Dual modulation of protein kinase C activity by sphingosine

Sphingosine is one of a number of cationic amphiphiles that inhibit the activity of protein kinase C (PKC) in commonly used assay conditions. This inhibition occurs only at high concentrations of this amphiphile. In the presence of excess negative charge from oleic acid, the addition of sphingosine surprisingly leads to activation of PKC. The results are explicable in terms of the dual role of charge and lipid phase propensity. When the positive charge on sphingosine is compensated by the negative charge on oleic acid, sphingosine, a hexagonal phase promoting amphiphile, becomes an activator of PKC. This does not occur with a bilayer stabilizing cationic amphiphile, N,N,N-Trimethyl-N'-cholesteryl amido-ethyl ammonium which is an inhibitor of PKC at all mol fractions, as well as in the presence of oleic acid. The results indicate that effects of sphingosine on more complex biological systems should be interpreted with caution because of this dual role of the amphiphile.