Calcium modulates cornified envelope formation, involucrin content, and transglutaminase activity in cultured human...

The entrance to the cervical canal, the ectocervix, is covered by a stratified squamous epithelium. In the present study we demonstrate that the extracellular calcium levels regulate the differentiation of cultured human ectocervical epithelial cells (ECE cells). ECE cells growing in serum-free medium containing 0.09 mM calcium remain as individual cells that do not spread on the culture dish or form cornified envelopes. Increasing the extracellular calcium concentration results in a dose-dependent increase in ECE cell differentiation that is manifest by an increase in cornified envelope (superficial cell) formation. These morphological changes are accompanied by a twofold increase in the levels of involucrin, a precursor of the cornified envelope. The extent of increase in involucrin mRNA is similar to that of involucrin protein, suggesting that the regulation is at the level of involucrin mRNA synthesis and/or stability. Transglutaminase, a calcium-activated enzyme responsible for assembly of the covalent cross-links that form the cornified envelope, increases to a similar extent to the increase in cornified envelope formation, suggesting that in ECE cells, activation of transglutaminase may be the rate-limiting step in envelope assembly. These results and the finding of a calcium gradient in other stratifying epithelia in vivo, indicate that the calcium is likely to be an important regulator of ectocervical epithelial cell differentiation in vivo.