ABSTRACT
- mTOR and AMPK modulation by rapamycin, metformin, and α-KG induces anagen hair growth
- Autophagy induction is necessary and sufficient for anagen entry and hair growth
- Autophagy is increased during anagen phase of the natural hair follicle cycle
- Aged mice fed the autophagy-inducing metabolite α-KB are protected from hair loss
Summary
The biological and psychological importance of hair is well recognized. Hair loss affects millions worldwide and can occur because of aging, hormonal dysfunction, or autoimmunity or as a side effect of cancer treatment. Mammalian hair growth consists of cyclic repetitions of telogen (quiescence), anagen (regeneration), and catagen (degeneration) phases of the hair follicle. This hair follicle cycle is regulated by both intrinsic and extrinsic signals that control quiescence and activation of hair follicle stem cells (HFSCs). Inadequate hair follicle stem cell activation and proliferation underlie alopecia in numerous biological and pathological conditions, including aging. Molecules that can promote hair follicle stem cell activation and anagen initiation have been intensely searched for, as they may both help reveal how hair regeneration is regulated and provide therapeutic and cosmetic interventions. Here, we postulate that telogen hair follicles may be induced to enter anagen by pharmacologically triggering autophagy.
As a fundamental process for degrading and recycling cellular components, autophagy is critical for adaptation to nutrient starvation and other adverse environmental conditions, and it is regulated by such signals. Autophagy is also important for quality control of proteostasis through the elimination of misfolded or damaged proteins and damaged organelles. The loss of autophagy may be causally related to neurodegeneration and other diseases. Autophagy declines with age, likely contributing to the higher prevalence of autophagy-related diseases (e.g., cancer, neurodegenerative diseases) in the elderly. Autophagic clearing of active, healthy mitochondria in hematopoietic stem cells is required to maintain quiescence and stemness
, and autophagy fulfills the nutrient demand of quiescent muscle stem cell activation. In the skin, autophagy is required for self-renewal and differentiation of epidermal and dermal stem cells, but its role in hair follicle stem cells has remained controversial. On one hand, autophagy may be required for hair growth as skin grafts from the autophagy-related gene 7 (Atg7)-deficient mice exhibit abnormal hair growth. On the other hand, Atg7 deficiency in epithelial cells of the skin and hair was reported to be compatible with growth of hair, although sebaceous glands were affected, and male mutant mice developed an oily coat when they aged. It was also reported that psychological stress induced autophagy and delay of hair cycle.
Previously, alterations in intrinsic signaling, gene expression, and circadian function were implicated to prevent anagen entry in aged hair follicle stem cells and result in alopecia. The unforeseen finding that supplementation of a metabolite α-ketobutyrate (α-KB) in old mice can increase longevity and prevent alopecia suggests that rejuvenating aging or aging associated deficiencies may restore hair follicle stem cell function and hair growth in skin. We report herein that autophagy is increased during anagen phase of the natural hair follicle cycle and demonstrate that specific small molecules that induce autophagy can be used to promote anagen entry and hair growth from quiescent telogen phase.
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