Control of murine hair follicle regression (catagen) by TGF-β1 in vivo

The regression phase of the hair cycle (catagen) is an apoptosis-driven process accompanied by terminal differentiation, proteolysis, and matrix remodeling. As an inhibitor of keratinocyte proliferation and inductor of keratinocyte apoptosis, transforming growth factor β1 (TGF-β1) has been proposed to play an important role in catagen regulation. This is suggested, for example, by maximal expression of TGF-β1 and its receptors during late anagen and the onset of catagen of the hair cycle. We examined the potential involvement of TGF-β1 in catagen control. We compared the first spontaneous entry of hair follicles into catagen between TGF-β1 null mice and age- matched wild-type littermates, and assessed the effects of TGF-β1 injection on murine anagen hair follicles in vivo. At day 18 p.p., hair follicles in TGF-β1 -/- mice were still in early catagen, whereas hair follicles of +/+ littermates had already entered the subsequent resting phase (telogen). TGF- β1-/- mice displayed more Ki-67-positive cells and fewer apoptotic cells than comparable catagen follicles from +/+ mice. In contrast, injection of TGF-β1 into the back skin of mice induced premature catagen development. In addition, the number of proliferating follicle keratinocytes was reduced and the number of TUNEL + cells was increased in the TGF-β1-treated mice compared to controls. Double visualization of TGF-β type II receptor (TGFRII) and TUNEL reactivity revealed colocalization of apoptotic nuclei and TGFRII in catagen follicles. These data strongly support that TGF-β1 ranks among the elusive endogenous regulators of catagen induction in vivo, possibly via the inhibition of keratinocyte proliferation and induction of apoptosis. Thus, TGF-βRII agonists and antagonists may provide useful therapeutic tools for human hair growth disorders based on premature or retarded catagen development (effluvium, alopecia, hirsutism).