Distinct utilization of biotin in and between adipose and brain during aging is associated with a lipogenic shift in Wistar rat brain.

Tissue-specific metabolism determines their functions that collectively sense and respond to numerous stress cues to achieve systemic homeostasis. Chronic stress skews such metabolic profiles and leads to failure of organs as evidenced by a bias towards lipid synthesis and storage in the aging brain, muscle, and liver under Alzheimer's disease, sarcopenia, and non-alcoholic fatty liver disease, respectively. In contrast, the tissue destined for lipid synthesis and storage, such as adipose, limits its threshold and develops diabetes mellitus. However, the underlying factors that contribute to this lipogenic shift between organs are unknown. From this perspective, differential biotin utilization between lipid-rich tissues such as adipose and brain during aging was hypothesized owing to the established role of biotin in lipogenesis. The same was tested using young and aged Wistar rats. We found that adipose-specific biotin content was much higher than the brain irrespective of aging status, as well as its associated cues. However, within tissues, the adipose fails to maintain its biotinylation levels during aging whereas the brain seizes more biotin and exhibits lipid accumulation. Furthermore, mimicking the age-related stress cues in vitro such as high glucose and endoplasmic reticulum stress deprive the astroglial biotin content, but not that of adipocytes. Lipid accumulation in the aging brain was also correlated with increased S-adenosylmethionine levels and biotin utilization by astrocytes. In summary, differential biotin utilization between adipose and brain under aging and their respective cell types like adipocytes and astrocytes under age-associated stress cues connects well with the lipogenic shift in rat brain.

Astroglial biotin deprivation under endoplasmic reticulum stress uncouples BCAA-mTORC1 role in lipid synthesis to prolong autophagy inhibition in the aging brain.

Autophagy delays the onset of endoplasmic reticulum (ER) stress by recycling cellular debris. However, the cues that elicit autophagy under the emergence of ER stress and their dysregulation during aging remains obscure. Amino acids, notably branched-chain amino acids (BCAA), get accumulated in the cells once protein synthesis is inhibited by ER stress. The BCAA mimic satiety to inhibit autophagy via mechanistic targets of rapamycin complex 1 (mTORC1) activation and, in contrast, their catabolism supplements de novo lipogenesis for the formation of autophagosome membranes. Thus promoting BCAA utilization is hypothesized to induce autophagy to alleviate ER stress. Nevertheless, except protein synthesis, the rest of BCAA utilization and lipogenesis depends on the co-enzyme biotin. Hence, the levels of biotinylated carboxylases and lipids were assessed in the aging brain of Wistar rats. Despite the increased levels of biotinylated carboxylases and lipids, the aging brain accumulates BCAA. Since astrocytes are the primary site of BCAA and lipid metabolism and the increased expression of glial fibrillary acidic protein (GFAP) denotes astroglial ER stress, co-localization studies were performed to determine the extent of biotinylation in GFAP positive cells. Although total biotin intensity was higher in aged brain slices, the astrocytes specific decrease in biotinylation is attributed to BCAA accumulation, mTORC1 overactivation, autophagy inhibition, and ER stress in the aging brain. The ER stress in primary astrocytes using tunicamycin also mimic the in vivo phenotype. Biotin supplementation ameliorated the changes observed in vitro, corroborating the significance of astrocytes biotin availability to promote autophagy under ER stress in aging.

Autophagy inhibition by biotin elicits endoplasmic reticulum stress to differentially regulate adipocyte lipid and protein synthesis.

Biotin is an indispensable adipogenic agent, and its ability to coordinate carbohydrate, lipid, and amino acid metabolism sensitizes insulin signaling in adipocytes. This enables the organism to adapt and survive under nutrient stress by synthesis and storage of lipids. Biotin deficiency mimics insulin resistance with alterations in cellular intermediary metabolism. Though the mechanism of lipogenesis is well established across cell types, considering its predisposition to accumulate only lipids, it is necessary to elucidate the mechanism that minimizes the effects of biotin on adipocyte protein synthesis. In order to determine the differential metabolic phenotype by biotin, the primary cultures of adipocytes were induced to differentiate in the presence and absence of excess biotin. Serum pre-incubated with avidin was used to limit biotin availability in cultured cells. Biotin restricts cellular signaling associated with protein synthesis without altering total protein content. The decline in autophagy elicits endoplasmic reticulum stress to inhibit protein synthesis by eIF2α phosphorylation possibly via accumulation of misfolded/long-lived proteins. Furthermore, the compensatory increase in Unc51 like autophagy activating kinase 1 possibly competes with eukaryotic initiation factor 4E-binding protein 1 and ribosomal p70 S6kinase phosphorylation by mechanistic targets of rapamycin complex 1 to uncouple its effect on protein synthesis. In conclusion, autophagy inhibition by biotin uncouples protein synthesis to promote lipogenesis by eliciting endoplasmic reticulum stress and differential phosphorylation of mechanistic targets of rapamycin complex 1 substrates.