Role of Ca2+ in stimulation of DNA synthesis by epidermal growth factor and tumor promoters in cultured rat hepatocytes.
This study examines the effects of extracellular Ca2+ concentrations, [Ca2+]o, and of treatments known to modulate intracellular Ca2+ levels on the extent and timing of DNA synthesis in primary cultures of adult rat hepatocytes. In cultures exposed to insulin and EGF, the extent of DNA synthesis between 40 h and 70 h in culture was independent of [Ca2+]o in the range 25-1,800 microM, although the peak of DNA synthesis occurred 5-10 h earlier with 1.2 mM Ca2+ than with 25 microM Ca2+. Complete removal of extracellular Ca2+ using EGTA blocked DNA synthesis if Ca2+ was removed on the second day after EGF addition but not if Ca2+ was absent only on day 1. Treatment of cultures in 1.2 mM Ca(2+)-containing media with Ca(2+)-ionophore A23187 or with thapsigargin, agents expected to raise cytosolic [Ca2+], failed to augment the stimulation of DNA synthesis by EGF. These observations suggest that hepatocytes may have a permissive requirement for [Ca2+]o > 0 at least late in the sequence of events leading from growth factor stimulation to DNA synthesis. However, sustained elevation of cytosolic [Ca2+] does not appear to be important as an early signalling event either in mediating or augmenting EGF action in hepatocytes. The ability of liver tumor promoters alpha-hexachlorocyclohexane or DDT to stimulate DNA synthesis in combination with EGF was independent of [Ca2+]o. By contrast, the skin tumor-promoting phorbol ester, TPA, or liver tumor promoter, phenobarbital, were without effect or inhibitory at low [Ca2+]o but in combination with EGF, stimulated DNA synthesis at [Ca2+]o > 0.4 mM, suggesting that Ca2+ may have some role in mediating or modulating the stimulatory effects of these agents.
Petronijevic
T
T
Department of Medical Biochemistry, School of Medicine, Flinders University of South Australia, Adelaide.
Edwards
A M
AM
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002273:Carcinogens / Q000494:pharmacology*; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis*; D004815:Epidermal Growth Factor / Q000494:pharmacology*; D005110:Extracellular Space / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008297:Male; D009994:Osmolar Concentration; D051381:Rats; D017208:Rats, Wistar; D013268:Stimulation, Chemical
D002273:Carcinogens; D007328:Insulin; D004815:Epidermal Growth Factor; D004247:DNA; D002118:Calcium