Effects of organometals on cellular signaling. I. Influence of metabolic inhibitors on metal-induced arachidonic acid liberation.

Organic lead and tin compounds stimulate an increase of free arachidonic acid (AA) in HL-60 cells. This fatty acid is involved in numerous health problems and physiological mechanisms. Three major pathways result in a liberation of AA from membrane phospholipids and there is evidence that G-proteins serve as couplers within all three pathways. Therefore we investigated the influence of pertussis toxin (PT) on the organometallic-induced AA liberation. The effect of all studied compounds (organotin and organo-lead) was diminished by PT. We conclude that the organometals activate PLA2 to some extent via a PT-sensitive pathway. The ionophor A 23187 (1-10 microM) led to an increase of free AA by raising the intracellular Ca2+ level. One of the postulated ways of AA release is via Ca2+ channel activation; phospholipases are Ca2+ dependent. Thus, we examined the necessity of free intracellular Ca2+ for the organometallic effect. The Ca2+ chelator EGTA inhibited the increase of free AA induced by organometals. This is true also for verapamil, a Ca2+ channel blocker. Quinacrine, which is thought to be an inhibitor of phospholipase A2 (PLA2), prevented the AA liberation from membrane phospholipids induced by organometals. This could be due to the inhibition of PLA2, but it could also be the result of an inhibited Ca2+ influx.

The stimulation of arachidonic acid metabolism by organic lead and tin compounds in human HL-60 leukemia cells.

The liberation of fatty acids, above all arachidonic acid, in human blood cells is involved in numerous health problems or physiological mechanisms. The activity of cellular phospholipases leads to lipid metabolites such as eicosanoids, platelet activating factor, diacylglycerol, and inositolphosphates that are capable of mediating such pathological symptoms. The results presented here demonstrate that organic heavy metal compounds induce arachidonic acid liberation or its rearrangement within the lipid classes of HL-60 cells before a loss in viability can be detected. Four of the compounds tested, triethyllead (Et3Pb+), diethyllead (Et2Pb2+), trimethyllead (Me3Pb+), and trimethyltin (Me3Sn+), show a threshold concentration at which the viability of the cells is drastically decreased after 60 to 180 min incubation, whereas dibutyltin (But2Sn2+) induces a constant increase of cell death during the whole incubation time. In the case of threshold concentrations, the compounds stimulate a loss of arachidonic acid within the phospholipids and an increase of free fatty acid and eicosanoids before cell death could be detected. An important fact is the rearrangement of arachidonic acid within the lipid classes of these cells induced by metal concentrations that were not able to kill the cells within the given time. Primarily affected is phosphatidylethanolamine which loses arachidonic acid and, to a minor extent, phosphatidylcholine. Portions of the liberated fatty acid were then metabolized and/or shifted into neutral lipids and other phospholipids. All compounds tested show comparable effects, although at different concentrations. The toxicities of the compounds can be ordered as follows: Et3Pb+ greater than or equal to Et2Pb2+ greater than But2Sn2+ greater than or equal to Me3Pb+ much greater than Me3Sn+ greater than or equal to Pb2+. The cellular shape change following incubation with metal compounds is a further strong indication of a change in the membrane lipids. The cells lose their characteristic microvilli and/or blebs and become round without a loss in viability.