Use of strontium to separate calcium-dependent pathways for proliferation and differentiation in human keratinocytes

The role of extracellular calcium (Caex) in modulating keratinocyte differentiation has been well documented, but its role in proliferation has been harder to define due to the confounding effect of terminal differentiation. Because strontium (Sr) does not induce terminal differentiation in murine keratinocytes but does mimic the stimulatory effect of Caex on DNA synthesis in chick fibroblasts, experiments were undertaken to determine if Sr could be used to separate the presumably opposing effects of Caex on the proliferation and differentiation of cultured human keratinocytes. In response to additions of SrCl2, keratinocytes in a serum-free hormone-supplemented basal medium containing 0.03 mM Ca showed a dose-dependent increase in day 7 cell yields. Cell yield in the optimal concentration of SrCl2 (1.8 mM) was approximately twice that obtained in any concentration of CaCl2. Maximally stimulatory additions of CaCl2 varied from 0.05 to 1.8 mM, but 0.03 and 0.05 mM additional CaCl2 always increased cell yield relative to unsupplemented controls. Keratinocytes grown in low levels of CaCl2 or any level of SrCl2 have minimal contact with each other regardless of cell density in contrast to the colonies of tightly apposed and stratified cells grown in 1.8 mM CaCl2. Transmission electron micrographs of vertically sectioned confluent cultures in low or high levels of SrCl2 or in low levels of CaCl2 revealed abundant ribosomes and keratin filaments but no stratification or desmosomes, while cultures in 1.8 mM CaCl2 were stratified with numerous desmosomes. These results suggest that Caex may separately stimulate keratinocyte proliferation and terminal differentiation and that Srex can substitute for Caex in the former but not the latter process.