Glucocorticoids confer normal serum/growth factor-dependent growth regulation to Fu5 rat hepatoma cells in vitro. Sequential expression of cell cycle-regulated genes without changes in intracellular calcium or pH.

Abstract

Glucocorticoid hormones induced a stringent dependence on serum for the in vitro proliferation of Fu5 rat hepatoma cells by suppressing the growth rate and final quiescent cell density. Treatment of dexamethasone-suppressed quiescent Fu5 with serum plus insulin caused a rapid reinitiation of cellular proliferation and DNA synthesis that peaked at 16 h. RNA dot blot analysis of this time course showed that the transcript levels for the proto-oncogenes c-fos, c-myc, and c-rasKi peaked at 0.5, 2, and 4 h, respectively, while expression of c-rasHa and ornithine decarboxylase transcripts rose steadily during 16 h. Microspectrofluorimetric measurements of cytosolic calcium (Ca2+i) with fura-2 showed that insulin and serum, alone or in combination, elicited no changes in Ca2+i over a 50-min time course, although ATP, which is not a mitogen, induced large increases in Ca2+i. Cytosolic pH, pHi, was also measured using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Insulin and serum, alone or in combination, did not cause pHi to increase in either 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) (pHi 7.17)- or HCO3/CO2 (pHi 7.19)- buffered media. Acid-loading of cells with NH4Cl indicated that both quiescent and proliferating Fu5 cells have equally active, amiloride-sensitive Na/H exchangers. Therefore, induction of DNA synthesis and proto-oncogene expression occurs in Fu5 epithelial tumor cells in the absence of any short term increases of pHi or Ca2+i.