Lindane decreases forskolin-stimulated cyclic AMP accumulation but does not modify Gs in rat enterocytes

Abstract

The treatment of isolated rat enterocytes with the halogenated insecticide lindane (the γ-isomer of hexachlorocyclohexane, HCCH) did not modify the general membrane fluidity (as estimated by a fluorescence polarization technique) nor the guanine nucleotide binding regulatory protein Gs (as studied by both ADP-ribosylation of its α subunit by cholera toxin and Gpp[NH]p stimulation of membrane adenylate cyclase activity). However, lindane decreased in a dose-dependent manner the effect of the diterpene forskolin on direct activation of the adenylate cyclase catalytic subunit. After 5 min of cell treatment with 0.5 mM lindane, the maximal stimulatory effect of forskolin (at 100 μM) decreased by about 50%. There was a certain degree of specificity since δ-HCCH was indeed more potent, whereas dieldrin and endrin (non-lindane related halogenated compounds) behaved as lindane, and α- and β-HCCH were poorly efficient on the inhibition of forskolin stimulation of adenylate cyclase activity. A similar effect of lindane was observed on receptor-stimulated cyclic AMP accumulation by using vasoactive intestinal peptide instead of forskolin. The results on a non-receptor mediated effect of lindane on the adenylate cyclase catalytic subunit itself could be related to: (i) alterations of membrane microdomains surrounding this and other integral proteins which would result in modifications of their activities; and/or (ii) a reciprocal relation between the two main routes of signal transduction so that the activation of protein kinase C (or other Ca 2+-dependent protein kinases) by lindane would lead to phosphorylation of the adenylate cyclase catalytic subunit. The simultaneous preincubation of enterocytes with lindane and a tumour-promoting phorbol ester resulted in additive inhibitory effects on forskolin stimulation of cyclic AMP accumulation which is apparently against the suggested involvement of protein kinase C in the mechanism of action of the insecticide. However, it cannot be discarded since this enzyme activity represents a family of isoforms so lindane and the phorbol ester could act on different isoenzymes.