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Date of Award

Spring 2012

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Sciences

Supervisor

Richard C. Austin

Language

English

Abstract

The endoplasmic reticulum (ER) is a specialized organelle that facilitates correct protein folding and maturation. Disruptions in ER homeostasis lead to ER stress and activation of a series of signal transduction cascades known as the unfolded protein response (UPR), which acts to restore ER homeostasis. In recent years, ER stress and UPR dysfunction have been linked to obesity, fatty liver and insulin resistance. Lipid-laden adipocytes, the main cellular component of white adipose tissue (WAT), play a critical role in whole body energy homeostasis as well as lipid and carbohydrate metabolism. Mature adipocytes, which are metabolically active endocrine cells, differentiate from precursor fibroblast-like preadipocytes, through a process called adipogenesis, leading to formation of cells capable of secreting numerous proteins, cytokines and hormones. ER homeostasis and UPR activation are essential to the function/differentiation of highly secretory cells, however, the role of ER stress/UPR activation in adipogenesis had previously not been examined. We hypothesized that adipogenesis may rely on physiological UPR activation to accommodate the demand on the ER for increased folding and secretion of proteins.

Initial experiments examining UPR activation during 3T3-L1 adipogenesis identified that expression of ER stress/UPR markers was modulated during adipocyte differentiation. Furthermore, inhibition of ER stress/UPR activation by the chemical chaperone, 4-phenyl butyric acid (4-PBA), inhibited adipogenesis and blunted high fat-diet induced weight gain in 4-PBA supplemented mice. These findings suggested that UPR activation modulates adipogenesis and adipose tissue metabolism.

Subsequently, we sought to identify novel candidate ER stress/UPR responsive genes that may be involved in adipogenesis and WAT metabolism. The expression of a recently recognized ER stress-responsive gene, T-cell death associated gene 51 (TDAG51) was identified to be differentially regulated during adipogenesis. However, the function of TDAG51 in adipogenesis or energy regulation was not known. Studies from this thesis showed that TDAG51 protein expression is attenuated by ER stress/UPR activation in preadipocytes and declines during adipogenesis. Based on these results, and given the importance of adipogenesis in WAT function and whole body energy metabolism, it was hypothesized that TDAG51 may be a novel regulator of adipogenesis and energy homeostasis. Indeed, as reported here, knock-down or absence of TDAG51 (TDAG51-/-) in pre-adipocytes increased lipogenesis and lead to earlier and more potent expression of adipogenic markers.

Finally, we investigated whether absence of TDAG51 in mice affected adiposity and metabolic outcomes. Consistent with the in vitro results, we found that TDAG51-/- mice fed a standard chow diet, exhibited an age-associated increase in WAT, developed fatty liver, and exhibited insulin resistance as compared to wild-type mice.

Taken together, the findings in this thesis indicate that physiological UPR activation and the UPR-responsive gene TDAG51 play important roles in regulating adipogenesis, lipogenesis and whole-body energy metabolism. Thus, therapeutic approaches aimed at modulating ER folding capacity, UPR activation and/or TDAG51 expression may have great potential in the treatment of obesity and its co-morbidities.

McMaster University Library

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