ALUMINUM FLUORIDE STIMULATES INOSITOL PHOSPHATE-METABOLISM AND INHIBITS EXPRESSION OF DIFFERENTIATION MARKERS IN MOUSE KERATINOCYTES

Mouse keratinocytes are induced to differentiate in vitro by elevating the level of extracellular calcium from 0.05 mM, where keratinocytes express a basal cell phenotype, to > 0.10 mM, where they express the differentiated phenotype. This process has been associated with a rapid, sustained increase in inositol phosphate (InsP) turnover, which precedes the expression of differentiation-specific proteins. In 0.05 mM Ca2+ medium, aluminum and fluoride salts (AlF4-), which combine to activate nonspecifically heterotrimeric guanine nucleotide-binding (G) proteins, cause a concentration-dependent increase in InsP metabolism in keratinocytes and generate elevated intracellular diacylglycerol levels. This is associated with an inhibition of cell growth. Treatment with both AlF4- and Ca2+ > 0.10 mM resulted in an additive increase in InsP turnover, implying the presence of at least two responsive InsP pools. AlF4- inhibited the expression of differentiation markers induced by Ca2+ > 0.10 mM and altered the morphology of keratinocytes from squamous to dendritic, which was reversible upon withdrawal of AlF4-. Neoplastic keratinocytes, in which basal levels of InsP metabolism are higher than in normal cells, do not differentiate in response to Ca2+. Neoplastic keratinocytes responded to AlF4- treatment with an even greater rise in InsP metabolism. AlF4- also inhibited cell growth and reversibly altered morphology in neoplastic keratinocytes. These data suggest that InsP metabolism in keratinocytes is at least partially regulated by a G protein mechanism. Furthermore, an increase in InsP metabolism is not sufficient to stimulate differentiation and may be inhibitory to differentiation if exceeding limited increases. However, these observations cannot exclude the possibility that other AlF4--stimulated pathways involving G or non-G proteins can also influence keratinocyte biology.