Effects of calcium on HPV16 gene transcription in cultured laryngeal epithelial cells

Human papillomavirus (HPV) gene transcription is closely linked to the differentiation status of infected epithelial cells. A variety of physiological agents, including calcium, regulates the differentiation of cultured epithelial cells. The expression of cytokeratin No.13 (CK13) can be used as a marker for differentiation in cultured laryngeal epithelial cells (HLEC cells). The purpose of this study is to examine the effects of calcium on CK13 expression and HPV16 gene transcription in HLEC cells. We analyzed two types of HPV16-containing HLEC cells: HPV 16-immortalized HLEC cells (HLEC16 cells) and HPV16-positive (infected) cultured laryngeal papilloma cells (HLP16 cells). In the HLEC16 cells, the viral genes were integrated into the host cell chromosomes, while the HLP16 cells contained extra-chromosomal viral genes. The effects of increasing calcium concentrations on CK13 expression were then evaluated using immunocytochemistry. Both the HLP16 and the HLEC16 cells responded to an increased calcium concentration by inducing CK13 expression. In HLP16 and HLEC16 cells, the CK13 expression was undetectable at low calcium concentrations (0.1 mM) but became clearly detectable at high calcium concentrations (1.0 mM). The level of viral RNA was elevated in HLP16 cells with added calcium (1.0 mM) but was similar in HLEC16 cells grown in either low (0.1 mM) or high (1.0 mM) calcium concentrations. These results suggest that a calcium-induced differentiation results in the up-regulation of HPV16 gene transcription in HLP16 cells. The integration of viral gene into the host cell chromosomes may be an important determinant for the differentiation-independent transcription of HPV16 genes.