Background/Purpose: Fibrosis, the hallmark of systemic sclerosis (SSc), is associated with metabolic and epigenetic alterations that are incompletely characterized. Our laboratory had previously implicated dysregulated nicotinamide adenine dinucleotide (NAD) metabolism associated with upregulation of the NAD hydrolase CD38 in SSc pathogenesis. Nicotinamide N-methyl transferase (NNMT) is a cellular enzyme with key roles in NAD homeostasis via de novo NAD generation through the salvage pathway. By irreversibly methylating the NAD precursor nicotinamide (NAM) and reducing its availability for NAD salvage synthesis while also depleting the universal methyl donor S-adenosylmethionine, NNMT uniquely links cellular energy consumption and epigenetic modifications. Moreover, NNMT promotes stromal cancer-associated fibroblast differentiation, and elevated NNMT is implicated in metastasis. In this study, we sought to examine the expression and role of NNMT in SSc fibrosis.

Methods: Gene expression in SSc skin biopsies was determined by bulk and single cell RNA-Seq and immunolabelling. To investigate the impact of NNMT, human skin fibroblasts in cultures were incubated with transforming forming growth factor (TGF-β) and NNMT inhibitors followed by PCR, immunolabelling, immunofluorescence (IF), NAD assays and migration assays using Incucyte™ Live Cell Imaging. Genomewide transcript level changes and chromatin remodeling were determined.

Results: NNMT expression is significantly elevated in SSc skin biopsies, and in explanted skin fibroblasts. NNMT mRNA levels in the skin are positively associated with TGF-β activity in multiple SSc datasets. Moreover, NNMT levels are strongly associated with CD38 levels in SSc skin biopsies, and a combined gene score for NNMT plus CD38 robustly correlated with the skin score (MRSS). The number of NNMT-immunopositive interstitial cells was elevated in both papillary and reticular dermis from SSc patients. TGF-β strongly increased the expression of NNMT in normal fibroblasts. Targeted pharmacological inhibition of NNMT markedly attenuated TGF-β-induced cellular fibrotic responses.

Conclusion: NNMT, which represents a nexus linking cellular energy metabolism and epigenetic chromosome modeling, is highly expressed in explanted SSc fibroblasts and SSc skin biopsies. NNMT is inducible by TGF-β and directly contributes to the induction of fibrotic cellular responses, while its pharmacological blockade mitigates fibrosis in vitro. The profibrotic effects of NNMT potentially implicate altered NAD metabolism as well as changes in DNA methylation and related epigenetic mechanisms. Targeting NNMT with selective inhibitors therefore represents a novel therapeutic approach for fibrotic diseases such as SSc.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/targeting-nnmt-as-a-novel-metabolic-approach-to-treat-systemic-sclerosis/