Background/Purpose: Glycoprotein nonmetastatic melanoma protein B (GPNMB) is widely expressed on stromal cells and immune cells. It is involved in various cell functions such as cell adhesion, migration, proliferation, and differentiation, and has been implicated in various pathological conditions including cancer and neurodegenerative diseases. This membrane bound protein can be cleaved into a soluble form by ADAM10. By interacting with various cell surface receptors including CD44, soluble (s)GPNMB has the ability to activate various cell types, including fibroblasts. In this study, we set forth to determine the role of sGPNMB in fibroblast functions in systemic sclerosis (SSc).

Methods: Dermal fibroblasts were isolated from skin biopsies from healthy subjects and patients with diffuse cutaneous (dc)SSc. GPNMB expression was analyzed by qPCR, Western blotting, and immunofluorescence. sGPNMB was measured using ELISA. The effects of sGPNMB (0.01-100ng/ml) on fibroblast function were analyzed using Western blotting, proliferation, migration, and gel contraction assays. To induce a myofibroblast phenotype, normal fibroblasts were incubated with TGFβ for 72 hours. To determine the differences between groups, Mann–Whitney U test, Wilcoxon test, Kruskal–Wallis test, or two-way ANOVA were performed. P values of less than 0.05 were considered statistically significant.

Results: In dcSSc fibroblasts, GPNMB was upregulated compared to normal fibroblasts. In addition, dcSSc fibroblasts secreted higher levels of sGPNMB (147.4 ± 50.2 pg/ml vs. 84.8 ± 14.8 pg/ml, p< 0.05), partly due to increased ADAM10. sGPNMB (1 and 10 ng/ml) downregulated profibrotic genes, including collagen I and α-smooth muscle actin in dcSSc fibroblasts. In addition, it inhibited cell proliferation and gel contraction, while it had minimal effect on fibroblast migration. The anti-fibrotic effect of sGPNMB is, at least in part, mediated through CD44, as CD44 knockdown inhibits response to sGPNMB. GPNMB is regulated by its own soluble form and also through epigenetic mechanisms. TGFβ downregulated GPNMB and decreased the release of its soluble form in normal fibroblasts. We also confirmed the role of ADAM10 as the sheddase for GPNMB in dermal fibroblasts, as inhibition of ADAM10 decreased sGPNMB levels significantly after concomitant TGFβ treatment.

Conclusion: We showed for the first time, an anti-fibrotic role of sGPNMB in dcSSc fibroblasts. We also established the ADAM10-sGPNMB-CD44 axis in dermal fibroblasts; ADAM10 cleaves off the soluble form of GPNMB, which mediates its effect through CD44 in both paracrine and autocrine fashions. The futile anti-fibrotic effect in dcSSc fibroblasts might be due to the low levels of sGPNMB released from these cells (pg/ml range) vs. the dose needed (ng/ml range) for the anti-fibrotic effect. We also revealed a complex regulatory network for GPNMB in dcSSc fibroblasts, as it appears to be suppressed by TGFβ.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/functional-characterization-of-glycoprotein-nonmetastatic-melanoma-protein-b-in-scleroderma-fibrosis/