Dexamethasone inhibits the hexose monophosphate shunt in activated rat peritoneal macrophages by reducing hexokinase-dependent sugar uptake.
Dexamethasone decreases 2-D-deoxyglucose (2-dGlc) uptake and accumulation into rat peritoneal macrophages in vitro in a concentration- and time-dependent manner (Ki for 1 microM-dexamethasone after a 2 h exposure = 0.71 +/- 0.21 microM; Ki for 0.1 microM-dexamethasone after exposure for 4 h = 0.10 +/- 0.06 microM). The inhibition of 2-dGlc uptake is consistent with a decrease in the coupling between endofacial hexokinase activity and the sugar transporter. The evidence for this is: (1) the Km for zero-trans 2-dGlc uptake in quiescent macrophages was increased by dexamethasone, but there was no significant effect on the Vmax.; (2) dexamethasone increased the rate of exit of sugar from cells preloaded with 2-dGlc; (3). the free sugar accumulation within the cytosol of the cells above the external solution concentration was significantly decreased by dexamethasone. These effects of dexamethasone on 2-dGlc transport were antagonized by simultaneous exposure to the steroid RU 38486 (Ki = 0.04 +/- 0.01 microM; 4 h incubation). Although dexamethasone inhibited zero-trans uptake, the maximum rate of infinite-trans exchange uptake of 2-dGlc into cells preloaded with 3-O-methyl-D-glucose (40 mM) was unaltered by dexamethasone or RU 38486, indicating that the dexamethasone-dependent decrease in zero-trans uptake was not due to a change in the number of transporters in the plasma membrane. Dexamethasone also inhibited the phorbol myristate acetate-induced stimulation of hexose monophosphate shunt (HMPS) activity, and this was reversed by RU 38486. Cytochalasin B, the potent sugar-transport inhibitor, inhibited HMPS activity and 2-d[2,6-3H]Glc uptake equally, indicating a single site of action. By contrast, dexamethasone showed differential inhibition of HMPS activity and 2-d[2,6-3H]Glc uptake, suggesting that it not only acts by decreasing the coupling between hexokinase and sugar transport, but also at one or more additional points.
Rist
R J
RJ
Biomedical Sciences Division (Physiology), King's College London, U.K.
Naftalin
R J
RJ
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D003571:Cytochalasin B / Q000494:pharmacology; D003847:Deoxyglucose / Q000378:metabolism*; D003907:Dexamethasone / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006593:Hexokinase / Q000378:metabolism*; D007700:Kinetics; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism*; D015735:Mifepristone / Q000008:administration & dosage / Q000494:pharmacology; D010427:Pentose Phosphate Pathway / Q000187:drug effects*; D010529:Peritoneal Cavity / Q000166:cytology; D051381:Rats; D011919:Rats, Inbred Strains
D015735:Mifepristone; D003571:Cytochalasin B; D003907:Dexamethasone; D003847:Deoxyglucose; D006593:Hexokinase