Control of myofibroblast gene expression by transcriptional co-regulators
Luchsinger, Larry L
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Type I collagen overexpression is a major contributor to fibrosis. This thesis investigates the function of two transcriptional co-regulator complexes involved in collagen expression. Interferon γ induces class II transactivator (CIITA), which is a potent activator of major histocompatibility class (MHC) II expression while concomitantly repressing collagen transcription. It was hypothesized that mutation of CIITA would alter the immune response and increase collagen transcription during bleomycin-induced fibrosis. Two CIITA partial knockout strains had decreased macrophage and T cell recruitment after bleomycin injury. Compared to WT, mutant CIITA mice had elevated IL-4 and IL-10 cytokine expression coinciding with decreased lung disease. Since CIITA hypomorphic mice maintained the collagen repression domain, there was equivalent fibrosis as judged by morphology and biochemical quantification of collagen. These studies determined that CIITA mediates injury and inflammatory responses in the lung. Myofibroblasts, characterized by the expression of smooth muscle cell actin (SMA) and collagen, are important regulators of the repair of tissue injury and fibrosis progression. The transcriptional co-activator, myocardin related transcription factor A (MRTF-A) is known to regulate SMA gene expression. It was hypothesized that MRTF-A might also activate expression of type I collagen in fibrotic cells. MRTF-A strongly transactivated type I collagen gene reporters in lung fibroblasts. Expression of a dominant negative MRTF-A or shRNA targeting MRTF-A resulted in a significant reduction in type I collagen synthesis. MRTF-A is known to activate gene transcription via serum response factor (SRF), which binds to CArG boxes within responsive promoters. Analysis of the COL1A2 promoter revealed a non-canonical CArG box [CCAAACTTGG] as well as several specific protein 1 (Sp1) sites that were important for MRTF-A activation. Fibroblasts from MRTF-A knockout mice exhibited impaired myofibroblast differentiation compared to WT controls. These findings indicate that MRTF-A is an important regulator of collagen synthesis in lung fibroblasts and it depends on both SRF and Sp1 to enhance collagen expression. Taken together, these studies identified two important transcriptional co-regulator complexes with potential regulatory roles in lung fibrosis.
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