Down‐regulation of Shh in the hair follicles of mice during chemotherapy‐induced hair loss is mediated by the <scp>JAK</scp> / <scp>STAT1</scp> signaling pathway

Chemotherapy-induced alopecia (CIA) is a major unresolved adverse effect in clinical oncology. We have previously shown that the Sonic hedgehog (Shh) signaling pathway is targeted by cyclophosphamide (CYP) treatment, but the detailed mechanism by which this chemotherapy drug induces alopecia still remains largely unknown. To answer this question, in the present study, we used Shh-GFP^+/- mice and analyzed Shh-expressing cells (Shh⁺ cells) in hair follicles at different times post-CYP treatment. Through flow cytometry assays, we showed that Shh⁺ cells decreased significantly after CYP treatment. To investigate the molecular events involved in this decrease, we carried out gene set enrichment analysis of RNA sequencing data of Shh⁺ cells, which revealed that the expression levels of most Janus-activated kinase/signal transducer and activator of transcription 1 (JAK/STAT1) signaling pathway-related genes were upregulated compared to the controls. Furthermore, through a chromatin immunoprecipitation assay, we showed that STAT1 could bind to the promoter of the Shh gene during CIA in hair follicles and the binding strength increased upon CYP treatment. Treatment with JAK inhibitors also rescued hair loss and upregulated Shh expression in mice with CIA, further supporting the role of the JAK/STAT1 signaling pathway in the regulation of Shh during CIA. Taken together, our results provide new insights into the molecular mechanisms of CIA.