Calcium regulates the differentiation of human papillomavirus type 16 (HPV16) immortalized ectocervical epithelial cells, but not the expression of the papillomavirus E6 and E7 oncogenes.

Calcium is an important regulator of normal human cervical cell differentiation; however, it is not known whether the calcium responsiveness of the cells would be altered by immortalization with human papillomavirus. In the present study we examine the effects of extracellular calcium on morphology, expression of biochemical markers of cervical cell differentiation, and HPV16 oncogene transcription in an HPV16-immortalized human cervical cell line, ECE16-1. ECE16-1 cells respond to increased calcium with increased levels of mRNA encoding cytokeratin K13, transglutaminase, and involucrin. Dose-response studies indicate that these markers are coordinately regulated and that maximal levels are present at calcium concentrations of > or = 0.4 mM. RNA levels begin to increase within 24 h after cells are shifted to medium containing high calcium and are maximal by 3 days. Protein levels change directly with the change in mRNA, suggesting that mRNA level determines protein concentration; however, transcription runoff experiments indicate that the increased mRNA does not result from new synthesis. Parallel studies indicate that expression of the HPV16 oncogenes, E6 and E7, are not regulated by extracellular calcium. These results are consistent with in vivo experiments showing that in advanced cervical intraepithelial neoplasia, where HPV16 DNA is integrated in host cell DNA, E6/E7 transcript production is not regulated in a differentiation-dependent manner.