INHIBITION OF THE MEMBRANE TRANSLOCATION AND ACTIVATION OF PROTEIN-KINASE-C, AND POTENTIATION OF DOXORUBICIN-INDUCED APOPTOSIS OF HEPATOCELLULAR-CARCINOMA CELLS BY TAMOXIFEN

Hepatocellular carcinoma (HCC) is characterized by high drug resistanc e to currently available chemotherapeutic agents. In a prospective cli nical study, we have demonstrated that high-dose tamoxifen significant ly enhanced the therapeutic efficacy of doxorubicin in patients with f ar-advanced HCC. In a search for a possible mechanism, we found that t amoxifen at a clinically achievable concentration (2.5 mu M) significa ntly enhanced doxorubicin-induced cytotoxicity and apoptosis of Hep-3B cells, a multidrug resistance (MDR)-1 expressing HCC cell line. This synergistic cytotoxic effect of tamoxifen, at this concentration, howe ver, was not mediated by MDR inhibition. Instead, as evidenced by both western blot and immunofluorescence studies, tamoxifen inhibited the cytoplasmic membrane translocation of protein kinase C (PKC)-alpha. 12 -O-Tetradecanoylphorbol-13-acetate (TPA) restored the membrane translo cation of PKC-alpha and abrogated the synergistic cytotoxicity of tamo xifen. We also showed that tamoxifen, at this concentration,did not di rectly affect the enzyme activity of PKC. Further, membrane translocat ion of other membrane-bound proteins, such as Ras protein, was similar ly inhibited by tamoxifen, but could not be restored by the addition o f TPA. Together, these data suggested that tamoxifen may act On the cy toplasmic membrane, and thereby inhibit PKC-alpha translocation to the membrane where it is activated. We hypothesize that high-dose tamoxif en may be an effective modulator of doxorubicin in the treatment of HC C, and suggest that biochemical modulation of PKC as a measure to impr ove systemic chemotherapy for HCC deserves further investigation. (C) 1998 Elsevier Science Inc.