Uncoupling of the calcium-sensing mechanism and differentiation in squamous carcinoma cell lines

Increases in the intracellular calcium (Cai) levels, induced either by extracellular calcium or by calcium ionophores, stimulate the terminal differentiation of normal human keratinocytes in culture (NHK). Despite extensive differences in phenotypic expression, squamous carcinoma cell lines (SCC lines) display only partial terminal differentiation even in the presence of normal extracellular calcium. Therefore, in this study, we evaluated whether the inability of SCC lines to differentiate normally is due to a defect in achieving adequate levels of Cai. Membrane-bound transglutaminase activity and involucrin levels of the various SCC lines were lower than those of NHK and correlated with their low extent of cornified envelope formation. Ionomycin, a calcium ionophore, acutely increased cornified envelope formation of NHK 60- to 70-fold, but only initiated a 1- to 5-fold increase in SCC lines. Yet resting Cai levels in and the Cai response to various agents of SCC lines were similar or higher than those of NHK. Extracellular calcium evoked a rapid, transient and a slower, sustained increase of Cai. Extracellular ATP increased Cai by a rapid release from intracellular sources. Ionomycin, on the other hand, increased Cai from both intracellular compartments and extracellular sources. Thus, these studies indicate that the abnormalities in differentiation among SCC lines do not appear to involve their calcium-sensing mechanism. An uncoupling of the Cai changes to the synthesis of the precursor molecules required for differentiation may be responsible for the defect in differentiation displayed by these SCC lines.