Obesity is a significant health problem, lacking effective pharmacological treatments. A deeper understanding of adipocyte function is crucial. Adipocytes store and release energy, acting as an energy depot. Pyruvate dehydrogenase kinase 2 (PDK2) regulates the pyruvate dehydrogenase complex. Previous research showed PDK2 deficiency reduced body weight gain, prevented hepatic steatosis, and increased hepatic fatty acid oxidation. This study aimed to confirm and extend these findings by investigating the role of adipose-specific PDK2 deficiency in body fat accumulation, adipocyte size, and insulin sensitivity. It also explored the role of PDK1 and PDK2 in adipocyte differentiation in 3T3-L1 cells, investigating the potential involvement of epigenetic modifications and the lactate-HIF1α axis. Finally, the study examined the correlation between PDK1/2 expression and adipogenic markers in human adipose tissue.

Existing research highlights the challenges in treating obesity due to a lack of effective pharmacological interventions. Studies on PDK2 have shown its involvement in maintaining euglycemia during starvation and its contribution to hyperglycemia in diabetes. Inhibition or deletion of PDK2 improves insulin sensitivity and lowers blood glucose. PDK2 also affects apoptosis, cell proliferation, HIF1α signaling, angiogenesis, and macrophage polarization. Previous work from the authors demonstrated that PDK2 deficiency reduces body weight gain, prevents hepatic steatosis, and increases hepatic fatty acid oxidation and ketogenesis in high-fat diet (HFD)-fed mice. This provided the foundation for investigating the role of PDKs in adipogenesis and obesity.

The study used both in vivo and in vitro approaches. For the in vivo experiments, Pdk2lox/+ mice were generated and intercrossed to create Pdk2loxlox mice. Adipocyte-specific PDK2 knockout (PDK2ad-/-) mice were generated by crossing Pdk2loxlox mice with adiponectin-Cre mice. Mice were fed either a control diet (low-fat diet; LFD) or a high-fat diet (HFD). Body composition, glucose and insulin tolerance tests (GTT & ITT), histological analysis (H&E staining and immunofluorescence), and serum analysis were performed. In vitro studies used 3T3-L1 preadipocytes. Cells were differentiated, and mRNA and protein expression levels of PDK isoforms were measured using qPCR and Western blotting. Chromatin immunoprecipitation (ChIP) assays were used to analyze epigenetic modifications. Gain- and loss-of-function studies were conducted using retroviral overexpression and shRNA knockdown. Lactate release was measured. Human visceral adipose tissue samples were analyzed for PDK and adipogenic marker expression. Statistical analysis included Student's t-tests and ANOVAs.

Global Pdk2 knockout mice exhibited significantly decreased adiposity compared to wild-type mice on an HFD, with reduced body weight and fat mass. Adipocyte-specific Pdk2 deficiency also decreased fat mass and slightly improved insulin sensitivity in HFD-fed mice, without affecting energy expenditure. PDK1 and PDK2 expression significantly increased during 3T3-L1 adipocyte differentiation, showing epigenetic regulation (increased H3K27ac, H3K4me3, and H3K36me3). Overexpression of PDK1 or PDK2 stimulated adipogenesis, while knockdown inhibited it. PDK1/2 deficiency reduced glucose consumption and lactate production. Exogenous lactate increased HIF1α protein levels and promoted adipogenesis. HIF1α inhibition abolished PDK1/2-mediated adipogenesis. In human adipose tissue, PDK1/2 expression correlated positively with PPARγ and negatively with BMI. PDK1/2 protein levels were decreased in HFD-fed mice compared to control diet fed mice.

The findings demonstrate that PDK1 and PDK2 are important regulators of adipogenesis. PDK2 deficiency protects against diet-induced obesity by reducing adipocyte differentiation and size. The increased lactate production associated with PDK1/2 activity appears to stabilize HIF1α, promoting adipogenesis. The inverse correlation between PDK1/2 expression and BMI in humans supports the potential of PDK inhibition as a therapeutic target for obesity. The study highlights the PDK-lactate-HIF1α axis as a novel pathway in adipogenesis and obesity.

This study identifies PDK1 and PDK2 as crucial regulators of adipogenesis, particularly in the context of diet-induced obesity. The PDK-lactate-HIF1α axis is implicated as a key driver of adipocyte differentiation. These findings suggest that pharmacological inhibition of PDK1/2 could be a promising strategy for obesity treatment. Future research could focus on developing and testing specific PDK inhibitors for therapeutic applications and further characterizing the role of the PDK-lactate-HIF1α axis in various metabolic contexts.

The study primarily used mouse models and 3T3-L1 cells, which may not fully represent human physiology. The human study used a relatively small sample size, limiting the generalizability of the findings. While the study showed correlation between PDK1/2 expression and obesity, causality was not directly established. Further research with larger human cohorts and different adipose tissue depots is needed.