Thymosin ß4 is a 4.9 kDa molecule that functions as a major actin-sequestering protein in cells. It is up-regulated during endothelial cell differentiation; when added exogenously, it promotes endothelial cell differentiation and migration. In vivo, it promotes wound repair and is a potent anti-inflammatory agent. A related family member, thymosin ß15, is also important in the metastasis of certain tumor types. It was reported that thymosin ß4 exerts its effects on cell locomotion through specific interactions with actin that regulate cytoskeletal organization. We show that exogenously delivered thymosin ß4 promotes hair growth in normal rats and mice. When examining the distribution of endogenous thymosin ß4 through sequential phases of depilation-induced hair growth, we found that in the resting (telogen) follicle it is expressed in the small number of cells originating in the bulge region of the outer root sheath. As the follicles enter active growth phase (anagen), the subset of thymosin ß4-expressing cells in the outer root sheath is expanded toward the base of the follicle. At the peak of anagen, a significant number of thymosin ß4-expressing cells are found in the bulb area, in the outer root sheath, and among the hair matrix cells. Isolated clonogenic hair follicle keratinocytes, closely related (if not identical) to the hair follicle stem cells, produce thymosin ß4 when cultured in vitro for 7–10 days. The presence of exogenous thymosin ß4 caused a dose-dependent decrease in the expression of the stem cell marker K15 by clonogenic keratinocytes, suggesting that thymosin ß4 may promote early stem cell differentiation (i.e., transition to the transi-amplifying cell phenotype). Most important, treatment of the bulge-derived clonogenic keratinocytes with exogenous thymosin ß4 increased migration and production of MMP-2.
A critical step in the hair growth cycle is the movement of some of the bulge-residing stem cells downward, where their differentiated progeny contribute to complete regrowth or regeneration of the lower, cycling portion of the follicle. Our data indicate that thymosin ß4 facilitates this movement of the stem cells and their immediate progeny and, thus, exerts its promoting effect on hair growth (Fig. 3 ). The effect of thymosin ß4 on MMP-2 expression appears to play an important role in this system. MMP-2 has been shown to contribute to cellular migration by degrading extracellular matrix barriers for cell movement and through direct effects on cell locomotion in vitro. MMP-2 is involved in hair cycle-associated remodeling of the basement membrane, the specialized extracellular matrix structure surrounding the epithelial core of the follicle. Basement membrane remodeling is necessary for signaling between epithelial and stromal elements of the growing follicle and for elongation and invasion of the lower follicle into subcutaneous tissue during the anagen phase.
Hair growth acceleration by thymosin ß4 may also be attributed to proangiogenic and other biological activities of this molecule. It was recently reported that VEGF promotes hair follicle development presumably due to its angiogenic activity. Thymosin ß4 is angiogenic (like VEGF), and the activity of thymosin ß4 may be due to its angiogenic activity. Another angiogenic molecule, hepatocyte growth factor, has been found to promote hair growth. Hepatocyte growth factor up-regulates thymosin ß4 expression and may be acting by increasing thymosin ß4 and/or synergizing with it. Steroids have been used to treat certain types of hair loss. Thymosin ß4 is the anti-inflammatory molecule identified as increased in steroid-treated monocytes. Thus, treatment with steroids may involve the activity of thymosin ß4 on hair growth.
Taken together, our results suggest that thymosin ß4 exerts a profound hair-promoting effect through a combined action on several critical events in hair follicle growth such as stem cell migration, ECM-degrading enzyme production, and differentiation.
