Systemic sclerosis and AHR: Shedding light on a hidden connections

Systemic sclerosis (SSc) is a complex and debilitating autoimmune disease marked by fibrosis of the skin and inner organs, alongside chronic inflammation, and vascular abnormalities. SSc pathogenesis involves both genetic and environmental factors, such as silica dust or benzene exposure but the underlying molecular mechanisms regulating fibrogenesis and organ involvement are not fully understood. In part due to this knowledge gap, treatment options are limited. In this review we look at the possible role of the aryl hydrocarbon receptor (AHR), a transcription factor involved in immunomodulation, fibrosis and drug metabolism and inflammatory responses, especially in barrier organs. AHR activation by binding to one of its many small molecular weight ligands can result in gene-expression changes in the nucleus (its role as a transcription factor) but also lead to knock-on effects on other signaling pathways via direct binding (e.g., to NFkB) or via AHR's protein degradation capacity (E3 ligase). In some cell types transcription target genes include the fibrogenic cytokine TGF-β or metalloproteinases responsible for extracellular matrix remodeling. AHR has been shown to be highly expressed in all cutaneous cell populations, and to be critical for skin homeostasis. Given its context-dependent effects, AHR may act as both a pro- and anti-fibrotic regulator in SSc, depending on ligand availability and cellular environment. This dual role highlights AHR as a potential therapeutic target, where selective agonists or antagonists could help restore immune and fibrotic homeostasis. Here, we explore these mechanisms and discuss the potential of AHR as a therapeutic target for modulating disease progression and improving patient outcomes.