Liver injury of different etiologies leads to a wound healing process involving activation of hepatic stellate cells (HSCs). However, an ongoing hepatocyte injury and inflammation results in an uncontrolled activation and proliferation of HSCs and development of hepatic fibrosis leading to cirrhosis and even hepatocellular cancer. Despite the advances made, gaps remain in our understanding of the mechanisms involved in the process of HSC transformation from quiescent to activated phenotype. Recently, we discovered that the phosphodiesterase 4 (PDE4) subfamily of enzymes play a pathogenic role in the development of cholestatic liver injury and fibrosis. Notably, PDE4s are not present in quiescent HSCs and are rapidly induced upon activation in vitro. Further, the PDE4 specific inhibitor, rolipram, effectively attenuates SMA, collagen expression and accompanying morphological changes in HSCs. PDE4 is the largest sub-family among cAMP- hydrolyzing PDEs, which tightly regulate the levels of cellular cAMP. cAMP, through its effector molecules protein kinase A (PKA) and Exchange Protein directly Activated by cAMP (EPAC), has been shown to down- regulate cytokine induced fibrogenic genes in non-hepatic cells. Hence, we hypothesize that induction of PDE4 expression and activity plays a causal role in HSC activation by decreasing cAMP-PKA/EPAC activities and promoting fibrogenic signaling. We postulate that during HSC activation, promoter associated epigenetic changes and post-translational modifications play a significant role in the regulation of PDE4 expression and activity. We also postulate that PDE4 inhibition will restore PKA/EPAC activities and attenuate TGF-Smad signaling through: (i) inactivation of relevant MAPKs; and (ii) de-repressing PPAR leading to decreased expression of SMA and Col1A1. Importantly, inhibition of PDE4 activity may be a significant therapeutic approach for liver fibrosis.
The specific aims of this proposal are to: 1) Determine the role of PDE4 in the regulation of fibrogenic signaling in HSCs; 2) Determine promoter-associated epigenetic modifications contributing to the induction of PDE4 isoforms during HSC activation; and 3) Determine the post-translational modifications (PTMs) relevant for PDE4 isoform function during HSC activation. Importantly, the results of this COBRE-funded project will provide proof-of-principle and mechanistic rationale for in vivo translational studies (R01) to examine PDE4 targeted strategies for prevention and treatment of hepatic fibrosis.
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