Date of Award

2020

Embargo Period

8-8-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biochemistry and Molecular Biology

College

College of Graduate Studies

First Advisor

L. Ashley Cowart

Second Advisor

Joe B. Blumer

Third Advisor

Samar M. Hammad

Fourth Advisor

David T. Long

Fifth Advisor

Martin E. Young

Abstract

The adipocyte, widely known for its large lipid droplets, is a crucial signaling cell at the crux of metabolism. Programs within the adipocyte itself may have massive impacts upon the body. For example, the internal cell-autonomous circadian clock within the adipocyte regulates such lipid droplet-associated processes as lipolysis (triacylglycerol breakdown) and lipogenesis (lipid uptake, triacylglycerol synthesis), which influence circulating lipids, thus impacting other organs. Circadian rhythms (molecular, metabolic, and behavioral patterning over a day) and lipid droplets are both highly evolutionarily conserved biological phenomena that also happen to be intertwined in the system about which we care today, the global human obesity pandemic. Excessive adiposity is a hallmark of obesity, and humans live in an extraordinarily artificially-lit world, which serves to disrupt circadian clocks, thereby disrupting metabolism. Characterized by their unique sphingosine backbone, sphingolipids, which may derive, in part, from the excess of adiposity and lipid free fatty acids, are well known to affect metabolic disease phenotypes. As the Cowart lab has shown in the past, and as shown in this work, sphingosine kinase 1 (SPHK1) and its enzymatic product sphingosine-1-phosphate (S1P) affect cellular signaling pathways that directly influence metabolism. SPHK1 is a lipid kinase which phosphorylates sphingosine (an N-acylated amino alcohol) to S1P, a bioactive autocrine and paracrine signaling lipid, to influence various processes including cellular survival, angiogenesis, endothelial permeability, immune cell trafficking, and oncogenesis. Since SPHK1 is highly involved in metabolism, such as glucose metabolism, and since metabolism is a circadian-governed process, we hypothesized that SPHK1 is involved in the regulation of the adipocyte circadian clock, which may impact circulating metabolites, such as glucose, fatty acids, and adipokines, thereby affecting weight gain and glucose tolerance. Using a wide variety of techniques and methodologies including RNA sequencing, mass spectrometry coupled with lipidomics and proteomics, chromatin immunoprecipitation (ChIP), protein co-immunoprecipitation, microscopy, gene and protein expression analysis, we conclude that novel properties of SPHK1 and S1P elicit changes in nuclear chromatin remodeling events, leading to circadian clock disruption and subsequent impairment of key metabolic physiological parameters in the mouse. To our knowledge, his is the first report of a lipid metabolite directly affecting the circadian clock.

Rights

All rights reserved. Copyright is held by the author.

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