Isothermal regulation of membrane fluidity in murine fibroblasts with altered phospholipid polar head groups

Abstract Fatty acyl chain and polar head group metabolism in a transformed murine fibroblast cell line, LM cells, may be linked and provide a mechanism for homeoviscous adaptation. Fluorescence polarization studies with β-parinaric acid, 1,6-diphenylhexatriene, and N -phenylnaphthylamine indicated that the microenvironments reported by each probe were different in plasma membranes isolated from LM cells grown in the presence of choline. In addition, each fluorescence probe molecule had markedly different polarization values in the plasma membranes, microsomes, and mitochondria. β-Parinaric acid incorporated into plasma membranes, microsomes, or mitochondria isolated from LM cells grown in the presence of N , N ′-dimethylethanolamine or N -monomethylethanolamine had essentially the same polarization values as in the corresponding membranes from choline-grown cells. However, polarization values of β-parinaric acid in microsomes and mitochondria from ethanolamine supplemented LM cells were markedly higher. These differences in polarization of the fluorescence probes in the isolated membranes may in part be due to a relationship between phospholipid polar head group and acyl group metabolism by which isothermally grown cells attempt to maintain the fluidity of their membranes. Data from choline-grown cells indicated that: (a) The fatty acid composition of phosphatidylcholine varied with respect to chain length and degree of unsaturation in the three membrane fractions. (b) The acyl chain composition of the other phospholipid species differed markedly from phosphatidylcholine in each membrane fraction and also exhibited major differences in chain length and degree of unsaturation depending on the membrane source. (c) The fatty acid composition of the zwitterionic phospholipids with a no net charge (phosphatidylcholine, sphingomyelin, and lysophosphatidylcholine) as well as the negatively charged phospholipids (phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin) increased or decreased in degree of unsaturation and/or chain length depending on the phospholipid species as well as the membrane source. Secondly, supplementation of the culture medium with the choline analogues N , N ′-dimethylethanolamine, N -monomethylethanolamine, or ethanolamine resulted in incorporation of the bases into membrane phospholipids and in alteration of acyl chain distribution: (a) Newly synthesized phosphatidyl- N , N ′-dimethylethanolamine and phosphatidyl- N -monomethylethanolamine from all three LM cell membrane fractions had fatty acid composition, intermediate between those of the phosphatidylcholine and phosphatidylethanolamine in that membrane fraction. (b) The choline analogues also affected the acyl chain composition of the other membrane phospholipid species, e.g. phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin. Phosphatidylcholine, phosphatidylethanolamine and cardiolipin acyl groups became more unsaturated in microsomes and mitochondria of analogue supplemented cells grown in suspension. (c) Neither of the alterations in a or b occurred to the same extent and in the same direction in all three membrane fractions.