# All-trans retinoic acid modulates proliferation and apoptosis of secondary hair follicle–dermal papilla cells in cashmere goats via the TGF-β2/Smad2/3 pathway

## TL;DR

This study shows that all-trans retinoic acid affects the growth and death of skin cells in cashmere goats by influencing a specific signaling pathway.

## Contribution

The study identifies the TGF-β2/Smad2/3 pathway as a novel mechanism through which ATRA regulates SHF-DPC proliferation and apoptosis.

## Key findings

- ATRA inhibits SHF-DPC proliferation by inducing G1-phase cell cycle arrest.
- ATRA promotes apoptosis by upregulating Bax and downregulating Bcl-2.
- TGF-β2 is upregulated by ATRA, and its pathway is involved in mediating these effects.

## Abstract

Cashmere, a fiber of high economic value, is produced by secondary hair follicles (SHFs), whose growth depends on the proliferation and apoptosis of SHF-derived dermal papilla cells (SHF-DPCs). All-trans retinoic acid (ATRA), a metabolite of vitamin A, has shown inconsistent effects on hair follicle biology, with reports of both inhibition and stimulation. To clarify the role of ATRA in follicular development, we investigated its effects on the proliferation and apoptosis of in vitro–cultured SHF-DPCs isolated from Capra hircus (Albas cashmere goats). SHF-DPCs were obtained from scapular skin, and the optimal ATRA concentration (10−4 M) and treatment duration (24 h) were identified using Cell Counting Kit-8, 5-ethynyl-2′-deoxyuridine, and terminal deoxynucleotidyl transferase dUTP nick end labeling assays. Cells were then assigned to an ATRA-treated group (10−4 M) or a control group. Proliferation, apoptosis, and cell cycle progression were evaluated, followed by transcriptome sequencing. Transcriptomic analysis indicated enrichment of the transforming growth factor-β (TGF-β) signaling pathway. Therefore, mRNA and protein levels of TGF-β2—a follicle growth–related regulator—and Smad2/3 phosphorylation were examined. The TGF-β type I/II receptor inhibitor LY2109761 was used to further validate the involvement of this pathway. ATRA inhibited SHF-DPC proliferation by inducing G1-phase cell cycle arrest and promoted apoptosis by upregulating Bax expression while downregulating Bcl-2 protein expression. Transcriptomic analysis showed that ATRA upregulated TGFβ2 expression in SHF-DPCs. Gene Ontology analysis revealed enrichment of genes related to proliferation and apoptosis, such as “cell migration” and “cell cycle.” Kyoto Encyclopedia of Genes and Genomes pathway analysis identified the TGF-β signaling pathway as a major regulatory target. Treatment with LY2109761 partially reversed ATRA-induced reductions in proliferation, increases in apoptosis, and elevations in Smad2/3 phosphorylation. Overall, ATRA inhibits proliferation and promotes apoptosis in SHF-DPCs, likely through the TGF-β2/Smad2/3 pathway. These findings provide novel insights into ATRA-mediated regulation of hair growth and offer a theoretical basis for future research on cashmere goat SHF development.