Glutamine deficiency drives transforming growth factor‐β signaling activation that gives rise to myofibroblastic carcinoma‐associated fibroblasts

Tumor‐promoting carcinoma‐associated fibroblasts (CAFs), abundant in the mammary tumor microenvironment (TME), maintain transforming growth factor‐β (TGF‐β)‐Smad2/3 signaling activation and the myofibroblastic state, the hallmark of activated fibroblasts. How myofibroblastic CAFs (myCAFs) arise in the TME and which epigenetic and metabolic alterations underlie activated fibroblastic phenotypes remain, however, poorly understood. We herein show global histone deacetylation in myCAFs present in tumors to be significantly associated with poorer outcomes in breast cancer patients. As the TME is subject to glutamine (Gln) deficiency, human mammary fibroblasts (HMFs) were cultured in Gln‐starved medium. Global histone deacetylation and TGF‐β‐Smad2/3 signaling activation are induced in these cells, largely mediated by class I histone deacetylase (HDAC) activity. Additionally, mechanistic/mammalian target of rapamycin complex 1 (mTORC1) signaling is attenuated in Gln‐starved HMFs, and mTORC1 inhibition in Gln‐supplemented HMFs with rapamycin treatment boosts TGF‐β‐Smad2/3 signaling activation. These data indicate that mTORC1 suppression mediates TGF‐β‐Smad2/3 signaling activation in Gln‐starved HMFs. Global histone deacetylation, class I HDAC activation, and mTORC1 suppression are also observed in cultured human breast CAFs. Class I HDAC inhibition or mTORC1 activation by high‐dose Gln supplementation significantly attenuates TGF‐β‐Smad2/3 signaling and the myofibroblastic state in these cells. These data indicate class I HDAC activation and mTORC1 suppression to be required for maintenance of myCAF traits. Taken together, these findings indicate that Gln starvation triggers TGF‐β signaling activation in HMFs through class I HDAC activity and mTORC1 suppression, presumably inducing myCAF conversion.