Changes in inositol phosphate metabolism are associated with terminal differentiation and neoplasia in mouse keratinocytes

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Abstract
Cultured murine keratinocytes respond to specific Ca2+ levels in medium (Ca0) by expressing markers of terminal differentiation. A Ca0, of 0.05 mM selects for a basal cell phenotype, whereas spinous cell characteristics occur in 0.12 mM Ca2+ and cornified envelopes develop in 1.0 mM Ca2+. An increase in inositol phosphate (InsP) metabolism is associated with higher Ca2+ in the medium. The magnitude of Ca2+-stimulated InsP turnover is Ca0-dependent, whereby Ca0 of 0.05, 0.12 or 1.4 mM resulted in a graded, sustained (>24 h) increase in InsPs. Diacylglycerol (DAG) levels similarly increased in a graded manner. The major inositol trisphosphate (InsP3) to accumulate was Ins-1,3,4P3 while Ins-1,4,5-P3 increased transiently. Neoplastic keratinocyte cell Lines, 308 and SP-1, which produce benign tumors and have a mutated c-rasHa gene, do not express markers of differentiation in response to Ca2+. Basal InsP and DAG are 2- and 5-fold higher respectively in the neoplastic cells relative to normal keratinocytes. However, the metabolic profiles of InsP were similar in normal and neoplastic cells. In neoplastic cells, InsP metabolism was stimulated even further following a Ca2+ increase, and this was graded to the Ca0,. The unusual, sustained Ca2+-graded InsP response in normal cells is consistent with the turnover of InsP contributing to the signals controlling expression of markers of differentiation. Very high InsP turnover and DAG levels, as in neoplastic cells, may be inhibitory to marker expression.

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