Degradation by Stratum Corneum Proteases Prevents Endogenous RNase Inhibitor from Blocking Antimicrobial Activities of RNase 5 and RNase 7

The antimicrobial defense of the skin is partially mediated by RNase 7, an abundant ribonuclease of the stratum corneum (SC). Here, we investigated the expression and regulation of members of the RNase A family and of the endogenous RNase inhibitor (RI) protein in epidermal keratinocytes (KCs). Reverse transcription-PCR screening revealed that KCs expressed not only RNase 7 but also RNase 5, which was shown earlier to kill the yeast Candida albicans, as well as RNase 1, RNase 4, and RI. The mRNA and protein levels of RNase 5, RNase 7, and RI increased during KC differentiation. When RNase 5 and RNase 7 were incubated with RI in vitro, not only their ribonucleolytic activities but also their antimicrobial activities were strongly suppressed. Immunochemical analyses revealed that SC contains RNase 5, whereas RI was not detectable. Unlike recombinant RNase 5, recombinant RI was degraded when exposed to SC extract. The addition of aprotinin prevented the degradation of RI, indicating that serine proteases of the SC cleave RI. Taken together, this study adds RNase 5 to the list of antimicrobial factors present in the SC and suggests that proteases contribute indirectly to the defense function of the SC by releasing the RI-mediated inhibition of RNase 5 and RNase 7.