Cell membrane signaling as target in cancer therapy: inhibitory effect of N,N-dimethyl and N,N,N-trimethyl sphingosine derivatives on in vitro and in vivo growth of human tumor cells in nude mice.

Sphingosine (SPN) has been claimed to be a negative modulator of transmembrane signaling through protein kinase C (PK-C) or some yet unidentified mechanism [for review see Y. A. Hannun and R. M. Bell, Science (Washington DC), 243: 500-507, 1989]. N,N-Dimethylsphingosine (DMS) was recently found to be a physiological cellular component and, in comparison to SPN, to show a stronger and stereospecific inhibitory effect on PK-C activity of A431 cells (for review see Y. Igarashi, Trends Glycosci. Glycotechnol., 2: 319-332, 1990; and S. Hakomori, J. Biol. Chem., 265: 18713-18716, 1990). (4E)-N,N,N-Trimethyl-D-erythro-sphingenine (TMS) is not detectable as a normal cellular component; however, it is expected to exhibit potent activity because of its quaternary ammonium ion structure, and in fact it showed much stronger inhibitory effect than DMS or SPN on PK-C activity (which plays an important role in cell growth regulation) in vitro. In view of these findings, we investigated the effects of SPN, DMS, and TMS on in vitro growth of various human carcinoma cell lines and on in vivo tumor growth in athymic nu/nu mice. Both DMS and TMS showed similar in vitro and in vivo growth inhibitory effects on tumor cells, despite the fact that TMS showed a much stronger inhibitory effect than DMS on PK-C activity of A431 cells. In contrast, SPN showed only a weak effect on in vitro cell growth and no effect on in vivo tumor growth. Tumor growth following s.c. inoculation of mice with human gastric carcinoma cell line MKN74 was inhibited in a dose-dependent manner by DMS, and tumor size was decreased after three or four consecutive daily injections of 0.5-mg doses of DMS or TMS. Increased tumor growth occurred after administration of these compounds was stopped; however, size of tumor remained significantly smaller than in groups treated with SPN or control saline. The effect of DMS or TMS on in vitro or in vivo MKN74 cell growth was stronger than that of 8-chloro-adenosine-cyclic 3':5'-monophosphate dihydrate, the most promising agent currently being used in clinical trials for inhibition of tumor growth by induction of differentiation. These results suggest that DMS or TMS could be useful anticancer agents through modification of transmembrane signaling related to cancer cell growth.