Interleukin-4 induces apoptosis in cultured human follicular keratinocytes, but not in dermal papilla cells.

The importance of apoptosis in hair follicle cycling is still not clearly understood, however, its regulation in follicular keratinocytes (FK) during bulb regression (catagen) may be essential for hair regrowth. So far, the control of FK apoptosis remains unknown. In this study, the anti-inflammatory cytokine IL-4 was found to induce apoptosis dose and time dependently in cultured human FK, in contrast to other agents known to inhibit hair growth such as IL-1alpha, IL-1beta, TNFalpha and TGFbeta, as shown by DNA fragmentation. On the other hand, cytokines reported to be involved in hair follicle cycling including IL-4 were not able to induce apoptosis in dermal papilla cells (DPC), in contrast to staurosporine. This PKC inhibitor revealed dose-dependent apoptotic signals not only for DPC but also for FK in vitro. In further experiments the expression of apoptosis regulating proteins, possibly involved in catagen formation, was analyzed in FK and DPC. However, no striking difference in RNA expression was seen in either cell population under culture conditions and after incubation with IL-4. We conclude, therefore, that IL-4 mediated apoptosis may participate in regulating catagen formation in the hair follicle, acting selectively on cultured FK and being independent of bcl-2 and bax expression.