Graduate Studies

 

Date of Award

12-5-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Biomedical Engineering

First Advisor

Jung Yul Lim

Abstract

Obesity has reached global epidemic levels in recent decades and is the leading risk factor for type-2 diabetes (T2D). Adipose tissue behaves as a link between obesity and T2D, with dysfunctional extracellular matrix (ECM) remodeling, adipokine secretion and lipid metabolism leading to insulin resistance. The extracellular mechanophysical milieus is understood to regulate adipocyte differentiation and function through activation of mechanosensory machinery and related pathways. Therefore, exploring the adipocytic response to mechanical loading can provide great pathophysiological insight into obesity and T2D. Adipose tissue is exposed to various compound forces due to weight-bearing and movement, and adipocyte stiffness influences stress distribution and deformation across the tissue. Our findings with atomic force microscopy (AFM) confirmed that differentiated adipocytes display higher stiffness than preadipocytes, and further showed that adipocyte stiffness is governed by cytoskeletal contractility modulated by Rho/RhoA Kinase (Rho/ROCK) mechanosensor. We also demonstrated that cytoskeletal rearrangements made to accommodate for lipid droplets during adipogenesis is associated with diminished force transmission at cell-substrate sites, as illustrated by traction force microscopy (TFM). Next, we investigated the role of adipocyte mechanosensors in modulating insulin signaling and found that cyclic stretch loading induced basal AKT activation and enhanced GLUT4 surface translocation. Impeding both Rho/ROCK and focal adhesion kinase (FAK) mechanosensor activity abrogated the stretch-activation of AKT, implying their regulatory role in the metabolic response of adipocytes to mechanical loading. In the final study, we found that cyclic stretch loading significantly altered adipokine expression and increased key adipogenic/lipogenic gene expression. Recent research has established transcriptional co-activator Yes-associated protein (YAP) as a molecular switch regulating adipose tissue fibrosis, lipid metabolism and adipokine secretion. Immunofluorescence and immunoblotting data showed that YAP is transiently activated by cyclic stretch loading. While YAP silencing did not affect AKT activation by stretch, YAP was required for the stretch-induced dephosphorylation of AMPK. These findings suggest that YAP may be responsible for metabolically shifting adipocytes towards a more anabolic state at the expense of catabolic activity when exposed to cyclic stretch loading.

Comments

Copyright 2023, Tasneem Bouzid

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