Ubc9 Impairs Activation of the Brown Fat Energy Metabolism Program in Human White Adipocytes

Abstract

Insulin resistance and type 2 diabetes mellitus (T2DM) result from an inability to efficiently store and catabolize surplus energy in adipose tissue. Subcutaneous adipocytes protect against insulin resistance and T2DM by coupling differentiation with the induction of brown fat gene programs for efficient energy metabolism. Mechanisms that disrupt these programs in adipocytes are currently poorly defined, but represent therapeutic targets for the treatment of T2DM. To gain insight into these mechanisms, we performed a high-throughput microscopy screen that identified ubiquitin carrier protein 9 (Ubc9) as a negative regulator of energy storage in human sc adipocytes. Ubc9 depletion enhanced energy storage and induced the brown fat gene program in human sc adipocytes. Induction of adipocyte differentiation resulted in decreased Ubc9 expression commensurate with increased brown fat gene expression. Thiazolidinedione treatment reduced the interaction between Ubc9 and peroxisome proliferator-activated receptor (PPAR)γ, suggesting a mechanism by which Ubc9 represses PPARγ activity. In support of this hypothesis, Ubc9 overexpression remodeled energy metabolism in human sc adipocytes by selectively inhibiting brown adipocyte-specific function. Further, Ubc9 overexpression decreased uncoupling protein 1 expression by disrupting PPARγ binding at a critical uncoupling protein 1 enhancer region. Last, Ubc9 is significantly elevated in sc adipose tissue isolated from mouse models of insulin resistance as well as diabetic and insulin-resistant humans. Taken together, our findings demonstrate a critical role for Ubc9 in the regulation of sc adipocyte energy homeostasis.

Mitochondrial Activity in Human White Adipocytes Is Regulated by the Ubiquitin Carrier Protein 9/microRNA-30a Axis

“…5E). Together, our findings demonstrate that the depletion of Ubc9 stimulates PPAR␥ activity (13) and stabilizes PRDM16, corresponding to activation of the BAT energy metabolism program in human adipocytes.…”

“…Previous data established that PRMD16 and PPAR␥ interact to promote expression of thermogenic genes in subcutaneous adipocytes (11,24). These data, combined with our recent observation that Ubc9 interacts with PPAR␥ (13), suggested that Ubc9 may interact with PRDM16 as part of a complex. As shown in Fig.…”

“…Our previous studies established Ubc9 as a regulator of the brown adipocyte gene program in human adipocytes (13). Analysis of regions upstream of the Ubc9 gene failed to identify any typical modes of transcriptional regulation.…”

“…diverse stimuli, subcutaneous human white fat depots demonstrate substantial metabolic flexibility and are capable of acquiring features of brown fat, including increased mitochondrial biogenesis and oxidative metabolism (3). Ubc9 inhibits the acquisition of brown fat features, including mitochondriogenesis and fatty acid oxidation (13). Based on the inverse correlation between miR-30a and Ubc9 expression in adipose tissue and our recent studies positioning Ubc9 as a regulator of oxidative metabolism in human adipocytes, we hypothesized that miR-30a blocks Ubc9 expression to stimulate mitochondrial activity.…”

“…We recently identified the E2 SUMO ligase Ubc9 as a factor that represses acquisition of brown fat features in human adipocytes (13). Ubc9 expression is increased in WAT isolated from mice and humans exhibiting insulin resistance, which motivated our studies to investigate the regulation of Ubc9 in human adipocytes and whether endogenous modes of suppression play a role in browning of white fat.…”