Human Fibroblast Growth Factor 9 (FGF9) Induces Hair Follicle Cycle Transition via TGF-β/BMP/Smad Pathway

Introduction

Hair growth progresses through a series of cycle phases, anagen, catagen and telogen. Dysregulation of these cycles can lead to alopecia. There are currently few potent therapies available to treat hair loss. Cycling of hair follicles and follicle development is controlled by multiple cell types and signaling pathways. The transforming growth factor (TGF) superfamily are key modulators of a wide range of signalling cascades. TGFb members in combination with bone morphogenic proteins (BMP) bind to cell receptors and activate SMADs. Fibroblast growth factors are also important components of many biological processes such as developmental regulation, biological homeostasis and disease progression. Recently, there have been studies linking FGF9 and the TGFb/BMP/SMAD pathways to regulation of hair follicle growth and cycling.

This study aimed to investigate the effects of recombinant human FGF9 (rhFGF9) on hair follicle growth and cycle transformation in mice after intradermal injection. Additionally, this study also examined the role of the TGF-β/BMP/ Smad signaling pathway in this process, aiming to offer new insights into the potential treatment strategies for hair disorders.

Main Points

• In C57BL/6N mice when hair changes from telogen to anagen (growth) phase there is an increase in black pigmentation in the skin. This was used as a marker for hair growth. At day 6 post epilation rhFGF9 resulted in increased pigmentation on the dorsal skin compared to controls. The majority of mice treated with rhFGF9 had reached the anagen phase by day 13, whilst only a mild increase in growth was observed in the controls. Most animals had complete hair growth at day21 in treated mice, with significantly less growth in the controls.
• Histological analysis on day 6 confirmed that whilst hair follicles from control mice were in the telogen phase, follicles from the treated mice were longer and more numerous and with many entering the early anagen phase. On days 13 and 21 there was a significant increase in skin thickness in the treated mice, hair follicles extended deeper into the skin and were larger, longer and more mature than controls.
• RNA-seq analysis from skin tissue resulted in 126 differentially expressed genes (DEGs) between the two groups with 50 upregulated and 76 downregulated. KEGG pathway and Gene Ontology analysis showed that DEGs were associated with signaling pathways including RAS, Hippo, MAPK, HIF-1 and cAMP.
• The DEGs were significantly associated with the TGFb signaling pathway. rhFGF9 activation of TGFb/BMP/SMAD was confirmed by qPCR.
• IHC also confirmed significant increases in TGFβ2 protein expression 13 days after rhFGF9 treatment and increased BMP4 levels at days 6 and 13. SMAD2 phosphorylation was increased whilst SMAD4 and Noggin expression was reduced, consistent with TGFb and BMP activation. These results were confirmed by western blotting.

Conclusion

Alternative therapies are needed for the treatment of hair loss, which affects approximately 150 million people worldwide. There is significant evidence to suggest that FGFs 1, 2, 7 and 10 influence hair follicle regeneration driving follicles into the anagen (growth) phase and promoting cell growth. In this study the authors demonstrated that recombinant FGF9 activated the TGFb/BMP/SMAD pathway promoting the telogen to anagen transition and promoting hair growth. The authors concluded that compounds targeting FGF9 degradation may be a promising avenue for therapeutics to treat hair loss.

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