Novel adipose genes in substrate- and depot-dependent lipid accumulation

Abstract

Altered adipose tissue function in obesity is linked to increased risk of type 2 diabetes (T2D) and cardiometabolic diseases via insulin resistance, visceral adiposity, hypertension, and other traits. This thesis studied novel mechanisms for lipid storage in abdominal subcutaneous (SC) and visceral (omental, OM) adipocytes and metabolic alterations in these depots that may promote lifestyle-related diseases. We studied cellular energy metabolism dependent on glucose, fatty acid and amino acid uptake, storage, and/or utilization, including potential adipose depot-specific mechanisms, integrating data from cellular models (human primary adipocyte cultures, murine cell lines), animal models (zebrafish), and multiple human cohorts of people with obesity and insulin resistance.

In Paper I, we identified SLC7A10, a neutral amino acid transporter, as a novel candidate that regulates adipocyte metabolism. Loss of SLC7A10 promoted weight gain and adipocyte hypertrophy with nutrient excess in mutant zebrafish. SLC7A10 transports serine in adipocytes and is important for maintaining cellular GSH levels. Loss of SLC7A10 activity also promoted reactive oxygen species (ROS) generation, lower mitochondrial respiratory capacity, and higher lipid accumulation despite impaired insulin-stimulated glucose uptake.

In Paper II, we showed that SLC7A10 inhibition promotes lipid accumulation in adipocytes partly by SREBP-1c-dependent mechanisms and potentially by channeling amino acids as substrates for de novo lipogenesis. The flux of branched-chain amino acids (BCAAs) and the valine catabolite 3-HIB was strongly affected by SLC7A10 inhibition, and expression of SLC7A10 in SC adipose tissue showed inverse correlations with BCAAs, 3-HIB, and insulin sensitivity in humans, suggesting that altered BCAA metabolism in adipocytes might contribute to excess adipocyte lipid storage and potentially whole-body insulin resistance.

In Paper III, we supplemented a standard high-glucose adipogenic medium with free fatty acids (FFA) to promote differentiation of OM adipocyte cultures. The addition of FFA promoted lipid accumulation in otherwise poorly differentiating OM adipose cultures and induced transcriptomic changes in processes, including mitochondrial translation and regulation of mTOR signaling by amino acids. These processes were also upregulated in mature OM adipocytes isolated directly from biopsies compared to the stromal vascular fraction (SVF) cells, supporting the in vivo relevance of the OM adipocyte cultures. We also observed differences in consumption of amino acids between SC and OM cultures, partly influenced by FFA availability during differentiation. Finally, in the population-based Hordaland Health Studies (HUSK), we found supportive evidence that genetic variants associated with visceral fat mass influence blood concentrations of BCAAs and other lipogenic amino acids.

Overall, in this thesis, we identified genes and pathways involved in altered adipocyte function, increased lipid accumulation, and insulin resistance. The findings help to understand how adipocytes from SC and OM adipose depots preferentially utilize different substrates in visceral adiposity, insulin resistance, and related diseases, shedding new light on mechanisms that regulate cellular nutrient and energy metabolism.