Date of Award

Winter 12-15-2019

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Molecular Cell Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Targeting adipose tissue function to decrease adiposity and improve insulin sensitivity could treat obesity and diabetes. How the nuclear receptor PPARγ regulates the development of two functionally distinct types of adipose tissue, brown and white fat (BAT and WAT), as well as the browning of white fat, remains unclear. Our previous studies suggest that PexRAP, a peroxisomal lipid synthetic enzyme, regulates PPARγ signaling and white adipogenesis.Here, we show that PexRAP is an inhibitor of brown adipocyte gene expression. PexRAP inactivation promoted adipocyte browning, increased energy expenditure, and decreased adiposity. Identification of PexRAP-interacting proteins suggests that PexRAP function extends beyond its role as a lipid synthetic enzyme. Notably, PexRAP interacts with importin-β1, a nuclear import factor, and knockdown of PexRAP in adipocytes reduced the levels of nuclear phospholipids. PexRAP also interacts with PPARγ, as well as PRDM16, a critical transcriptional regulator of thermogenesis, and disrupts the PRDM16-PPARγ complex, providing a potential mechanism for PexRAP-mediated inhibition of adipocyte browning. These results identify PexRAP as an important regulator of adipose tissue remodeling.Consistent with the idea that PexRAP is a transcription co-regulator, we also identified the mediator complex component Med19 as a PexRAP-interacting protein. Mediator is a multi-subunit complex that acts as a bridge between transcription factors and RNA PolII. The subunit composition of Mediator can vary, and different subunits interact with different transcription factors in a cell context-dependent manner. Many of the specific roles of Mediator subunits, including Med19, remain unknown.Here, we demonstrate that Med19 plays a critical role in the development and maintenance of white fat both in vitro and in vivo, but it is dispensable for brown fat development. Lentivirus-mediated knockdown of Med19 in iWAT SV cells inhibited adipogenesis. However, Med19 knockdown in brown adipocytes had no effect on adipogenesis and appeared to increase thermogenic gene expression. Adipose-specific deletion of Med19 (Med19-AKO) in mice resulted in near ablation of WAT leading to hepatic insulin resistance and steatosis. BAT mass was unaffected, but Med19-AKO mice manifested a significant whitening of the thermogenic fat, most likely due to a secondary lipid “spillover” effect caused by white adipose lipodystrophy. To study the effect of Med19 KO in mature adipose tissue, we created an adipose-specific, tamoxifen-inducible Med19 knockout (Med19-iAKO) mouse line, which possessed a similar phenotype to Med19-AKO animals. RNA-seq analysis of Med19-iAKO gWAT painted a picture of an unhealthy tissue struggling to maintain adipose-specific gene expression, resulting in loss of adipose tissue, likely due to the upregulation of genes involved in apoptosis, necroptosis, and inflammation. Chromatin immunoprecipitation (ChIP) in iWAT SV cells indicated that the decrease in adipose-specific gene expression in Med19-iAKO gWAT is likely due to reduced residency of PPARγ on adipose-specific target genes. These results identify Med19 as a crucial facilitator of PPARγ-mediated gene expression that is selectively required for the development and maintenance of WAT.

Language

English (en)

Chair and Committee

Irfan J. Lodhi

Committee Members

Clay F. Semenkovich, Brian N. Finck, Charles A. Harris, Babak Razani,

Comments

Permanent URL: https://doi.org/10.7936/d8rt-9c96

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