Background/Purpose: Epigenetic regulation, including histone acetylation, plays an important role in scleroderma (SSc) fibrosis. The binding of the bromodomain and extra-terminal domain proteins (BRDs) to acetylated histone residues is critical for gene transcription. It has been shown that BRD inhibitor JQ1 prevents fibrosis in various tissues. However, the involvement of BRDs in SSc fibrosis has not been examined. This study sought to determine the expression and functions of BRDs in SSc fibroblasts. We hypothesize that modulating BRDs by JQ1 will alleviate SSc fibrosis.

Methods: Dermal fibroblasts were isolated from biopsies from healthy subjects or patients with diffuse cutaneous SSc (dcSSc). Fibroblasts were treated with JQ1 (0.01-22µM) for 48 hours. Knockdown of BRD2, BRD3, or BRD4 was achieved by siRNA transfection. Gene expression was determined by qPCR. A scratch wound assay and gel contraction assay were used to evaluate fibroblast function. Cell proliferation was assessed by ki67 immunofluorescence, BrdU incorporation, as well as Incucyte imaging. The in vivo anti-fibrotic efficacy of JQ1 was determined in a bleomycin-induced skin fibrosis mouse model. A t-test was used to compare differences between groups, and a p-value of < 0.05 was considered significant.

Results: BRD inhibitor JQ1 dose-dependently downregulated pro-fibrotic genes including ACTA2, COL1A1, and CTGF in dcSSc fibroblasts. It also downregulated BRD4 but upregulated BRD2. In addition, JQ1 treatment inhibited cell migration, proliferation, and gel contraction mediated by dcSSc fibroblasts. Daily administration of JQ1 (50mg/kg) in mice prevented bleomycin-induced skin fibrosis. The expression of BRD2, BRD3, and BRD4 were similar in fibroblasts from healthy controls and dcSSc patients. To determine which BRD is involved in the anti-fibrotic effect of JQ1, we knocked down BRD2, BRD3, or BRD4 in dcSSc fibroblasts. Surprisingly, knockdown of BRD2, 3, or 4 had minimal effect on cell proliferation. BRD4 knockdown in dcSSc fibroblasts resulted in downregulation of ACTA2 and COL1A1 and relaxed gel contraction. In contrast, BRD2 knockdown led to upregulation of COL1A1 while it had no effect on gel contraction.

Conclusion: BRD modulation by JQ1 showed promising anti-fibrotic effects both in vitro and in vivo. JQ1 decreased profibrotic gene expression in dcSSc fibroblasts, and inhibited cell migration, proliferation, and gel contraction, which are the three manifestations of dcSSc fibroblasts. Our results suggested that BRD4 is pro-fibrotic while BRD2 might be anti-fibrotic. We believe that BRD modulators have the potential of becoming therapeutics for SSc patients in the future. These results also revealed the specific involvement of BRD2 and BRD4 in SSc fibrosis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/inhibition-of-histone-readers-bromodomain-and-extraterminal-domain-proteins-alleviates-scleroderma-fibrosis/