Deletion of Trim28 in committed adipocytes promotes obesity but preserves glucose tolerance

Obesity is a global health concern linked to numerous metabolic complications. Lipotoxicity, caused by excessive lipid deposition in non-adipose tissues, is a major contributor. Promoting healthy white adipose tissue (WAT) expansion is a potential therapeutic strategy. Previous research implicated tripartite motif-containing 28 (Trim28) in adiposity regulation, primarily through developmental epigenetic mechanisms. However, the precise role of Trim28 in post-developmental adipocyte function remains unclear. This study aimed to investigate the consequences of Trim28 deletion specifically in committed adipocytes, exploring its potential contribution to obesity and metabolic syndrome.

Trim28, a transcriptional corepressor, has been associated with fat mass in mice and humans. Studies using global Trim28 haploinsufficiency showed varying results, with some reporting obesity and metabolic syndrome, while others reported a metabolically healthy phenotype. These discrepancies might be due to developmental vs. post-developmental effects of Trim28. While previous research suggested that Trim28's role in adiposity is primarily developmental, the effects of post-developmental, adipose-specific Trim28 deletion have not been extensively studied. This lack of clarity provided the impetus for the present study. The study also examines the previously identified link between Trim28, Klf14, and obesity and sex specific effects.

The researchers generated mice with adipocyte-specific Trim28 deletion (Trim28 adi-KO) using the Cre-Lox system by crossing floxed Trim28 mice with AdipoQ-Cre mice. They analyzed Trim28 expression in 3T3-L1 adipocytes during differentiation. Both male and female Trim28 adi-KO and wild-type (WT) littermates were fed either a chow diet or a high-fat diet (HFD) for various durations. Body weight, body composition (EchoMRI), glucose metabolism (OGTT, fasting glucose), energy expenditure (CLAMS), and plasma and tissue lipid profiles (lipidomics) were assessed. Western blotting and qPCR were used to analyze protein and gene expression levels of key metabolic pathways. RNA sequencing was performed on WAT from chow-fed mice to identify differentially expressed genes. In vitro studies used 3T3-L1 cells with stable Trim28 knockdown to investigate lipolysis.

Trim28 expression decreased during 3T3-L1 adipocyte differentiation. Trim28 adi-KO mice showed increased adiposity, particularly in females, on both chow and HFD. However, glucose tolerance was largely preserved, even in obese female adi-KO mice, with no significant difference in OGTT or ITT results. Lipidomics revealed altered triglyceride (TG) composition in WAT, with increased saturated and decreased polyunsaturated TG species in female adi-KO mice. Reduced phosphorylation of HSL at serine 563 and decreased ACC protein abundance were found in adi-KO mice, suggesting impaired lipolysis. Increased DJ-1 protein expression was also observed, further supporting altered lipid metabolism. RNA sequencing showed significant changes in genes involved in lipid metabolism, adipogenesis, and differentiation. Notably, Klf14 expression was almost completely absent in adi-KO mice, potentially explaining sex-specific differences in the phenotype. In vitro studies using Trim28-depleted 3T3-L1 cells confirmed impaired lipolysis.

This study demonstrates that post-developmental Trim28 deletion in adipocytes leads to obesity, primarily in females, without glucose intolerance. This challenges the previous notion that Trim28's role in obesity is solely developmental. The findings suggest that Trim28 plays a crucial role in regulating adipocyte lipid homeostasis, impacting lipolysis, lipid storage, and possibly substrate switching, thus contributing to metabolic inflexibility. The loss of Klf14 expression, a known sex-specific regulator of adipocyte function, likely accounts for the more pronounced phenotype in female mice. The altered TG composition observed in WAT could contribute to improved liver lipid profiles and fasting blood glucose levels.

Adipose-specific Trim28 deletion causes obesity, particularly in females, without impaired glucose tolerance. This is linked to altered lipid metabolism, impaired lipolysis, and Klf14 downregulation. Future research should investigate the Trim28-Klf14 interaction and the precise mechanisms driving metabolic inflexibility in Trim28-deficient adipocytes. Further studies are also needed to determine the impact of food intake on the observed phenotype.

The study did not measure food intake, a factor that could influence the observed adiposity. The interpretation of OGTT results in HFD-fed mice was complicated by the difference in body mass between the WT and adi-KO groups. While the in-vitro experiments support the in-vivo findings, the extrapolation from in-vitro results to in-vivo conditions should be interpreted with caution. The mechanisms underlying the Trim28-Klf14 interaction remain to be fully elucidated.