PHGDH supports liver ceramide synthesis and sustains lipid homeostasis

Kang, Yun Pyo; Falzone, Aimee; Liu, Min; González-Sánchez, Paloma; Choi, Byungjoo; Coloff, Jonathan L.; Saller, James J.; Karreth, Florian A.; DeNicola, Gina M.

doi:10.1186/s40170-020-00212-x

Cited by 21 publications

(18 citation statements)

References 50 publications

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“…Despite evident regulation of the serine synthesis genes PHGDH, PSAT1, and PSPH by insulin signaling through mTORC1 and ATF4 in primary hepatocytes, we failed to detect any regulated flux through the serine synthesis pathway, which was extremely low under all conditions tested. Our findings are consistent with data from a recent study that showed the fractional enrichment of serine (m+3) synthesized from 13 C 6 -glucose in the liver was minimal (<1%) [ 71 ]. Thus, despite clear regulation of the serine synthesis enzymes, hepatocytes might rely predominantly on exogenous serine.…”

Section: Discussionsupporting

confidence: 93%

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

Byles

Cormerais

Kalafut

et al. 2021

Molecular Metabolism

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Objective The mechanistic target of rapamycin complex 1 (mTORC1) is dynamically regulated by fasting and feeding cycles in the liver to promote protein and lipid synthesis while suppressing autophagy. However, beyond these functions, the metabolic response of the liver to feeding and insulin signaling orchestrated by mTORC1 remains poorly defined. Here, we determine whether ATF4, a stress responsive transcription factor recently found to be independently regulated by mTORC1 signaling in proliferating cells, is responsive to hepatic mTORC1 signaling to alter hepatocyte metabolism. Methods ATF4 protein levels and expression of canonical gene targets were analyzed in the liver following fasting and physiological feeding in the presence or absence of the mTORC1 inhibitor, rapamycin. Primary hepatocytes from wild-type or liver-specific Atf4 knockout ( LAtf4 KO ) mice were used to characterize the effects of insulin-stimulated mTORC1-ATF4 function on hepatocyte gene expression and metabolism. Both unbiased steady-state metabolomics and stable-isotope tracing methods were employed to define mTORC1 and ATF4-dependent metabolic changes. RNA-sequencing was used to determine global changes in feeding-induced transcripts in the livers of wild-type versus LAtf4 KO mice. Results We demonstrate that ATF4 and its metabolic gene targets are stimulated by mTORC1 signaling in the liver, in a hepatocyte-intrinsic manner by insulin in response to feeding. While we demonstrate that de novo purine and pyrimidine synthesis is stimulated by insulin through mTORC1 signaling in primary hepatocytes, this regulation was independent of ATF4. Metabolomics and metabolite tracing studies revealed that insulin-mTORC1-ATF4 signaling stimulates pathways of nonessential amino acid synthesis in primary hepatocytes, including those of alanine, aspartate, methionine, and cysteine, but not serine. Conclusions The results demonstrate that ATF4 is a novel metabolic effector of mTORC1 in the liver, extending the molecular consequences of feeding and insulin-induced mTORC1 signaling in this key metabolic tissue to the control of amino acid metabolism.

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Section: Discussionsupporting

confidence: 93%

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

Byles

Cormerais

Kalafut

et al. 2021

Molecular Metabolism

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show abstract

“…Despite evident regulation of the serine synthesis genes PHGDH, PSAT1, and PSPH by insulin signaling through mTORC1 and ATF4 in primary hepatocytes, we failed to detect any regulated flux through the serine synthesis pathway, which was extremely low under all conditions tested. Our findings are consistent with data from a recent study that showed the fractional enrichment of serine (m+3) synthesized from 13 C6-glucose in the liver was minimal (<1%) [64]. Thus, despite clear regulation of the serine synthesis enzymes, hepatocytes might rely predominantly on exogenous serine.…”

Section: Discussionsupporting

confidence: 92%

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

Byles

Cormerais

Kalafut

et al. 2021

Preprint

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Objective: The mechanistic target of rapamycin complex 1 (mTORC1) is dynamically regulated by fasting and feeding cycles in the liver to promote protein and lipid synthesis while suppressing autophagy. However, beyond these functions, the metabolic response of the liver to feeding and insulin signaling orchestrated by mTORC1 remains poorly defined. Here, we determine whether ATF4, a stress responsive transcription factor recently found to be independently regulated by mTORC1 signaling in proliferating cells, is responsive to hepatic mTORC1 signaling to alter hepatocyte metabolism. Methods: ATF4 protein levels and expression of canonical gene targets were analyzed in the liver following fasting and physiological feeding in the presence or absence of the mTORC1 inhibitor rapamycin. Primary hepatocytes from wild-type or liver-specific Atf4 knockout (LAtf4KO) mice were used to characterize the effects of insulin-stimulated mTORC1-ATF4 function on hepatocyte gene expression and metabolism. Both unbiased steady-state metabolomics and stable-isotope tracing methods were employed to define mTORC1 and ATF4-dependent metabolic changes. RNA-sequencing was used to determine global changes in feeding-induced transcripts in the livers of wild-type versus LAtf4KO mice. Results: We demonstrate that ATF4 and its metabolic gene targets are stimulated by mTORC1 signaling in the liver in response to feeding and in a hepatocyte-intrinsic manner by insulin. While we demonstrate that de novo purine and pyrimidine synthesis is stimulated by insulin through mTORC1 signaling in primary hepatocytes, this regulation was independent of ATF4. Metabolomics and metabolite tracing studies revealed that insulin-mTORC1-ATF4 signaling stimulates pathways of non-essential amino acid synthesis in primary hepatocytes, including those of alanine, aspartate, methionine, and cysteine, but not serine. Conclusion: The results demonstrate that ATF4 is a novel metabolic effector of mTORC1 in liver, extending the molecular consequences of feeding and insulin-induced mTORC1 signaling in this key metabolic tissue to the control of amino acid metabolism.

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“…These observations show some parallels to other approaches aimed at limiting the supply of NEAAs to cancers that are already in clinical use, such as treatment of patients with ASS1 deficient cancers (so blocking the de novo pathway of arginine synthesis) with ADI-PEG20, to degrade circulating arginine 41 . Consistent with a recent study showing no overt toxicity associated with depletion of PHGDH in adult mice (sparing the brain) 42 , mice fed on a normal diet tolerated treatment with PH755 well, showing that in the presence of exogenous serine, inhibition of the SSP was not detrimental to health. Similarly, PH755 did not affect the growth of cells in serine and glycine containing medium.…”

Section: Discussionsupporting

confidence: 89%

Serine synthesis pathway inhibition cooperates with dietary serine and glycine limitation for cancer therapy

et al. 2021

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Many tumour cells show dependence on exogenous serine and dietary serine and glycine starvation can inhibit the growth of these cancers and extend survival in mice. However, numerous mechanisms promote resistance to this therapeutic approach, including enhanced expression of the de novo serine synthesis pathway (SSP) enzymes or activation of oncogenes that drive enhanced serine synthesis. Here we show that inhibition of PHGDH, the first step in the SSP, cooperates with serine and glycine depletion to inhibit one-carbon metabolism and cancer growth. In vitro, inhibition of PHGDH combined with serine starvation leads to a defect in global protein synthesis, which blocks the activation of an ATF-4 response and more broadly impacts the protective stress response to amino acid depletion. In vivo, the combination of diet and inhibitor shows therapeutic efficacy against tumours that are resistant to diet or drug alone, with evidence of reduced one-carbon availability. However, the defect in ATF4-response seen in vitro following complete depletion of available serine is not seen in mice, where dietary serine and glycine depletion and treatment with the PHGDH inhibitor lower but do not eliminate serine. Our results indicate that inhibition of PHGDH will augment the therapeutic efficacy of a serine depleted diet.

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PHGDH supports liver ceramide synthesis and sustains lipid homeostasis

Cited by 21 publications

References 50 publications

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

Hepatic mTORC1 signaling activates ATF4 as part of its metabolic response to feeding and insulin

Serine synthesis pathway inhibition cooperates with dietary serine and glycine limitation for cancer therapy

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