PGE2 and PGA2 affect the allosteric properties and the activities of calmodulin-dependent guanylate cyclase and Ca2+-stimulated ATPase of Walker-256 tumour microsomal membranes.
PGE2 and PGA2 incubated for 30 min at 25 degrees C with microsomal membranes isolated from Walker-256 tumour, in the presence of 50 microM indomethacin increase the lipid fluidity estimated by steady-state fluorescence anisotropy [(r0/r)-1]-1, using 1,6-diphenyl-1,3,5-hexatriene (DPH) as probe. The microsomal preparations of Walker-256 tumour contained calcium-stimulated and magnesium-dependent ATPase as well as calmoduling-dependent guanylate cyclese activities. A considerable decrease (approx. 65%) in the activity of the Ca2+-stimulated ATPase was observed when preparations were treated with 10 microM PGE2 and PGA2. A dramatic gradual decrease of the calmodulin-dependent guanylate cyclase activity was also observed at different concentrations of PGE2 and PGA2 (0.25-10 microM). The ATP-dependent uptake of calcium was reduced by approximately 60% in microsomal membranes treated with PGE2 and PGA2. The allosteric properties of Ca2+-stimulated ATPase by Na+, and of guanylate cyclase by Mn.GTP (as reflected by changes in the Hill coefficients, h) were modulated by PGE2 and PGA2. The apparent cooperativity of the Ca2+-ATPase (h + 1.73 +/- 0.21) in control membranes was abolished (h + 1.1 +/- 0.11 and h = 0.9 +/- 0.09) in membranes treated by PGE2 and PGA2 (10 microM), while the allosteric stimulation of guanylate cyclase by Mn.GTP was reduced from h = 2.78 +/- 0.24 in control membranes to h = 1.92 +/- 0.16 and h = 1.73 +/- 0.15 in membranes treated by PGE2 and PGA2 (10 microM), respectively, suggesting that the physical state of Ca2+-stimulated ATPase and guanylate cyclase lipid microenvironments changed from a gel phase to a liquid-crystalline phase. In conclusion, it is suggested that PGE2 and PGA2 promote a phase separation in Walker-256 tumour microsomal membranes. This may be relevant to the Ca2+-calmodulin system and tumour growth inhibition.
Deliconstantinos
G
G
Department of Experimental Physiology, University of Athens, Medical School, Greece.
Kopeikina
L
L
Ramantanis
G
G
D000494:Allosteric Regulation; D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000032:analysis*; D002147:Calmodulin / Q000494:pharmacology*; D002279:Carcinoma 256, Walker / Q000201:enzymology*; D015232:Dinoprostone / Q000378:metabolism / Q000494:pharmacology*; D006162:Guanylate Cyclase / Q000032:analysis*; D008297:Male; D008560:Membrane Fluidity / Q000187:drug effects; D008861:Microsomes / Q000201:enzymology; D011454:Prostaglandins A / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013696:Temperature
D002147:Calmodulin; D011454:Prostaglandins A; D006162:Guanylate Cyclase; D000252:Calcium-Transporting ATPases; C100008:prostaglandin A2; D015232:Dinoprostone; D002118:Calcium