The Effect of Microenvironmental Cues on Adipocyte Cytoskeletal Remodeling

Abstract

Obesity, a disease characterized by excess adipose tissue (AT), is a growing worldwide epidemic. The Centers for Disease Control and Prevention (CDC), in 2017-2018, reported the prevalence of obesity in adults in the United States was 42.4% . Obesity increases the risk for many other serious health conditions such as type 2 diabetes, cardiovascular diseases, stroke, and some cancers. In individuals with obesity, the hypertrophic expansion of adipocytes, the main cell type within AT, is not matched by new vessel formation, leading to AT hypoxia. As a result, hypoxia inducible factor-1⍺ (HIF-1⍺) accumulates in adipocytes inducing a transcriptional program that upregulates profibrotic genes and biosynthetic enzymes such as lysyl oxidase (LOX) synthesis. This excess synthesis and crosslinking of extracellular matrix (ECM) components cause AT fibrosis. Although fibrosis is a hallmark of obese AT, the role of fibroblasts, cells known to regulate fibrosis in other fibrosis-prone tissues, is not well studied. Adipocytes are mechanoresponsive and affected by different microenvironmental cues, including hypoxia and mechanical (un)loading. Yet, no study has focused on the role of the aforementioned factors on the adipocyte mechanical response, including actin cytoskeletal remodeling. This dissertation aims to develop an in vitro model of healthy/diseased AT to explore the effect of microenvironmental cues on adipocyte function and actin cytoskeletal remodeling. The first aim is to study (1) the crosstalk between fibroblasts and adipocytes in a co-culture model and (2) the effect of hypoxia on the ras homolog gene family member A (RhoA)/Rho-associated coiled-coil kinases (ROCK) mechanical pathway and actin cytoskeletal remodeling in adipocytes. We confirmed that hypoxia creates a diseased phenotype by inhibiting adipocyte maturation and inducing actin stress fiber formation facilitated by myocardin-related transcription factor A (MRTF-A/MKL1) nuclear translocation. The second aim explores the effects of mechanical unloading (simulated microgravity) on key adipocyte functions and actin cytoskeletal remodeling. This study demonstrated that mechanical unloading enhances adipocyte maturation via increased lipogenesis and lipolysis and cortical actin remodeling, which together further enhanced glucose uptake. However, disrupting cortical actin remodeling by using inhibitors or exposure to a high concentration of free fatty acids (FFAs) diminished enhanced adipocyte functions observed in simulated microgravity. Overall, the results of these studies support the importance of microenvironmental cues on adipocyte actin cytoskeletal remodeling. Therefore, targeting mechanical pathways that regulate actin cytoskeletal remodeling can be used to improve adipocyte function and AT metabolism and possibly treat related diseases such as type 2 diabetes and obesity.