DETECTION OF NEUTRAL ACTIVE PHOSPHATIDYLCHOLINE-SPECIFIC PHOSPHOLIPASE-C IN FRIEND-LEUKEMIA CELLS BEFORE AND AFTER ERYTHROID-DIFFERENTIATION

With respect to normal tissues, P-31 NMR spectra of tumors usually exh ibit elevated phosphomonoester (PME) and phosphodiester (PDE) signals, arising from phospholipid metabolites such as phosphocholine (PCho) a nd glycerophosphocholine (GroPCho) (and/or ethanolamine analogues). PM E and PDE resonances may undergo significant alterations during tumor growth, at early stages of tumor response to treatment or following ce ll differentiation and maturation. The enzymatic mechanisms which regu late these alterations are scarcely understood. Recent studies on agon ist-induced phosphatidylcholine (PC) hydrolysis by PC-specific phospho lipase C (PC-plc) in cells stimulated by hormones or growth factors su ggest the hypothesis that repeated transient activations of this enzym e may also contribute to the elevation of PCho levels in tumor NMR spe ctra. This paper reports the first direct evidence on neutral active P C-plc activity in a tumour cell system, Friend leukemia cells, either in the undifferentiated (FLC) or differentiated state (dFLC). Cell hom ogenates were incubated in the presence of mixed diheptanoylphosphatid ylcholine/sphingomyelin unilamellar vesicles (SLUV), which were previo usly shown to represent a good substrate for bacterial plc. P-31 NMR a nalyses allowed the simultaneous detection and quantification of phosp horylated metabolites produced in tumor cell homogenates by PC-plc act ivity, as well by enzymes active in the PC deacylation pathway. With r espect to FLC, (dFLC homogenates exhibited higher PC-plc activity and lower accumulation of a deacylation product, GroPCho, in agreement wit h the elevation in the [PCho]/[GroPCho] ratio, already reported in P-3 1 NMR spectra of intact differentiated cells. The direct detection of PC-plc in this cell system opens novel biochemical interpretations on a series of oncological observations, such as a) transient increases i n the levels of PCho and PC-derived diacylglycerols reported in immatu re or in transformed cells in response to agonist-receptor interaction s and b) accumulation of mobile lipids at tumor cell membranes and tis sues.