Control of Systemic Lipid Metabolism by Adipocyte mTOR Signaling

Pharmacological agents targeting the mTOR complexes are used clinically as immunosuppressants and anticancer agents, and can extend lifespan in model organisms. An undesirable side effect of these drugs is hyperlipidemia. Raptor and Rictor are essential component of mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) and 2 (mTORC2), respectively. Despite multiple roles that have been described for mTOR complex 1 (mTORC1) in lipid metabolism, the etiology of hyperlipidemia remains incompletely understood. The objective of this study was to determine the influence of adipocyte mTORC1 signaling in systemic lipid homeostasis in vivo. We characterized systemic lipid metabolism in mice lacking the mTORC1 subunit raptor (RaptoraKO), the key lipolytic enzyme ATGL (ATGLaKO), or both (ATGL-RaptoraKO) in adipocytes. Mice lacking mTORC1 activity in adipocytes failed to completely suppress lipolysis in the fed state and displayed prominent hypertriglyceridemia and hypercholesterolemia. Blocking lipolysis in adipose tissue restored normal levels of triglycerides and cholesterol in the fed state, as well as the ability to clear triglycerides in an oral fat tolerance test. Unsuppressed adipose lipolysis in the fed state interferes with triglyceride clearance and contributes to hyperlipidemia. Adipose tissue mTORC1 activity is necessary for appropriate suppression of lipolysis and for the maintenance of systemic lipid homeostasis. Loss of mTORC1 signaling in adipose is sufficient to disrupt lipid homeostasis, resulting in hyperlipidemia caused by unrestrained lipolysis. However, studies to date examining the role of deletion of this complex, complex 2 or both in adipose tissue in combination with rapamycin have yet to be investigated. Here, we report the consequences of Raptor, Rictor or both deleted specifically in mature adipocytes driven by Adiponectin-Cre (RaptoraKO, RictoraKO, Raptor-RictoraKO). Concordant with the RaptoraKO mice, RictoraKO mice display pronounced hyperlipidemia and both KO models have a further increase in plasma lipids with rapamycin treatment. Genetic inhibition of lipolysis in mice with loss of mTORC1 (ATGL-RaptoraKO) treated with rapamycin prevents the further increase in plasma lipids seen in Raptorako mice treated with rapamycin. We propose a hypothetical mechanism that in the fed state, rapamycin inhibition of adipose mTORC1 leads to decreased C/EBP transcriptional activity. This decreased transcriptional activity results in decreased expression of perilipin and subsequent unrestrained lipolysis, leading to hyperlipidemia. Here we show that enhanced lipolysis upon refeeding increases plasma triglyceride levels in the context of rapamycin treatment and that both complexes are involved in regulating this lipolytic process. However, mTORC1, not mTORC2, is required for proper adipocyte lipolysis to maintain circulating plasma lipid levels. Additionally, we provide evidence that loss of adipocyte mTOR signaling is not solely responsible for the rapamycin induced hyperlipidemia

Control of Systemic Lipid Metabolism by Adipocyte mTOR Signaling

Pharmacological agents targeting the mTOR complexes are used clinically as immunosuppressants and anticancer agents, and can extend lifespan in model organisms. An undesirable side effect of these drugs is hyperlipidemia. Raptor and Rictor are essential component of mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) and 2 (mTORC2), respectively. Despite multiple roles that have been described for mTOR complex 1 (mTORC1) in lipid metabolism, the etiology of hyperlipidemia remains incompletely understood. The objective of this study was to determine the influence of adipocyte mTORC1 signaling in systemic lipid homeostasis in vivo. We characterized systemic lipid metabolism in mice lacking the mTORC1 subunit raptor (RaptoraKO), the key lipolytic enzyme ATGL (ATGLaKO), or both (ATGL-RaptoraKO) in adipocytes. Mice lacking mTORC1 activity in adipocytes failed to completely suppress lipolysis in the fed state and displayed prominent hypertriglyceridemia and hypercholesterolemia. Blocking lipolysis in adipose tissue restored normal levels of triglycerides and cholesterol in the fed state, as well as the ability to clear triglycerides in an oral fat tolerance test. Unsuppressed adipose lipolysis in the fed state interferes with triglyceride clearance and contributes to hyperlipidemia. Adipose tissue mTORC1 activity is necessary for appropriate suppression of lipolysis and for the maintenance of systemic lipid homeostasis. Loss of mTORC1 signaling in adipose is sufficient to disrupt lipid homeostasis, resulting in hyperlipidemia caused by unrestrained lipolysis. However, studies to date examining the role of deletion of this complex, complex 2 or both in adipose tissue in combination with rapamycin have yet to be investigated. Here, we report the consequences of Raptor, Rictor or both deleted specifically in mature adipocytes driven by Adiponectin-Cre (RaptoraKO, RictoraKO, Raptor-RictoraKO). Concordant with the RaptoraKO mice, RictoraKO mice display pronounced hyperlipidemia and both KO models have a further increase in plasma lipids with rapamycin treatment. Genetic inhibition of lipolysis in mice with loss of mTORC1 (ATGL-RaptoraKO) treated with rapamycin prevents the further increase in plasma lipids seen in Raptorako mice treated with rapamycin. We propose a hypothetical mechanism that in the fed state, rapamycin inhibition of adipose mTORC1 leads to decreased C/EBP transcriptional activity. This decreased transcriptional activity results in decreased expression of perilipin and subsequent unrestrained lipolysis, leading to hyperlipidemia. Here we show that enhanced lipolysis upon refeeding increases plasma triglyceride levels in the context of rapamycin treatment and that both complexes are involved in regulating this lipolytic process. However, mTORC1, not mTORC2, is required for proper adipocyte lipolysis to maintain circulating plasma lipid levels. Additionally, we provide evidence that loss of adipocyte mTOR signaling is not solely responsible for the rapamycin induced hyperlipidemia

SIRT3 is required for liver regeneration but not for the beneficial effect of nicotinamide riboside

Liver regeneration is critical to survival after traumatic injuries, exposure to hepatotoxins, or surgical interventions, yet the underlying signaling and metabolic pathways remain unclear. In this study, we show that hepatocyte-specific loss of the mitochondrial deacetylase SIRT3 drastically impairs regeneration and worsens mitochondrial function after partial hepatectomy. Sirtuins, including SIRT3, require NAD as a cosubstrate. We previously showed that the NAD precursor nicotinamide riboside (NR) promotes liver regeneration, but whether this involves sirtuins has not been tested. Here, we show that despite their NAD dependence and critical roles in regeneration, neither SIRT3 nor its nuclear counterpart SIRT1 is required for NR to enhance liver regeneration. NR improves mitochondrial respiration in regenerating WT or mutant livers and rapidly increases oxygen consumption and glucose output in cultured hepatocytes. Our data support a direct enhancement of mitochondrial redox metabolism as the mechanism mediating improved liver regeneration after NAD supplementation and exclude signaling via SIRT1 and SIRT3. Therefore, we provide the first evidence to our knowledge for an essential role for a mitochondrial sirtuin during liver regeneration and insight into the beneficial effects of NR

Nicotinamide adenine dinucleotide is transported into mammalian mitochondria

Mitochondrial NAD levels influence fuel selection, circadian rhythms, and cell survival under stress. It has alternately been argued that NAD in mammalian mitochondria arises from import of cytosolic nicotinamide (NAM), nicotinamide mononucleotide (NMN), or NAD itself. We provide evidence that murine and human mitochondria take up intact NAD. Isolated mitochondria preparations cannot make NAD from NAM, and while NAD is synthesized from NMN, it does not localize to the mitochondrial matrix or effectively support oxidative phosphorylation. Treating cells with nicotinamide riboside that is isotopically labeled on the nicotinamide and ribose moieties results in the appearance of doubly labeled NAD within mitochondria. Analogous experiments with doubly labeled nicotinic acid riboside (labeling cytosolic NAD without labeling NMN) demonstrate that NAD(H) is the imported species. Our results challenge the long-held view that the mitochondrial inner membrane is impermeable to pyridine nucleotides and suggest the existence of an unrecognized mammalian NAD (or NADH) transporter