Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Pathology

First Advisor

James G. Granneman

Abstract

Adipose tissue (AT) is a critical regulator of energy balance through its ability to store or oxidize free fatty acids (FFAs). White adipose tissue (WAT) functions as an anabolic organ to sequester and release FAs, in contrast brown adipose tissue (BAT) is a catabolic organ that oxidizes FAs. However, a comprehensive understanding of the role that FFAs play in the function of WAT and BAT is needed. Here we demonstrate that intracellular FAs enhance the expression of inflammatory cytokines by β3-AR activation in adipocytes, in which the expression of PAI-1 is partly mediated by the de novo synthesis of ceramides/sphingolipids. We also explored the relationship between lipolysis and oxidative gene expression in AT. β3-AR stimulation increased the expression of oxidative genes (PCG1á, UCP1 and NOR-1) in WAT of mice, which was greatly potentiated by inhibition of hormone sensitive lipase (HSL). In 3T3-L1 adipocytes, limiting lipolysis potentiated the induction of oxidative genes; while in contrast, promoting the accumulation of intracellular FAs suppressed their induction by Β-AR stimulation. Interrogation of the Β-adrenergic signalling pathway indicates that intracellular FAs inhibit adenylyl cyclase activity and thereby reduce PKA-mediated transcriptional activity. Partially limiting lipolysis enhanced the induction of brown fat markers and mitochondrial electron transport chain activity in WAT, and facilitated fat loss in mice treated with a β3-AR agonist for five days. In contrast to the results observed in WAT, HSL activity was required for the induction of PGC1α, PPARα, PDK4 and UCP1 by β3-AR activation in BAT of mice. Similarly, lipolysis was required for the maximal induction of oxidative genes in cultured brown adipocytes (BAs), while increasing endogenous FAs elevated their expression. Pharmacological antagonism and siRNA knockdown indicate that PPARα and δ mediate the induction of oxidative genes by Β-AR agonism. Dynamic imaging studies demonstrate that lipids droplets are a source of ligands that transcriptionally activate PPARα and δ. Finally, lipolysis was required to maintain the expression of mitochondrial genes, and FA oxidation in BA. Overall, results indicate that in WAT excess FFAs are toxic, and function to balance production with efflux, while in BAT FFAs promote oxidation, and match FA oxidation with supply. These finding suggest that limiting intracellular FAs is WAT, and activation of PPARα/δ would be of benefit in preventing the toxic effects of FAs, which could have implications for treating excess adiposity and diabetes.

Download

Share

COinS