The interrelationship between P-glycoprotein and glutathionyl S-conjugate transporter(s)

Glutathione-dependent bioactivation of haloalkenes

Several halogenated alkenes are nephrotoxic in rodents. A mechanism for the organ-specific toxicity of these compounds to the kidney has been elucidated. The mechanism involves hepatic glutathione conjugation to dihaloalkenyl or 1,1-difluoroalkyl glutathione S-conjugates, which are cleaved by gamma-glutamyltransferase and dipeptidases to cysteine S-conjugates. Haloalkene-derived cysteine S-conjugates may have four fates in the organism: (a) They may be substrates for renal cysteine conjugate beta-lyases, which cleave them to form reactive intermediates identified as thioketenes (chloroalkene-derived S-conjugates), thionoacyl halides (fluoroalkene-derived S-conjugates not containing bromide), thiiranes, and thiolactones (fluoroalkene-derived S-conjugates containing bromine); (b) cysteine S-conjugates may be N-acetylated to excretable mercapturic acids; (c) they may undergo transamination or oxidation to the corresponding 3-mercaptopyruvic acid S-conjugate; (d) finally, oxidation of the sulfur atom in halovinyl cysteine S-conjugates and corresponding mercapturic acids forms Michael acceptors and may also represent a bioactivation reaction. The formation of reactive intermediates by cysteine conjugate beta-lyase may play a role in the target-organ toxicity and in the possible renal tumorigenicity of several chlorinated olefins widely used in many chemical processes.

Hepatocanalicular organic-anion transport is regulated by protein kinase C

In order to investigate the regulation of canalicular organic-anion transport, we used a hepatocyte transport assay in which canalicular secretion of a model organic anion, dinitrophenyl-glutathione (GS-DNP), was measured in the presence of stimulators and inhibitors of the Ca2+/protein kinase C (PKC) second-messenger system and of the cyclic AMP (cAMP) second-messenger system. Vasopressin (24 nM) and the phorbol ester phorbol 12-myristate 13-acetate (1 microgram/ml), both stimulators of PKC, stimulated GS-DNP efflux by 65 +/- 36% and 55 +/- 28% respectively, whereas staurosporine (10 microM), an inhibitor of PKC, inhibited efflux by 53 +/- 13%. Glucagon and forskolin, both stimulators of the cAMP second-messenger system, as well as the cAMP analogue dibutyryl cAMP and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, did not significantly influence the GS-DNP efflux. It can be concluded that canalicular organic-anion transport in hepatocytes is either directly or indirectly regulated by PKC.