UCLA

Adipose tissue plays a pivotal role in energy homeostasis and metabolic regulation. However, in obesity, the remarkable adaptability of adipose tissue becomes impaired. The underlying mechanisms behind this limited adaptability remain poorly understood. Here, we initially discuss on chapter one an overview about adipose tissue plasticity in health and disease. Then, we investigate a novel layer of regulation involving translation in the adipose tissue, examining its response to obesity and acute PPARγ agonist treatment with rosiglitazone. First, using single-cell RNA sequencing, we establish a transcriptional profile atlas of stromal cellular remodeling from obese to lean-like states in inguinal and epididymal adipose tissue following rosiglitazone treatment. Notably, both stromal fractions exhibit a downregulation of inflammation-related transcripts and an upregulation of lipid-related metabolism and ribosomal transcripts. Adipocyte progenitor and preadipocyte populations display enhanced ex-vivo differentiation potential and upregulation in ribosome and peptide chain elongation pathways. This ribosomal remodeling is directly driven by PPARγ binding to gene promoters of ribosomal factors. Furthermore, we have characterized the translatome in the epididymal stromal fraction, highlighting a buffering response and fat-exclusive preferential translation after rosiglitazone treatment. Enhanced translation efficiency in rosiglitazone-elicited polysomes promotes the translation of transcripts containing G-rich sequences in their 5’ untranslated regions. Our findings shed light and provide a resource on how rosiglitazone remodels the adipose stromal vascular fraction, both dependent and independent of PPARγ. Importantly, we uncover translatome remodeling as a major new mechanism for maintaining translation homeostasis and preserving adipose tissue health in obesity. Second, we focused on the mature adipocytes, and how rosiglitazone modulates their translation machinery. Transcriptional analysis of brown and white adipose tissue after thiazolidinedione treatment has shown translation as a highly upregulated process. Therefore, we hypothesized that thiazolidinediones may have an uncharacterized mechanism of action by enhancing translation efficiency in adipocytes. A mechanism that may be mediated by specific induced proteins such as PIXL. Our laboratory has identified a largely uncharacterized PPARγ -responsive, X-linked gene, PIXL that is primarily expressed in the cytoplasm. Notably, PIXL and PPARγ expression is reduced in ob/ob mice and with high-fat feeding, results that are consistent with impaired expansion of adipose tissue. Our in vivo studies demonstrated that PIXL loss-of-function in mature adipocytes leads to a dysfunctional tissue characterized by adipocyte hypertrophy, impaired glucose metabolism, inflammation, hypertriglyceridemia, enhanced cold sensitivity, and decreased energy expenditure. Mechanistically, immunoprecipitation studies have uncovered PIXL interactome, highlighting the close association with eukaryotic initiation factors, ribonucleoprotein complexes, and 40S ribosomal proteins. Indeed, co-immunoprecipitation studies have demonstrated that PIXL associated with the eIF4E complex. Additionally, polysome profiling have shown PIXL at protein levels associated with initiation factors and subunits of ribosomes. Taken together, we propose that PPARγ induces the expression of PIXL, which in turn may be an important factor regulating translational control and directing the translation of defined mRNA networks. Finally, we propose a novel layer of molecular regulation of thiazolidinediones by targeting translation efficiency and adipocyte proteostasis - a process that may be impaired during obesity and restored after treatment with rosiglitazone.