E. Metzen et al., Evidence against a major role for Ca2+ in hypoxia-induced gene expression in human hepatoma cells (Hep3B), J PHYSL LON, 517(3), 1999, pp. 651-657
1. The human hepatoma cell line Hep3B is a widely used model for studies of
hypoxia-related gene expression. Cytosolic free calcium concentration ([Ca
2+](i)) has been implicated in cellular oxygen-sensing processes. We invest
igated whether calcium ions have a significant impact on the production of
erythropoietin (EPO) and vascular endothelial growth factor (VEGF).
2. We found that the calcium ionophore ionomycin induced a rapid and sustai
ned increase of [C2+](i) while thapsigargin, an inhibitor of endoplasmic re
ticulum calcium ATPase, only caused a 20% elevation of [Ca2+](i) within 10
min after application. However, the calcium content of intracellular stores
was considerably reduced by thapsigargin after an incubation period of 24
h.
3. Variations in [C2+](o) did not result in altered ETO or VEGF secretion r
ates. Ionomycin decreased EPO production while the lowering of VEGF product
ion was not statistically significant. In the presence of extracellular Ca2
+ the membrane permeant calcium chelator BAPTA-AM stimulated the production
of EPO (P < 0.05) but not of VEGF while EGTA-AM, a closely related agent,
affected neither EPO nor VEGF formation under these conditions. Incubation
with thapsigargin resulted in decreased EPO synthesis (P < 0.05) but stimul
ated VEGF secretion (P < 0.05).
4. In the absence of extracellular calcium, EGTA-AM led to an accumulation
of hypoxia inducible factor-1 alpha (HIF-1 alpha). This treatment significa
ntly stimulated VEGF synthesis but also decreased EPO secretion (P < 0.05).
5. Our data suggest that the calcium transient and the cytosolic Ca2+ conce
ntration do not play a key role in hypoxia-induced EPO and VEGF production
in Hep3B cells.