Nonalcoholic fatty liver disease (NAFLD) has become the most common liver disease, and decreased fatty acid oxidation is one of the important contributors to NAFLD. Mitochondrial trifunctional protein α-subunit (MTPα) functions as a critical enzyme for fatty acid β-oxidation, but whether dysregulation of MTPα is pathogenically connected to NAFLD is poorly understood. We show that MTPα is acetylated at lysine residues 350, 383, and 406 (MTPα-3K), which promotes its protein stability by antagonizing its ubiquitylation on the same three lysines (MTPα-3K) and blocking its subsequent degradation. Sirtuin 4 (SIRT4) has been identified as the deacetylase, deacetylating and destabilizing MTPα. Replacement of MTPα-3K with either MTPα-3KR or MTPα-3KQ inhibits cellular lipid accumulation both in free fatty acid (FFA)-treated alpha mouse liver 12 (AML12) cells and primary hepatocytes and in the livers of high-fat/high-sucrose (HF/HS) diet-fed mice. Moreover, knockdown of SIRT4 could phenocopy the effects of MTPα-3K mutant expression in mouse livers, and MTPα-3K mutants more efficiently attenuate SIRT4-mediated hepatic steatosis in HF/HS diet-fed mice. Importantly, acetylation of both MTPα and MTPα-3K is decreased while SIRT4 is increased in the livers of mice and humans with NAFLD. Our study reveals a novel mechanism of MTPα regulation by acetylation and ubiquitylation and a direct functional link of this regulation to NAFLD.

Background: Bone morphogenetic protein 4 (BMP4) can induce C3H10T1/2 mesenchymal stem cell commitment into the adipocyte lineage. Results: Overexpression of RhoGDIβ in C3H10T1/2 cells prevented BMP4-induced adipogenic commitment, whereas it facilitated expression of smooth muscle-like cell-specific markers. Conclusion: RhoGDIβ plays opposing roles in committing C3H10T1/2 cells to adipocytes and smooth muscle-like cells. Significance: This is a first report implicating a role of RhoGDIβ in C3H10T1/2 cells fate decisions. The integration of signals involved in deciding the fate of mesenchymal stem cells is largely unknown. We used proteomics profiling to identify RhoGDIβ, an inhibitor of the small G-protein Rho family, as a component that regulates commitment of C3H10T1/2 mesenchymal stem cells to the adipocyte or smooth muscle cell lineage in response to bone morphogenetic protein 4 (BMP4). RhoGDIβ is notably down-regulated during BMP4-induced adipocytic lineage commitment of C3H10T1/2 mesenchymal stem cells, and this involves the cytoskeleton-associated protein lysyl oxidase. Excess RhoGDIβ completely prevents BMP4-induced commitment to the adipocyte lineage and simultaneously stimulates smooth muscle cell commitment by suppressing the activation of Rac1. Overexpression of RhoGDIβ induces stress fibers of F-actin by a process involving phosphomyosin light chain, indicating that cytoskeletal tension regulated by RhoGDIβ contributes to determining adipocyte versus myocyte commitment. Furthermore, the overexpression of RacV12 (constitutively active form of Rac1) totally rescues the inhibition of adipocyte commitment by RhoGDIβ, simultaneously preventing formation of the smooth muscle-like phenotype and disrupting the stress fibers in cells overexpressing RhoGDIβ. Collectively, these results indicate that RhoGDIβ functions as a novel BMP4 signaling target that regulates adipogenesis and myogensis.

Obesity is associated with chronic low-level inflammation, especially in fat tissues, which contributes to insulin resistance and type 2 diabetes mellitus (T2DM). Protein inhibitor of activated STAT 1 (PIAS1) modulates a variety of cellular processes such as cell proliferation and DNA damage responses. Particularly, PIAS1 functions in the innate immune system and is a key regulator of the inflammation cascade. However, whether PIAS1 is involved in the regulation of insulin sensitivity remains unknown. Here, we demonstrated that PIAS1 expression in white adipose tissue (WAT) was downregulated by c-Jun N-terminal kinase in prediabetic mice models. Overexpression of PIAS1 in inguinal WAT of prediabetic mice significantly improved systemic insulin sensitivity, whereas knockdown of PIAS1 in wild-type mice led to insulin resistance. Mechanistically, PIAS1 inhibited the activation of stress-induced kinases and the expression of nuclear factor-κB target genes in adipocytes, mainly including proinflammatory and chemotactic factors. In doing so, PIAS1 inhibited macrophage infiltration in adipose tissue, thus suppressing amplification of the inflammation cascade, which in turn improved insulin sensitivity. These results were further verified in a fat transplantation model. Our findings shed light on the critical role of PIAS1 in controlling insulin sensitivity and suggest a therapeutic potential of PIAS1 in T2DM.