In a recent study published in Nature Metabolism, researchers found that feeding a high-fat diet (HFD) causes mitochondrial dysfunction and fragmentation in white adipocytes in mice.
Obesity has become a global epidemic, increasing the incidence of non-alcoholic steatohepatitis, diabetes, and other cardiometabolic disorders. White adipose tissue (WAT) expands chronically during the development of obesity, with metabolic changes characterized by fibrosis, inflammation, hormone insensitivity, and apoptosis. Obese individuals have impaired mitochondrial function, and the underlying mechanisms and their contribution to obesity remain unclear.
The study and findings
In the present study, researchers demonstrated increased expression and activity of Ras-like proto-oncogene A (RalA) in adipocytes from obese mice and attenuation of HFD-induced obesity upon targeted Rala deletion in white adipocytes. First, they noted upregulation of Rala expression in epididymal (eWAT) and inguinal WAT (iWAT) adipocytes during obesity development in HFD-fed mice relative to controls.
Further, RalA protein levels were elevated in iWAT adipocytes from obese mice. No changes in RalA were observed in brown adipose tissue (BAT) after HFD feeding. Next, RalA-floxed (Ralaf/f) mice and adiponectin-promoter-driven Cre transgenic mice were crossed to generate adipocyte-specific Rala knockout (KO) mice (RalaAKO). RalaAKO mice showed over 90% reduced RalA protein in primary adipocytes from BAT and WAT compared to Ralaf/f littermates.
RalA depletion reduced insulin-stimulated glucose uptake in BAT and iWAT. Additionally, brown adipocyte-specific KO mice (RalaBKO) were produced by crossing Ralaf/f mice and uncoupling protein 1 (Ucp1)-promoter-driven Cre transgenic mice. This reduced glucose uptake in the BAT of RalaBKO mice, and insulin-stimulated glucose uptake was mainly limited to brown fat.
Adipocyte-specific Rala deletion did not affect the body weight of chow-diet (CD)-fed mice, albeit they had reduced fat mass and depot weight. RalaAKO mice had smaller iWAT adipocytes than CD-fed controls. RalaAKO mice gained less weight than controls when fed 60% HFD. HFD-fed RalaAKO mice had smaller adipocytes in iWAT but not in BAT or eWAT compared to controls.
HFD-fed RalaAKO mice also showed improved glucose tolerance, without changes in insulin tolerance; they also had reduced insulin levels and improved homeostasis model assessment of insulin resistance (HOMA-IR) than controls. RalaAKO mice showed lower glucose excursions in a pyruvate tolerance test than controls, with downregulation of hepatic gluconeogenic genes.
HFD-fed RalaAKO mice had lower triglyceride levels and liver weight and less lipid accumulation in the liver than controls. Moreover, the expression of lipogenic, fibrosis-related, and inflammatory genes was reduced in the livers of RalaAKO mice. The team found that adipocyte Rala ablation did not affect food intake and energy metabolism in CD-fed mice.
However, HFD-fed RalaAKO mice had increased energy expenditure. In contrast, energy expenditure and food intake were identical in HFD-fed RalaBKO mice and controls, suggesting that WAT-specific Rala deficiency increased energy expenditure. Further, oxidative phosphorylation proteins were upregulated in the iWAT of RalaAKO mice but not in eWAT.
Next, the team explored mechanisms underlying increased energy metabolism in RalaAKO mice and mitochondrial activity in adipocytes. They observed an elevated oxygen consumption rate in iWAT mitochondria from KO mice relative to controls. Moreover, fatty acid oxidation was higher in KO adipocytes. The expression of mitochondrial biogenesis-related genes in WAT was comparable between HFD-fed RalaAKO and Ralaf/f mice.
Electron microscopy showed that HFD feeding of wild-type mice induced smaller, spherical iWAT mitochondria. iWAT mitochondria in CD-fed mice had an elongated shape, while those in HFD-fed mice had smaller mitochondria. Besides, adipocyte Rala ablation did not grossly impact mitochondrial morphology in the iWAT of CD-fed mice; in contrast, the HFD-induced morphological change in mitochondria was prevented in Rala KO iWAT.
Mitochondrial morphology in BAT was unaltered upon Rala deletion in HFD- or CD-fed mice. HFD feeding downregulated protein levels of long and short forms of optic atrophy 1 (Opa1), a mitochondrial fusion regulator, in iWAT. However, only the short form (S-Opa1) was downregulated in eWAT. Further, they focused on dynamin-related protein 1 (Drp1), which regulates mitochondrial fission, and found increased phosphorylation at the anti-fission site (S637) in Rala KO iWAT.
The researchers analyzed microarray data of WAT from non-obese and obese females to examine the relevance of Drp1 in human obesity. They found that the human Drp1 homolog, dynamin 1 like (DNM1L), was positively correlated with HOMA-IR and body mass index. DNM1L expression was upregulated in obese subjects.
Taken together, the study demonstrated that RalA was induced and activated in white adipocytes of HFD-fed mice. Targeted RalA deletion in white adipocytes prevented obesity-related mitochondrial fragmentation and resulted in resistance to HFD-induced weight gain through heightened energy expenditure.
HFD-fed RalaAKO mice showed improved liver function and pyruvate tolerance and reduced gluconeogenesis and hepatic lipids. Overall, chronically increased RalA activity plays a role in repressing energy expenditure in obese adipose tissue by shifting mitochondrial dynamics towards excessive fission and contributing to weight gain and metabolic dysfunction.