Thesis

The Role of Regulators of G Protein Signaling 2 and 4 in Human Adipogenesis and Osteogenesis

Understanding the molecular basis underlying the formation of bone-forming osteocytes and lipid-storing adipocytes will help provide insights into the cause of disorders originating in stem/progenitor cells and develop therapeutic treatments for bone- or adipose- related diseases. In this study, the role of RGS2 and RGS4, two members of the Regulators of G protein Signaling (RGS) family, were investigated during adipogenic and osteogenic differentiation of human mesenchymal stem cells (hMSCs). HMSCs are multipotent adult stem cells that normally reside in adipose tissue and bone marrow. They can be expanded and differentiated into mature adipocytes or osteocytes upon receiving the appropriate stimuli in vitro, making them an excellent cellular model for studying human adipogenesis and osteogenesis. Expression of RGS2 and RGS4 were found to be inversely regulated during adipogenesis induced by dexamethasone (DEX) and 3-isobutyl-methylxanthine (IBMX), regardless of the presence of insulin, with RGS2 up-regulated and RGS4 down-regulated. RGS2 expression was also up-regulated during osteogenesis induced by Ascorbic Acid-2-phosphate, B-glycerophosphate and DEX, at a level similar to that induced by treatment of DEX alone, a shared component of adipogenic and osteogenic differentiation inducing media, but significantly lower than the level induced by adipogenic inducing media. RGS4 expression was down-regulated during the first 48 hours of osteogenenesis but up-regulated afterwards, in both cases at levels similar to that induced by DEX alone. Expression knock-down using small interfering RNA against RGS2 resulted in decreased differentiation efficiency during both adipogenesis and osteogenesis. On the other hand, expression knock-down of RGS4 also resulted in decreased adipogenic differentiation but increased osteogenic differentiation. Taken together, my results demonstrate that RGS2 and RGS4 are differentially regulated during adipogenic and osteogenic differentiation of hMSCs, whose expression are regulated by both DEX and IBMX during adipogenesis but only by DEX during osteogenesis. In addition, both RGS2 and RGS4 play positive roles during adipogenesis, but opposing roles during osteogenesis, with RGS2 as a positive regulator and RGS4 as a negative regulator.

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