Psoriasis is a chronic skin disease characterized by keratinocyte hyperproliferation and inflammation, largely driven by the cytokines IL-22 and interferon (IFN)-γ. These cytokines activate the signal transducer and activator of transcription (STAT) 3 and STAT1 molecular pathways, leading to abnormal proliferation, impaired differentiation, and increased production of inflammatory mediators in keratinocytes. While the IL-22/STAT3 pathway primarily promotes de-differentiation in keratinocytes, IFN-γ/STAT1-3 signaling induces pronounced inflammation, despite exerting antiproliferative effects on these cells. Recent research has highlighted the role of serine/glycine metabolism in the pathogenesis of psoriasis, by supporting T cell and keratinocyte proliferation. Furthermore, pharmacological inhibition of serine catabolism through targeting serine hydroxymethyltranferase (SHMT)1/2 enzymes reduced the infiltration of inflammatory cells in the skin of the imiquimod-induced mouse model of psoriasis. This study investigates the role of serine catabolism in psoriasis, focusing on its influence on keratinocyte proliferation and inflammation. We examined how pharmacological inhibition of SHMT1/2, mediated by a folate-competitive cell-permeable inhibitor Serine Hydroxymethyltransferase INhibitor 1 (SHIN1), affects keratinocyte proliferation and inflammatory signaling pathways in response to psoriasis-associated cytokines IL-22 and IFN-γ, using both in vitro and ex vivo models of the disease. We found that SHIN1 reduced keratinocyte proliferation, particularly under IL-22 stimulation, and restored differentiation in ex vivo psoriasis skin explants by reversing the effects of IL-22. SHIN1 also inhibited IFN-γ-induced expression of pro-inflammatory genes (e.g., CXCL10, CXCL9, CCL5, CCL2, IL-6) and reduced STAT3 activation, with only modest effects on STAT1 and extracellular signal-regulated kinase 1/2 activation. In psoriasis explants, SHIN1 decreased the expression of Ki67, Keratin 16, and pro-inflammatory cytokines including IL-17A, IL-22, and IFN-γ. These findings support the therapeutic potential of SHIN1 as a metabolism-targeted agent for psoriasis and other cytokine-mediated skin disorders, providing a rationale for further exploration of novel treatment strategies.
