Abstract
TGF-β promotes excessive collagen deposition in fibrotic diseases such as idiopathic pulmonary fibrosis (IPF). The amino acid composition of collagen is unique due to its high (33%) glycine content. Here, we report that TGF-β induces expression of glycolytic genes and increases glycolytic flux. TGF-β also induces the expression of the enzymes of the de novo serine synthesis pathway (phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase 1 (PSAT1), and phosphoserine phosphatase (PSPH)) and de novo glycine synthesis (serine hydroxymethyltransferase 2 (SHMT2)). Studies in fibroblasts with genetic attenuation of PHGDH or SHMT2 and pharmacologic inhibition of PHGDH showed that these enzymes are required for collagen synthesis. Furthermore, metabolic labeling experiments demonstrated carbon from glucose incorporated into collagen. Lungs from humans with IPF demonstrated increased expression of PHGDH and SHMT2. These results indicate that the de novo serine synthesis pathway is necessary for TGF-β-induced collagen production and suggest that this pathway may be a therapeutic target for treatment of fibrotic diseases including IPF.
Highlights
Organ injury triggers a complex cascade of responses that results in fibrosis [1]
TGF- Up-regulates Glycolytic Enzyme Expression and Glycolytic Activity in Primary Human Lung Fibroblasts—Treatment of primary human lung fibroblasts (HLFs) (Lonza, Allendale, NJ) with active TGF- (1 ng/ml) for 48 h results in increasing amounts of ␣-smooth muscle actin (␣-SMA, a marker for myofibroblast differentiation) and type I collagen in a time-dependent manner (Fig. 1A)
To determine whether the increased lactate production induced by TGF- was due to up-regulation of specific glycolytic enzymes, we performed quantitative reverse transcription-polymerase chain reaction from HLFs 24 h after treatment of TGF-
Summary
Droxymethyltransferase (SHMT) genes; SHMT1, encoding the cytoplasmic isozyme (SHMT1), and SHMT2, encoding the mitochondrial one (SHMT2) [15,16,17]. We show that TGF- up-regulates glycolytic genes and flux in human fibroblasts. Inhibition of glycolysis resulted in the inhibition of myofibroblast differentiation and production of collagen protein without a significant change in transcription of ␣-1 type collagen and other TGF- target genes. TGF- induced the mRNA expression of most of the enzymes of the glycolytic pathway as well as mRNA and protein expression of enzymes that play a role in de novo synthesis of serine and glycine, including PHGDH and SHMT2. Our results suggest that de novo synthesis of serine and glycine is required for collagen synthesis. This pathway may be a therapeutic target for treatment of organ fibrosis including IPF
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