PROTEIN PHOSPHATASE-ACTIVITY IN HUMAN KERATINOCYTES CULTURED FROM NORMAL EPIDERMIS AND EPIDERMIS FROM PATIENTS WITH HARLEQUIN ICHTHYOSIS

We investigated serine/threonine protein phosphatase (PP) activity and the expression of PP2A during growth and differentiation of epidermal keratinocytes in culture. Keratinocyte PP activity was strongly inhib ited by calyculin A and okadaic acid, indicating that the activity was mainly due to PP2A and PP1. The phosphatase activity decreased to abo ut 20% of the initial (day 1) level by the time of confluence and to a bout 10% at day 7 postconfluence. In contrast to activity, the level o f expression of the PP2A catalytic subunit: protein and the mRNA for t he two isoforms increased slightly over the period of growth, Keratino cyte differentiation was shown by a significant increase in profilaggr in expression after confluence, Keratinocytes were also cultured from individuals affected with harlequin ichthyosis. This severe hyperkerat otic skin disorder has abnormal lipid structures and is blocked in the PP2A-dependent conversion of phosphorylated profilaggrin to the non-p hosphorylated filaggrin. The PP activity Tn harlequin cultures was low er than in normal cultures (about 20% of the subconfluent normal contr ol value) and decreased even further in confluent cultures, In contras t, the level of expression of the PP2A catalytic subunit protein and m RNA for the two isoforms was similar to that of normal keratinocytes a nd increased with confluence. These results suggest that PP activity i n keratinocytes is regulated in a post-translational manner: they also support the possibility of impaired or reduced function of PPs in har lequin ichthyosis.