ABSTRACT
Caspase-14 is the only member of the caspase family that shows a restricted tissue expression. It is mainly confined to epidermal keratinocytes and in contrast to other caspases, is not activated during apoptosis induced by ultraviolet irradiation or cytotoxic substances. As it is cleaved under conditions leading to terminal differentiation of keratinocytes we suggested that caspase-14 plays a part in the physiologic cell death of keratinocytes leading to skin barrier formation. Here we show that retinoic acid, at concentrations inhibiting terminal differentiation of keratinocytes, strongly suppressed caspase-14 mRNA and protein expression by keratinocytes in monolayer culture and in a three-dimensional in vitro model of differentiating human epidermis (skin equivalent). By contrast, the expression of the caspases 3 and 8, which are both activated during conventional apoptosis, was increased and unchanged, respectively, after retinoic acid treatment. In addition to inhibition of differentiation in skin equivalents, retinoic acid treatment led to keratinocyte apoptosis and activation of caspase-3, both of which were undetectable in differentiated control skin equivalents. As this occurred in the absence of detectable caspase-14, our data demonstrate that caspase-14 is dispensable for keratinocyte apoptosis. The fact that in contrast to caspase-3 and caspase-8, caspase-14, similarly to other keratinocyte differentiation-associated proteins, is downregulated by retinoids, strongly suggests that this caspase, but not caspase-3 and -8, plays a part in terminal keratinocyte differentiation and skin barrier formation.