Our previous study suggested that N,N-dimethylsphingosine, but not unsubstituted sphingosine, could be a modulator of protein kinase C in epidermoid carcinoma A431 cells, since N,N-dimethyl-D-erythrosphingenine showed a stronger stereospecific effect on protein kinase C activity in comparison with N,N-dimethyl-L-erythrosphingenine, unsubstituted D- or L-erythrosphingenine, and gangliosides (Igarashi, Y., Hakomori, S., Toyokuni, T., Dean, B., Fujita, S., Sugimoto, M., Ogawa, T., El-Ghendy, K.,

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... d Racker, E. (1989) Biochemistry 28, 6796-6800). Other studies also indicated that commercial sphingosine preparation has an enhancing effect on epidermal growth factor (EGF) receptor kinase activity in A431 cells (Davis, R. J., Girones, N., and Faucher, M. F. (1988) J. Biol. Chem. 263, 5373-5379; Faucher, M. F., Girones, N., Hannun, Y. A., Bell, R. M., and Davis, R. J. (1988) J. Biol. Chem. 263, 5319-5327). In the present paper, we report (i) the effect of N,N-dimethylsphingosine as compared with lyso-glycosphingolipids and other sphingolipid breakdown products on EGF receptor autophosphorylation and (ii) demonstration of endogenous N,N-dimethylsphingosine synthesis and the virtual absence of unsubstituted sphingosine in A431 cells. The autophosphorylation of EGF receptor in the absence of detergent was strongly enhanced by N,N-dimethyl-D-erythrosphingenine; this effect was even obvious in the absence of EGF and synergistic in the presence of EGF. Similar enhancing activity was not produced by N,N-dimethyl-L-erythrosphingenine, D- and L-erythrosphingenine, N-monomethyl-D-erythrosphingenine, N-acetyl-D-erythrosphingenine, or the five lyso-glycosphingolipids tested. Labeling of sphingosine in A431 cells by culturing in medium containing [3H]Ser for various durations, followed by extraction and isolation of sphingolipids by standard procedures, resulted in clear bands corresponding to N,N-dimethylsphingosine and ceramide, whereas the band corresponding to sphingosine was virtually absent. The bands corresponding to N,N-dimethylsphingosine and ceramide intensified when cells were treated with metabolic inhibitor for UDP-Glc:Cer beta-Glc transferase (which causes accumulation of ceramide). These results indicate that N,N-dimethylsphingosine acts as a stereospecific enhancer for EGF receptor kinase and is able to produce EGF-like activity in vitro even in the absence of EGF and detergent. Under physiological conditions, N,N-dimethylsphingosine is the major catabolite resulting from ceramide breakdown.