Regulation of the level and glycosylation state of plasminogen activator inhibitor type 2 during human keratinocyte differentiation

Abstract

Plasminogen activator inhibitor type 2 (PAI-2) is an unusual member of the serine proteinase inhibitor (serpin) family which has been implicated in the protection of cells from programmed cell death. Human epidermal keratinocytes synthesize large amounts of PAI-2 in two active forms: glycosylated and non-glycosylated. Calcium (Ca 2+), a well-known inducer of numerous aspects of keratinocyte terminal differentiation, increases the steady-state levels of PAI-2 mRNA and protein. As the cultures become more differentiated due to longer incubation with Ca 2+, the glycosylated form is preferentially elevated. Surprisingly, glycosylated as well as non-glycosylated PAI-2 remains predominantly cell-associated, in a trypsin-inaccessible form and thus most likely inside the cell. Tumor necrosis factor-α (TNF-α), an inflammatory mediator that induces some markers of keratinocyte differentiation, also increases PAI-2 mRNA and protein levels. Experiments using cultures in which protein kinase C has been downregulated suggest that Ca 2+, but not TNF-α, acts at least partially through one or more isozymes of this kinase for induction of PAI-2. This is consistent with the finding that the effect of simultaneous addition of Ca 2+ plus TNF-α is at least additive, compared with addition of either stimulant alone. These data demonstrate that the keratinocyte maintains multiple regulatory pathways for control of PAI-2 expression, at least one of which is related to terminal differentiation.