CHANGES IN SPHINGOSINE LEVEL IN RAT-LIVER CELLS AND NUCLEI DURING CYCLOHEXIMIDE-INDUCED HYPEREXPRESSION OF ONCOGENES

The dependence of sphingosine content on the level of nuclear oncogene expression induced by various doses of cycloheximide (0.1, 0.5, and 3 .0 mg/kg) was analyzed in rat liver cells and nuclei. Only sublethal d oses (3.0 mg/kg), which cause hyperexpression of c-fos and c-myc oncog enes, lead to accumulation of sphingosine in the cells. With enhanced expression of these nuclear oncogenes, the maximum content of free sph ingosine exceeds the control 1.5- and 3-fold in whole cells and nuclei , respectively. These values suggest more intensive sphingosine metabo lism in the nuclei. Accumulation of sphingosine in the nuclei depends on the sphingomyelinase activity and sphingomyelin content. The activi ty of sphingomyelinase, an enzyme of the sphingomyelin cycle, and that of phosphatidylinositol kinase, an enzyme of the phosphoinositides cy cle, were compared. In the nucleus the former is activated before the latter and before the maximum accumulation of cycloheximide-induced nu clear oncogene mRNAs. It is suggested that activation of oncogene expr ession is mediated by sphingosine-induced inhibition of protein kinase C and activation of casein kinase II, key enzymes of cell proliferati on and differentiation signaling.