Background/Purpose: Vascular abnormalities in systemic sclerosis (SSc) are characterized by injury to vascular wall and extensive damage of the microvessels. It has been shown that the endothelial cells (ECs) isolated from patient skin biopsies show dysregulated phenotypes including impaired angiogenesis, increased oxidative stress, barrier dysfunction, and endothelial-to-mesenchymal transition (Endo-MT). Interestingly, all the above-mentioned events are characteristics of the endothelial phenotype of sirtuin 1 (SIRT1)-deficient ECs. SIRT1 is a class III NAD-dependent histone deacetylase that maintains EC function by deacetylating both histones or non-histone proteins. SIRT1-deficient ECs show impaired angiogenesis, eNOS inactivation, increased oxidative stress, and accelerated senescence. It is possible that endothelial SIRT1 deficiency plays a fundamental role in endothelial dysfunction in SSc. In this study, the impact of SIRT1 on SSc EC is determined.

Methods: Dermal ECs were isolated from skin biopsies from healthy subjects and patients with diffuse cutaneous (dc)SSc. Senescence was measured by immunofluorescence, β-galactosidase assay, and senescence-associated secretory phenotype (SASP). EC functions were determined by Matrigel tube formation assay and proliferation assays. SIRT1 activators, including resveratrol, sodium hydrosulfide, or nicotinamide mononucleotide, or SIRT1 inhibitor EX527 were used to treat ECs. SIRT1 expression in ECs were modulated by siRNA knockdown or overexpression. P values of less than 0.05 were considered statistically significant.

Results: We first showed that endothelial SIRT1 was essential for normal endothelial function in human dermal microvascular ECs (HMVECs), as knockdown or inactivation of SIRT1 inhibited cell proliferation and angiogenesis while increased Endo-MT and senescence. In contrast, overexpression or activation of SIRT1 in HMVECs led to opposite observations. We found that SIRT1 was significantly downregulated in SSc ECs. This was accompanied with increased acetylation of SIRT1-target proteins including eNOS, p53, and NF-kB p65. SSc ECs also showed increased senescence, determined by increased senescent markers and SASP, and reduced eNOS activity. Enhanced staining of senescent markers was co-localized with blood vessels in SSc skin. Overexpression or activation of SIRT1 in SSc ECs significantly enhanced cell proliferation and angiogenesis, while decreased senescence. To determine the mechanism for SIRT1 downregulation in SSc ECs, we uncovered the p53-miR34a-SIRT1 axis in these cells. We found significantly elevated levels of TP53, acetylated-p53, and miR34a in SSc ECs, which are key members of the regulatory loop for SIRT1 that leads to downregulation of SIRT1 and elevated levels of p21.

Conclusion: In this study, we provided a link between epigenetic regulation and endothelial senescence in SSc, and presented a novel mechanism for the dysregulated endothelial phenotype that characterizes this disease. Class III histone deacetylases also show potent anti-fibrotic effects in the skin and lungs. Our results offer a strong scientific framework for repurposing SIRT1 activators as targeted therapeutics to treat SSc patients.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/sirt1-modulates-the-senescent-phenotype-in-scleroderma-endothelial-cells/