TGF-β Promotes Metabolic Reprogramming in Lung Fibroblasts via mTORC1-dependent ATF4 Activation

Abstract

Idiopathic pulmonary fibrosis is a fatal interstitial lung disease characterized by the TGF-b (transforming growth factor-b)dependent differentiation of lung fibroblasts into myofibroblasts, which leads to excessive deposition of collagen proteins and progressive scarring. We have previously shown that synthesis of collagen by myofibroblasts requires de novo synthesis of glycine, the most abundant amino acid found in collagen protein. TGF-b upregulates the expression of the enzymes of the de novo serine-glycine synthesis pathway in lung fibroblasts; however, the transcriptional and signaling regulators of this pathway remain incompletely understood. Here, we demonstrate that TGF-b promotes accumulation of ATF4 (activating transcription factor 4), which is required for increased expression of the serine-glycine synthesis pathway enzymes in response to TGF-b. We found that induction of the integrated stress response (ISR) contributes to TGF-b-induced ATF4 activity; however, the primary driver of ATF4 downstream of TGF-b is activation of mTORC1 (mTOR Complex 1). TGF-b activates the PI3K-Akt-mTOR pathway, and inhibition of PI3K prevents activation of downstream signaling and induction of ATF4. Using a panel of mTOR inhibitors, we found that ATF4 activation is dependent on mTORC1, independent of mTORC2. Rapamycin, which incompletely and allosterically inhibits mTORC1, had no effect on TGF-b-mediated induction of ATF4; however, Rapalink-1, which specifically targets the kinase domain of mTORC1, completely inhibited ATF4 induction and metabolic reprogramming downstream of TGF-b. Our results provide insight into the mechanisms of metabolic reprogramming in myofibroblasts and clarify contradictory published findings on the role of mTOR inhibition in myofibroblast differentiation.

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“…Transforming growth factor‐β (TGF‐β) is a common agent to induce the differentiation of fibroblasts into myofibroblasts, accompanied by the excessive secretion of extracellular matrix 15 . In this process, the PI3K/Akt/mTOR pathway is upregulated by TGF‐β to increase the expression of the enzymes that are required for the deposition of collagen proteins and progressive scarring 54 . Drugs such as isoliquiritigenin and Yifei Sanjie formula can improve TGF‐β induced pulmonary fibrosis through decreasing the phosphorylation levels of PI3K, Akt and mTOR 55,56 .…”

“…15 In this process, the PI3K/Akt/mTOR pathway is upregulated by TGF-β to increase the expression of the enzymes that are required for the deposition of collagen proteins and progressive scarring. 54 Drugs such as isoliquiritigenin and Yifei Sanjie formula can improve TGF-β induced pulmonary fibrosis through decreasing the phosphorylation levels of PI3K, Akt and mTOR. 55,56 Chronic radon exposure can cause lung injury and fibrosis, manifested as increasing lung epithelial cell proliferation and migration.…”

mTOR Signaling in Pulmonary Vascular Disease: Pathogenic Role and Therapeutic Target

“…In rats with HPH, the increased TGF-β 1 inactivated PTEN to trigger PASMC proliferation and resistance to apoptosis via AKT signaling contributing to pulmonary vascular remodeling [ 164 ]. In lung FB, TGF-β launched the metabolic reprogramming by driving mTORC1 [ 165 ]. Hypoxia-mediated EndMT provoked mTORC1 phosphorylation in PAEC while the treatment with recombinant human BMP-7 markedly suppressed both events [ 166 ].…”

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“…Furthermore, studies by Ringuette-Goulet et al have shown that invasive bladder cancer cells (T24) secrete more TGF-β than non-invasive bladder cancer cells (RT4) do [16]. Interestingly, TGF-β has been shown to activate the PI3K-Akt-mTOR pathway and promote the accumulation of activating transcription factor 4 (ATF4) in lung fibroblasts, leading to their metabolic reprogramming [36]. Another molecule that could be implicated is IL-6, which could impact the physiological activity of fibroblasts as well as potentialize the effects of TGF-β [37].…”