27me3 levels are increased. Chromatin immunoprecipitation assays confirmed that JMJD3 directly interacts with XIAP and survivin promoter regions in a glutamine-dependent manner. Exogenous α-KG partially restores JMJD3 function and its interaction with the XIAP and survivin promoter regions under glutamine-deficient conditions. Interestingly, α-KG upregulates XIAP, but not survivin, suggesting differential α-KG-dependent and -independent mechanisms by which glutamine regulates these IAPs. Our data demonstrate a novel mechanism of metabolic regulation in which glutaminolysis promotes apoptosis resistance of IPF fibroblasts through epigenetic regulation of XIAP and survivin.

Pathologic Regulation of Collagen I by an Aberrant Protein Phosphatase 2A/Histone Deacetylase C4/MicroRNA-29 Signal Axis in Idiopathic Pulmonary Fibrosis Fibroblasts

Idiopathic pulmonary fibrosis (IPF) is characterized by the relentless expansion of fibroblasts depositing type I collagen within the alveolar wall and obliterating the alveolar airspace. MicroRNA (miR)-29 is a potent regulator of collagen expression. In IPF, miR-29 levels are low, whereas type I collagen expression is high. However, the mechanism for suppression of miR-29 and increased type I collagen expression in IPF remains unclear. Here we show that when IPF fibroblasts are seeded on polymerized type I collagen, miR-29c levels are suppressed and type I collagen expression is high. In contrast, miR-29c is high and type I collagen expression is low in control fibroblasts. We demonstrate that the mechanism for suppression of miR-29 during IPF fibroblast interaction with polymerized collagen involves inappropriately low protein phosphatase (PP) 2A function, leading to histone deacetylase (HDA) C4 phosphorylation and decreased nuclear translocation of HDAC4. We demonstrate that overexpression of HDAC4 in IPF fibroblasts restored miR-29c levels and decreased type I collagen expression, whereas knocking down HDAC4 in control fibroblasts suppressed miR-29c levels and increased type I collagen expression. Our data indicate that IPF fibroblast interaction with polymerized type I collagen results in an aberrant PP2A/HDAC4 axis, which suppresses miR-29, causing a pathologic increase in type I collagen expression.