Moderate l-lactate administration suppresses adipose tissue macrophage M1 polarization to alleviate obesity-associated insulin resistance

As a crucial metabolic intermediate, l-lactate is involved in redox balance, energy balance, and acid-base balance in organisms. Moderate exercise training transiently elevates plasma l-lactate levels and ameliorates obesity-associated type 2 diabetes. However, whether moderate l-lactate administration improves obesity-associated insulin resistance remains unclear. In this study, we defined 800 mg/kg/day as the dose of moderate l-lactate administration. In mice fed with a high-fat diet (HFD), moderate l-lactate administration for 12 weeks was shown to alleviate weight gain, fat accumulation, and insulin resistance. Along with the phenotype alterations, white adipose tissue thermogenesis was also found to be elevated in HFD-fed mice. Meanwhile, moderate l-lactate administration suppressed the infiltration and proinflammatory M1 polarization of adipose tissue macrophages (ATMs) in HFD-fed mice. Furthermore, l-lactate treatment suppressed the lipopolysaccharide-induced M1 polarization of bone marrow-derived macrophages (BMDMs). l-lactate can bind to the surface receptor GPR132, which typically drives the downstream cAMP-PKA signaling. As a nutrient sensor, AMP-activated protein kinase (AMPK) critically controls macrophage inflammatory signaling and phenotype. Thus, utilizing inhibitors of the kinases PKA and AMPK as well as siRNA against GPR132, we demonstrated that GPR132-PKA-AMPKα1 signaling mediated the suppression caused by l-lactate treatment on BMDM M1 polarization. Finally, l-lactate addition remarkably resisted the impairment of lipopolysaccharide-treated BMDM conditional media on adipocyte insulin sensitivity. In summary, moderate l-lactate administration suppresses ATM proinflammatory M1 polarization through activation of the GPR132-PKA-AMPKα1 signaling pathway to improve insulin resistance in HFD-fed mice, suggesting a new therapeutic and interventional approach to obesity-associated type 2 diabetes.

A new perspective of lactatogenesis by isolated adipocytes

Increased adipose tissue mass exhibited greater capacity of glucose transformation in lactate, highlighting lactatogenesis as a crucial factor in body size. Classically, lactate produced by isolated adipocytes are expressed per million of cells and were never correlated with their size. Spontaneously hypertensive rats (SHR) have a lower body weight and smaller adipocytes when compared to Wistar-Kyoto. We evaluated basal lactate by epididymal 15^-weeks-old isolated adipocytes of SHR, Wistar-Kyoto and Wistar. Basal lactate was similar when expressed by one million cells. However, SHR adipocytes were smaller, so we adjusted the results by cell volume and SHR showed higher basal lactate production which was directly endorsed by hyperlactatemia in the presented conditions. Thereby, we suggest a new perspective on lactatogenesis analysis by adipocytes, which could be linked to the receptors density and associate enzymes. Moreover, we showed that the thin and hypertensive rats can be hyperlactemic in fasting conditions.