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Cellular Sensitization to cis-Diamminedichloroplatinum(II) by Novel Analogues of the Protein Kinase C Activator Lyngbyatoxin A¹

Department of Pharmacology, School of Medicine, and The Experimental Therapeutics Program, Pittsburgh Cancer Institute [A. B., J. S. L.], and Department of Chemistry, Chevron Science Center [A. P. K., K. S.], University of Pittsburgh, Pittsburgh, Pennsylvania 15261

Protein kinase C (PKC) has been implicated in enhancing cellular sensitivity to cis-diamminedichloroplatinum(II) (CP). We have synthesized a series of novel analogues of lyngbyatoxin A (7-linalylindolactam V), a natural tumor promoter and a potent activator of PKC, and investigated the effects of these synthetic compounds on PKC activity and the antiproliferative activity of CP. Lyngbyatoxin A was as effective as phorbol esters, such as 12-O-tetradecanoylphorbol-13-acetate, in enhancing the sensitivity of HeLa cells to CP. A 24-h pretreatment of HeLa cells with 1 to 100 nM lyngbyatoxin A caused an approximately 9-fold sensitization to CP. All analogues of lyngbyatoxin A that retained the lactam ring portion of the molecule but contained different hydrophobic substituents at C-7 including indolactam V (ILV), tert-butyl-ILV, or n-hexyl-ILV increased cellular sensitivity to CP in a concentration-dependent manner. Maximum cellular sensitization to CP (9-fold) was seen with 10 nM n-hexyl or tert-butyl compounds, and ILV devoid of any C-7 substitution required higher concentrations (1 µM) for equivalent sensitization. The ability of lyngbyatoxin A analogues to sensitize cells to CP correlated directly with their ability to activate PKC in vitro. Synthetic analogues that lacked the lactam ring structure neither activated PKC nor sensitized cells to CP. The C-9 epi analogue of n-hexyl-ILV was less effective than the corresponding natural stereoisomer in activating PKC as well as sensitizing cells to CP. Exposure of HeLa cells to 100 nM lyngbyatoxin A for 24 h caused a substantial decrease in cellular PKC activity to 20% of the untreated control value, but a similar treatment of cells with n-hexyl- or tert-butyl-ILV led to only a 25% reduction in PKC activity. Concentrations of ILV (e.g. 1 µM) that sensitized HeLa cells to CP caused no down-regulation of PKC. Thus, on the basis of results with these novel lyngbyatoxin A analogues, we conclude that activation but not down-regulation of PKC is necessary for sensitization of HeLa cells to CP.

¹ Supported in part by NIH Grants CA43917 (J. S. L.) and CA50175 (A. P. K.) and Grant CH486 (J. S. L.) from the American Cancer Society.

² To whom correspondence should be addressed, at Department of Pharmacology, University of Pittsburgh School of Medicine, E1346 Biomedical Science Tower, Pittsburgh, PA 15261.

³ Present address: Neurochemistry Research, Mayo Clinic Jacksonville, Jacksonville, FL 32224.

Received 11/19/90. Accepted 3/ 8/91.

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