Dietary methionine restriction (MR) produces an integrated series of biochemical and physiological responses that improve biomarkers of metabolic health, limit fat accretion, and enhance insulin sensitivity. Using transcriptional profiling to guide tissue-specific evaluations of molecular responses to MR, we report that liver and adipose tissue are the primary targets of a transcriptional program that remodeled lipid metabolism in each tissue. The MR diet produced a coordinated downregulation of lipogenic genes in the liver, resulting in a corresponding reduction in the capacity of the liver to synthesize and export lipid. In contrast, the transcriptional response in white adipose tissue (WAT) involved a depot-specific induction of lipogenic and oxidative genes and a commensurate increase in capacity to synthesize and oxidize fatty acids. These responses were accompanied by a significant change in adipocyte morphology, with the MR diet reducing cell size and increasing mitochondrial density across all depots. The coordinated transcriptional remodeling of lipid metabolism between liver and WAT by dietary MR produced an overall reduction in circulating and tissue lipids and provides a potential mechanism for the increase in metabolic flexibility and enhanced insulin sensitivity produced by the diet.

Dietary methionine restriction (MR) by 80% increases energy expenditure (EE), reduces adiposity, and improves insulin sensitivity. We propose that the MRinduced increase in EE limits fat deposition by increasing sympathetic nervous system-dependent remodeling of white adipose tissue and increasing uncoupling protein 1 (UCP1) expression in both white and brown adipose tissue. In independent assessments of the role of UCP1 as a mediator of MR's effects on EE and insulin sensitivity, EE did not differ between wild-type (WT) and Ucp1 2/2 mice on the control diet, but MR increased EE by 31% and reduced adiposity by 25% in WT mice. In contrast, MR failed to increase EE or reduce adiposity in Ucp1 2/2 mice. However, MR was able to increase overall insulin sensitivity by 2.2-fold in both genotypes. Housing temperatures used to minimize (28°C) or increase (23°C) sympathetic nervous system activity revealed temperature-independent effects of the diet on EE. Metabolomics analysis showed that genotypic and dietary effects on white adipose tissue remodeling resulted in profound increases in fatty acid metabolism within this tissue. These findings establish that UCP1 is required for the MR-induced increase in EE but not insulin sensitivity and suggest that diet-induced improvements in insulin sensitivity are not strictly derived from dietary effects on energy balance

Calorie restriction without malnutrition, commonly referred to as dietary restriction (DR), results in a well-documented extension of life span. DR also produces significant, long-lasting improvements in biomarkers of metabolic health that begin to accrue soon after its introduction. The improvements are attributable in part to the effects of DR on energy balance, which limit fat accumulation through reduction in energy intake. Accumulation of excess body fat occurs when energy intake chronically exceeds the energy costs for growth and maintenance of existing tissue. The resulting obesity promotes the development of insulin resistance, disordered lipid metabolism, and increased expression of inflammatory markers in peripheral tissues. The link between the life-extending effects of DR and adiposity is the subject of an ongoing debate, but it is clear that decreased fat accumulation improves insulin sensitivity and produces beneficial effects on overall metabolic health. Over the last 20 years, dietary methionine restriction (MR) has emerged as a promising DR mimetic because it produces a comparable extension in life span, but surprisingly, does not require food restriction. Dietary MR also reduces adiposity but does so through a paradoxical increase in both energy intake and expenditure. The increase in energy expenditure fully compensates for increased energy intake and effectively limits fat deposition. Perhaps more importantly, the diet increases metabolic flexibility and overall insulin sensitivity and improves lipid metabolism while decreasing systemic inflammation. In this chapter, we describe recent advances in our understanding of the mechanisms and effects of dietary MR and discuss the remaining obstacles to implementing MR as a treatment for metabolic disease.

Calorie restriction (CR) without malnutrition increases lifespan and produces significant improvements in biomarkers of metabolic health. The improvements are attributable in part to effects of CR on energy balance, which limit fat accumulation by restricting energy intake. Normal age-associated increases in adiposity and insulin resistance are associated with development of a systemic pro-inflammatory state, while chronic CR limits fat deposition and expression of inflammatory markers. Dietary methionine restriction (MR) has emerged as an effective CR mimetic because it produces a comparable extension in lifespan. MR also reduces adiposity through a compensatory increase in energy expenditure that effectively limits fat accumulation, but essentially nothing is known about the effects of MR on systemic inflammation. Here we review the relationships between these two interventions and discuss their transcriptional impact. In addition, using tissues from rats after long term consumption of CR or MR diets, transcriptional profiling was used to examine retrospectively the systems biology of 59 networks of molecules annotated to inflammation. Transcriptional effects of both diets occurred primarily in white adipose tissue and liver, and the responses to MR were far more robust than those to CR. The primary transcriptional targets of MR in both liver and white adipose tissue were phagocytes and macrophages, where expression of genes associated with immune cell infiltration and quantity was reduced. These findings support the conclusion that anti-inflammatory responses produced by CR and MR are not strictly dependent upon reduced adiposity, but are significantly influenced by the metabolic mechanisms through which energy balance is altered.

Dietary methionine restriction (MR) and calorie restriction (CR) each improve metabolic health and extend life span. We used comprehensive transcriptome profiling and systems biology analysis to interrogate the unique and overlapping molecular responses in rats provided these dietary regimens for 20 mo after weaning. Microarray analysis was conducted on inguinal white adipose (IWAT), brown adipose tissue (BAT), liver, and skeletal muscle. Compared to controls, CR-induced transcriptomic responses (absolute fold change ≥1.5 and P≤0.05) were comparable in IWAT, BAT, and liver (~800 genes). MR-induced effects were largely restricted to IWAT and liver (~2400 genes). Pathway enrichment and gene-coexpression analyses showed that induction of fatty acid synthesis in IWAT was common to CR and MR, whereas immunity and proinflammatory signaling pathways were specifically down-regulated in MR-treated IWAT and liver (FDR≤0.07-0.3). BAT demonstrated consistent down-regulation of PPAR-signaling under CR and MR, whereas muscle was largely unaffected. Interactome analysis identified CR-specific down-regulation of cytoskeletal matrix components in IWAT and MR-specific up-regulation of ribosomal genes in liver (FDR≤0.001). Transcriptomic down-regulation of inflammation genes by MR in IWAT was consistent with upstream inhibition of STAT3. Together, these results provide an integrated picture of the breadth of transcriptional responses to MR and CR among key metabolic tissues.