Phospholipid Metabolism in Embryonic Rat Fibroblasts following Stimulation by a Combination of the Serum Proteins S1 and S2

Abstract

Phospholipid metabolism has been investigated in embryonic rat fibroblasts which have been incubated in serum‐free medium and subsequently stimulated by the growth‐stimulating serum proteins S1 and S2: Incorporation of 32 P into phosphatidylinositol iancreases rapidly within a few minutes of stimulation; after 30 min it is 7.4‐fold higher in stimulated cells when compared with the controls. Phosphatidylcholine synthesis and uptake of 32 P into the acid‐soluble pool is not significantly enhanced during this time. Labelling of phosphatidylserine, phosphatidylethanolamine and sphingomyelin during this period is just above the background level. Incorporation of 32 P into phosphatidylinositol and phosphatidylethanolamine is strongly inhibited in the presence of dibutyryl‐adenosine 3′: 5′‐monophosphate and theophylline whereas the metabolism of phosphatidylcholine is not influenced. As demonstrated with [ 3 H] inositol, the inhibition of phosphatidylinositol synthesis is restricted to dibutyryladenosine 3′:5′‐monophosphate; adenosine 3′:5′‐monophosphate; guanosine 3′:5′‐monophosphate and dibutyryl guanosine 3′:5′‐monophosphate are ineffective. The growth‐stimulating serum factors must be present continuously for activation of phosphatidylinositol synthesis; within 1 h after serum deprivation the incorporation of 32 P into phosphatidylinositol decreases to about 20% of the original value; in contrast, phosphatidylcholine biosynthesis is not influenced significantly during this time period.