Transcription of the Human Loricrin Gene In Vitro Is Induced by Calcium and Cell Density and Suppressed by Retinoic Acid

We have previously shown that loricrin is a major component of the cornified cell envelope (CE) expressed late in epidermal differentiation in the granular layers of normal skin. Normal human keratinocytes were cultured under various conditions and loricrin mRNA levels were assessed at various time points. Only Ca++ concentrations above 0.1mM Ca++ were permissive for the expression of loricrin mRNA. Maximal mRNA levels were found at 0.35mM Ca++ and a critical cell density appeared to be required for optimal accumulation of lonicrin transcripts. Retinoic acid (RA) at 10−7 to 10−9 M completely blocked Ca++ tinduced loricrin mRNA synthesis when applied simultaneously. Furthermore, addition of RA to cultures already exposed to higher Ca++ levels resulted in the complete loss of loricrin mRNA within 48–72h. So far, no other components of the CE have been shown to be suppressed by RA. However, similar patterns of expression were reported for filaggrin, a matrix protein also expressed late in epidermal differentiation. Therefore, we compared the mRNA levels of loricrin and filaggrin and found them to change in parallel in response to the various culture conditions. These results suggest that Ca++, cell density, and RA are crucial regulators of loricrin expression in vitro and that the transcriptional control of loricrin and filaggrin expression in the epidermis are closely coordinated.