SUMMARY Circadian clocks and metabolism are inextricably intertwined, where central and hepatic circadian clocks coordinate metabolic events in response to light-dark and sleep-wake cycles. We reveal an additional key element involved in maintaining host circadian rhythms, the gut microbiome. Despite persistence of light-dark signals, germ-free mice fed low or high fat diets exhibit markedly impaired central and hepatic circadian clock gene expression and do not gain weight compared to conventionally-raised counterparts. Examination of gut microbiota in conventionally-raised mice showed differential diurnal variation in microbial structure and function dependent upon dietary composition. Additionally, specific microbial metabolites induced under low or high fat feeding, particularly short chain fatty acids, but not hydrogen sulfide, directly modulate circadian clock gene expression within hepatocytes. These results underscore the ability of microbially-derived metabolites to regulate or modify central and hepatic circadian rhythm and host metabolic function, the latter following intake of a Westernized diet.

The endoplasmic reticulum is the port of entry for proteins into the secretory pathway and the site of synthesis for several important lipids, including cholesterol, triacylglycerol, and phospholipids. Protein production within the endoplasmic reticulum is tightly regulated by a cohort of resident machinery that coordinates the folding, modification, and deployment of secreted and integral membrane proteins. Proteins failing to attain their native conformation are degraded through the endoplasmic reticulum–associated degradation (ERAD) pathway via a series of tightly coupled steps: substrate recognition, dislocation, and ubiquitin-dependent proteasomal destruction. The same ERAD machinery also controls the flux through various metabolic pathways by coupling the turnover of metabolic enzymes to the levels of key metabolites. We review the current understanding and biological significance of ERAD-mediated regulation of lipid metabolism in mammalian cells.

Background Growing evidence shows that dietary factors can dramatically alter the gut microbiome in ways that contribute to metabolic disturbance and progression of obesity. In this regard, mesenteric adipose tissue has been implicated in mediating these processes through the elaboration of pro-inflammatory adipokines. In this study, we examined the relationship of these events by determining the effects of dietary fat content and source on gut microbiota, as well as the effects on adipokine profiles of mesenteric and peripheral adipocytes. Methods Adult male C57Bl/6 mice were fed milk fat-, lard-(SFA sources), or safflower oil (PUFA)- based high fat diets for four weeks. Body mass and food consumption were measured. Stool 16S rRNA was isolated and analyzed via T-RFLP as well as variable V3-4 sequence tags via next gen sequencing. Mesenteric and gonadal adipose samples were analyzed for both lipogenic and inflammatory mediators via qRT-PCR. Results High-fat feedings caused more weight gain with concomitant increases in caloric consumption relative to low-fat diets. Additionally, each of the high fat diets induced dramatic and specific 16S rRNA phylogenic profiles that were associated with different inflammatory and lipogenic mediator profile of mesenteric and gonadal fat depots. Conclusions Our findings support the notion that dietary fat composition can both reshape the gut microbiota as well as alter host adipose tissue inflammatory/lipogenic profiles. They also demonstrate the interdependency of dietary fat source, commensal gut microbiota, and inflammatory profile of mesenteric fat that can collectively impact the host metabolic state.

Introduction Commensal gut microbiota play an important role in regulating metabolic and inflammatory conditions. Reshaping intestinal microbiota through pharmacologic means may be a viable treatment option. We sought to delineate the functional characteristics of glucocorticoid-mediated alterations on gut microbiota and their subsequent repercussions on host mucin regulation and colonic inflammation. Methods Adult male C57Bl/6 mice, germ-free (GF), Muc2-heterozygote (+/−), or Muc2-knockout (−/−) were injected with dexamethasone, a synthetic glucocorticoid, for four weeks. Fecal samples were collected for gut microbiota analysis via 16S rRNA T-RFLP and amplicon sequencing. Intestinal mucosa was collected for mucin gene expression studies. GF mice were conventionalized with gut microbes from treated- and non-treated groups to determine their functional capacities in recipient hosts. Results Exposure to DEX in WT mice led to substantial shifts in gut microbiota over a four-week period. Furthermore, a significant down-regulation of colonic Muc2 gene expression was observed after treatment. Muc2-knockout mice harbored a pro-inflammatory environment of gut microbes, characterized by the increase or decrease in prevalence of specific microbiota populations such as Clostridiales and Lactobacillaceae, respectively. This colitogenic phenotype was transmissible to IL10-knockout (IL10-KO) mice, a genetically susceptible model of colonic inflammatory disorders. Microbiota from donors pre-treated with DEX, however, ameliorated symptoms of inflammation. Conclusions Commensal gut bacteria may be a key mediator of the anti-inflammatory effects observed in the large intestine after GC exposure. These findings underscore the notion that intestinal microbes comprise a “microbial organ” essential for host physiology that can be targeted by therapeutic approaches to restore intestinal homeostasis.

Niacin treatment lowers serum total- and phosphofetuin-A concentrations in individuals with MetS, and these changes correlate with the beneficial changes in serum lipids. Because niacin is known to induce insulin resistance, these findings suggest that fetuin-A may not be a mediator of niacin-induced insulin resistance but it may blunt the insulin resistance induced by niacin by decreasing its circulating concentrations.