Tamoxifen inhibits particulate-associated protein kinase C activity, and sensitises cultured human glioblastoma cells not to etoposide but to gamma-radiation and BCNU

We investigated the potential mechanisms of tamoxifen cytotoxicity in the U -373, U-138, and U-87 human glioblastoma cell Lines, namely interference wi th protein kinase C (PKC) activity, the oestrogen receptor, and/or the prod uction of transforming growth factor beta 1 (TGF-beta 1). We further examin ed the effects of tamoxifen on the cytotoxicity exerted by gamma-radiation, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and etoposide in this cell li ne panel. Thus, the cells were treated for 4 days with tamoxifen, gamma-rad iation, purified recombinant human TGF-beta 1 (rhTGF-beta 1), BCNU, or etop oside, either alone or at certain combinations. Cellular responses were eva luated with the sulphorhodamine B assay, as well as by multiple drug effect analysis, and related to PKC activities in particulate and cellular fracti ons; cellular oestrogen receptor contents; and the influence of rhTGF-beta 1 on cell growth. Tamoxifen inhibited cell proliferation as well as the pho sphorylation capacity of the particulate, but not of the cytosolic fraction s dose-dependently, at comparable kinetics, and at IC50 values of approxima tely 15 mu M. At these concentrations, tamoxifen acted synergistically with gamma-radiation (4- to 6-fold) and additively with BCNU (approximately 2-f old), but did not affect etoposide cytotoxicity. The cells were negative to immunostaining for the oestrogen receptor, and rhRGF-beta 1 did not influe nce their growth up to 100 nm. Our data suggest that tamoxifen can sensitis e cultured glioblastoma cells not to etoposide but to gamma-radiation and B CNU, possibly through interference with membrane PKC, supporting its evalua tion in experimental protocols for primary malignant gliomas. (C) 1999 Else vier Science Ltd. All rights reserved.