Fatty acid composition of tissue phospholipids and prostaglandin excretion in hyperlipidemia induced in rats by implantation of the mammotropic pituitary tumor MtT-F₄

Abstract

A mammotropic pituitary tumor, MtT-F₄, was implanted into male Fisher 344 rats for a period of 4 weeks. This tumor induced growth retardation, hyperlipidemia, hepatic hypertrophy and adrenal hyperplasia. Lipids were extracted from various tissues. In tumor-bearing rats, phospholipid concentration was found to be increased in plasma, spleen and testis. Distribution among the various phospholipid classes was similar to that of controls except in liver and heart, where phosphatidylcholine was increased at the expense of phosphatidylinositol and phosphatidylserine. The main difference was in the fatty acid composition of major phospholipids. The proportion of ω6 fatty acids was lower and that of docosahexaenoic acid of the ω3 series (22∶6ω3) was higher in most tissues, especially in plasma, liver, heart and kidney. Concurrently, the urinary excretion of two endogenous metabolites of PGI₂ (2,3-dinor-6-keto-PGF_1α and 6,15-diketo-13,14-dihydro-2,3-dinor-PGF_1α) was found to be increased significantly in tumor-bearing rats. The results raise the hypothesis that hormonal changes induced by the MtT-F₄ tumor accelerate the conversion of arachidonic acid (20∶4ω6) to prostaglandins. This effect, perhaps coupled with a diversion of linoleic acid (18∶2ω6) towards other metabolic processes, would account for a partial depletion of membrane phospholipids in 18∶2ω6 and for the reduced production of longer chain ω6 unsaturated acids from 20∶4ω6, creating a state of “relative essential fatty acid deficiency.” As a result, the metabolism of ω3 fatty acids is altered towards an enhanced production of 22∶6ω3 which accumulates in the lipids of cell membranes to compensate for the depletion of unsaturated acids of the ω6 series.