Mechanism of desensitization of the Ca2(+)-mobilizing system to bombesin by Ha-ras. Independence from down-modulation of agonist-stimulated inositol phosphate production.

Expression of a transforming Ha-ras gene in NIH 3T3 cells transfected with an inducible Ha-ras construct leads to a rapid desensitization of the intracellular Ca2(+)-mobilizing system to bombesin and serum growth factors. Half-maximal depression of the Ca2+ response is observed 2 h after induction of p21ras. A maximum is obtained after 6 h. Bombesin-induced elevation of inositol 1,4,5-trisphosphate formation is also depressed in cells expressing Ha-ras. This, however, is a relatively late phenomenon and not yet detectable when maximal depression of the Ca2+ signal is observed. We conclude that the rapid densensitization of the Ca2(+)-releasing system to bombesin by Ha-ras is not caused by down-modulation or uncoupling of phospholipase C-coupled bombesin receptors. The inositol 1,4,5-trisphosphate-mediated release of intracellular Ca2+ is reduced in permeabilized cells expressing the Ha-ras oncogene. A depletion of intracellular Ca2+ stores by Ha-ras is unlikely since (i) the Ha-ras-induced growth factor-independent stimulation of inositol phosphate formation occurs several hours after reduction of the Ca2+ response and (ii) the Ca2+ load of intracellular nonmitochondrial Ca2+ stores was found to be unaffected by Ha-ras. We conclude that the desensitization of the Ca2(+)-mobilizing system is caused either by partial inhibition of inositol 1,4,5-trisphosphate-regulated Ca2+ channels or by interference of Ha-ras with Ca2+ translocation between intracellular Ca2+ compartments.