The antitumour agent 5-(3,3-dimethyl-1-triazeno) imidazole-4-carboxamide (DTIC) inhibits rat liver cAMP phosphodiesterase and amplifies hormone effects in hepatocytes and hepatoma cells

Abstract

Summary.-The antitumour agent 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (DTIC) was found to inhibit competitively the low-Km cyclic AMP phosphodiesterase activity in an ammonium-sulphate-precipitable fraction of the 2,000g supernatant of rat liver. With substrate concentration at 0-25 [M, 150 was 790 /tM for DTIC and 350 tM for theophylline. DTIC at 2 mm more than doubled the cAMP response to glucagon in hepatocytes and to adrenaline in MH1Cj hepatoma cells, indicating that it also exerts its inhibitory effect on the phosphodiesterase in intact cells. The possible contribution of the phosphodiesterase inhibition to the growthinhibitory and cytotoxic effects of DTIC is discussed.

Effect of the Antitumour Agent 5‐(3,3‐Dimethyl‐1‐triazeno imidazole‐4‐carboxamide (DTIC) on Cyclic AMP Levels in Intact MH1C1 Hepatoma Cells

“…The ability of DTIC to increase cAMP levels in the intact MHICl cells was probably due to inhibition of the CAMP-phosphodiesterase. As we have previously seen with liver preparations (Larsson et al 1979) DTIC inhibited the hydrolysis of a low concentration of cAMP (0.25 pM) by a crude homogenate from MHlCl cells (fig. 6A).…”

“…This could be of significance, because cAMP is strongly implicated in regulation of cell proliferation and differentiation (Ryan & Heidrick 1974;Pastan et al 1975;Friedman 1976;Tisdale 1979). There is a reasonably good correlation between the doseresponse curves for DTIC on phosphodiesterase activity (Larsson et al 1979) growth were seen at somewhat lower concentrations than those giving measurable cAMP elevation in the present study, the possibility should be considered that even small, hardly detectable, increases in total cellular cAMP may be sufficient to activate the protein kinase. However, a large number of substances are able to inhibit the cAMP phosphodiesterase and possibly elevate cellular cAMP (Chasin & Harris 1976) and it is doubtful whether all these agents exert their pharmacological effects through this mechanism.…”

“…The data presented provide further evidence that the inhibition by DTIC of the low-K, form of the 0-0 CONTROL 0 10 20 3 0 4 0 TIME (MINUTES) cAMP phosphodiesterase (Larsson et a/. 1979) is actually exerted in intact cells.…”