Modulation of cellular cholesterol and its effect on cornified envelope formation in cultured human epidermal keratinocytes.

When cultured human epidermal keratinocytes (NHK) reach confluence they start to differentiate and an increase in the total cellular cholesterol content is observed. This increase parallels the appearance of a characteristic feature of terminal keratinocyte differentiation, the spontaneous formation of cornified envelopes (CE). Synthesis of CE is catalyzed by the plasma membrane-associated transglutaminase (TGm). Supplementation of the medium with inhibitors of cholesterologenesis suppressed increase in cholesterol levels and CE formation but did not interfere with TGm expression or TGm activity. Modulation of the plasma membrane cholesterol-phospholipid ratio of confluent NHK cultures using either pure phospholipid liposomes or liposomes enriched in cholesterol strongly affected spontaneous CE formation. Pure phospholipid liposomes completely inhibited CE formation, whereas cholesterol-enriched liposomes ensured envelope formation, even in the presence of inhibitors of cholesterol synthesis. From these results we conclude that in differentiating NHK an increase in the cellular cholesterol level is part of the differentiation program and is essential for the spontaneous CE formation.