All ETDs from UAB

Advisory Committee Chair

Rosa Serra

Advisory Committee Members

Chenbei Chang

Amjad Javed

Michael Miller

David Schneider

Document Type

Dissertation

Date of Award

2017

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

One out of two people in the U.S. will experience some form of Osteoarthritis (OA) in their lifetime. Members of the Transforming Growth Factor-β (TGF-β) superfamily are important factors that stimulate chondrocyte matrix biosynthesis. Mice with a dominant-negative mutation of the TGF-βIIR develop a degenerative joint disease resembling OA. Another key chondrogenic factor, SOX9, is important for maintaining chondrocyte function. SOX9 and TGF-β receptors are decreased in OA patients. Our overall hypothesis is that TGF-β regulates SOX9 to maintain articular cartilage. We utilized two cell culture models for these experiments, ATDC5 cells, and primary bovine articular chondrocytes. We showed that TGF-β stabilizes SOX9 protein and increases phosphorylation of SOX9 in both ATDC5 and bovine articular chondrocytes. SOX9 can be phosphorylated on three Serine residues S64, S181, and S211. We mutated these serine sites to alanine and found that mutation of Serine 211 is required to maintain normal SOX9 protein levels and that TG-Fβ was not able to increase the stability of SOX9-S211A mutant. The results suggest that TGF-β may phosphorylate SOX9 on S211 to stabilize it. The S211 site is in a motif that is targeted by p38 kinase. TGF-β can signal through a canonical/Smad-mediated pathway or non-conical pathways, including p38. We tested the hypothesis that Smad2/3 and/or p38 regulate the phosphorylation and stability of SOX9. First, we determined that knock down of Smad2/3 using siRNA, blocked TGF-β-mediated stability of Sox9. Then we showed that stabilization of SOX9 by TGF-β was blocked in cells treated with p38 inhibitors suggesting that TGF-β requires both Smad2/3 and p38 activity to regulate SOX9 protein levels. Previously, we showed that Papss2 is a downstream target of TGF-β. We utilized siRNA for SOX9 to show that TGF-β requires SOX9 to regulate Papss2. We also showed that p38 is required for TGF-β-mediated regulation of PAPSS2 suggesting a new mechanism for TGF-β-mediated gene regulation in chondrocytes. Understanding how TGF-β stabilizes SOX9 may lead to identification of preventative targets of both age-related and post-traumatic OA.

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