Altered expression of RXFP1 receptor contributes to the inefficacy of relaxin-based anti-fibrotic treatments in systemic sclerosis

OBJECTIVES: Relaxin is a potent anti-fibrotic hormone that has been tested to ameliorate fibrosis in systemic sclerosis (SSc), but with controversial results. The aim of the study is to sequence relaxin receptor gene RXFP1 and to assess its mRNA expression and protein levels in the skin of SSc patients and healthy subjects. METHODS: Fibroblasts were isolated from unaffected/affected skin samples of (n=16) limited-cutaneous-SSc-(LcSSc) and from affected ones of (n=4) diffuse-cutaneous-SSc-(DcSSc) patients. Fibroblasts from healthy subjects were used as controls. Sequencing of exonic target regions of interest for RXFP1 gene was performed, coupled with mRNA transcript variant analysis. RXFP1 mRNA and protein levels were assessed by quantitative-real-time-PCR-(qRT-PCR) and by immunocytochemistry-(ICC). Alpha-smooth-muscle-actin-(α-SMA) synthesis induced by transforming-growth-factor-beta-1-(TGF-β1) stimulation was investigated in all fibroblasts with and without pre-treatment with serelaxin (a recombinant form of human relaxin-2 targeting the receptor RXFP1). RESULTS: Sequencing of RXFP1 gene showed no relevant mutations in all fibroblast populations. The analysis of mRNA transcripts revealed the presence of 13 different mRNA isoforms of RXFP1 (7 coding and 6 non-coding) upregulated in LcSSc/DcSSc-affected samples and not in LcSSc-unaffected and in healthy ones. On the contrary, ICC demonstrated the absence of RXFP1 in LcSSc/DcSSc-affected fibroblasts and the presence in LcSSc-unaffected and in healthy ones. To prove these findings, serelaxin pre-incubation was unable to counteract TGF-β1-driven upregulation of α-SMA in LcSSc/DcSSc-affected fibroblasts only, but not in LcSSc-unaffected and healthy ones. CONCLUSIONS: The absence/altered expression of relaxin receptor RXFP1 in the affected fibroblasts of SSc patients could explain the inefficacy of relaxin-based anti-fibrotic treatments in the disease.