mTORC1 regulates a lysosome-dependent adaptive shift in intracellular lipid species.

The mechanistic target of rapamycin complex 1 (mTORC1) senses and relays environmental signals from growth factors and nutrients to metabolic networks and adaptive cellular systems to control the synthesis and breakdown of macromolecules; however, beyond inducing de novo lipid synthesis, the role of mTORC1 in controlling cellular lipid content remains poorly understood. Here we show that inhibition of mTORC1 via small molecule inhibitors or nutrient deprivation leads to the accumulation of intracellular triglycerides in both cultured cells and a mouse tumor model. The elevated triglyceride pool following mTORC1 inhibition stems from the lysosome-dependent, but autophagy-independent, hydrolysis of phospholipid fatty acids. The liberated fatty acids are available for either triglyceride synthesis or ß-oxidation. Distinct from the established role of mTORC1 activation in promoting de novo lipid synthesis, our data indicate that mTORC1 inhibition triggers membrane phospholipid trafficking to the lysosome for catabolism and an adaptive shift in the use of constituent fatty acids for storage or energy production.

Multiomics assessment of dietary protein titration reveals altered hepatic glucose utilization.

Dietary protein restriction (PR) has rapid effects on metabolism including improved glucose and lipid homeostasis, via multiple mechanisms. Here, we investigate responses of fecal microbiome, hepatic transcriptome, and hepatic metabolome to six diets with protein from 18% to 0% of energy in mice. PR alters fecal microbial composition, but metabolic effects are not transferable via fecal transplantation. Hepatic transcriptome and metabolome are significantly altered in diets with lower than 10% energy from protein. Changes upon PR correlate with calorie restriction but with a larger magnitude and specific changes in amino acid (AA) metabolism. PR increases steady-state aspartate, serine, and glutamate and decreases glucose and gluconeogenic intermediates. 13C6 glucose and glycerol tracing reveal increased fractional enrichment in aspartate, serine, and glutamate. Changes remain intact in hepatic ATF4 knockout mice. Together, this demonstrates an ATF4-independent shift in gluconeogenic substrate utilization toward specific AAs, with compensation from glycerol to promote a protein-sparing response.

Short-Term Ketogenic Diet Induces a Molecular Response That Is Distinct From Dietary Protein Restriction.

There is increasing interest in utilizing short-term dietary interventions in the contexts of cancer, surgical stress and metabolic disease. These short-term diets may be more feasible than extended interventions and may be designed to complement existing therapies. In particular, the high-fat, low-carbohydrate ketogenic diet (KD), traditionally used to treat epilepsy, has gained popularity as a potential strategy for weight loss and improved metabolic health. In mice, long-term KD improves insulin sensitivity and may extend lifespan and healthspan. Dietary protein restriction (PR) causes increased energy expenditure, weight loss and improved glucose homeostasis. Since KD is inherently a low-protein diet (10% of calories from protein vs. >18% in control diet), here we evaluated the potential for mechanistic overlap between PR and KD via activation of a PR response. Mice were fed control, protein-free (PF), or one of four ketogenic diets with varying protein content for 8 days. PF and KD both decreased body weight, fat mass, and liver weights, and reduced fasting glucose and insulin levels, compared to mice fed the control diet. However, PF-fed animals had significantly improved insulin tolerance compared to KD. Furthermore, contrary to the PF-fed mice, mice fed ketogenic diets containing more than 5% of energy from protein did not increase hepatic Fgf21 or brown adipose Ucp1 expression. Interestingly, mice fed KD lacking protein demonstrated greater elevations in hepatic Fgf21 than mice fed a low-fat PF diet. To further elucidate potential mechanistic differences between PF and KD and the interplay between dietary protein and carbohydrate restriction, we conducted RNA-seq analysis on livers from mice fed each of the six diets and identified distinct gene sets which respond to dietary protein content, dietary fat content, and ketogenesis. We conclude that KD with 10% of energy from protein does not induce a protein restriction response, and that the overlapping metabolic benefits of KD and PF diets may occur via distinct underlying mechanisms.