Abstract
The nonestrogen receptor-mediated antiproliferative action of antiestrogen binding site (AEBS) ligands, including triphenylethylene antiestrogens and phenothiazines, has been linked to their ability to inhibit protein kinase C (PKC). Recent studies indicate that some diphenylmethane derivatives inhibit growth, are potent AEBS ligands, and antagonize histamine binding at an AEBS-related histamine site different from H1 and H2. Three novel diphenylmethane derivatives, N,N-diethyl-2-[4-(phenylmethyl)phenoxyl]ethanamine·HCl (DPPE), 4-decanoyl-DPPE (dec-DPPE), and 4-benzylphenyl decanoate (BPD) were studied in an attempt to determine whether PKC or histamine interactions best correlate with their antiproliferative effects. Platelet aggregation and the phosphorylation of a platelet Mr 47,000 protein (p47) induced by phorbol-12-myristate-13-acetate (PMA) represent two processes mediated by PKC. DPPE inhibits PMA-induced aggregation [50% inhibitor concentration (IC50) = 31.2 ± 2.4 (SEM) × 10-6 m] but does not significantly inhibit either PMA-induced phosphorylation of Mr 47,000 protein (IC50 > 500 × 10-6 m), or binding of [3H]phorbol dibutyrate to platelets. dec-DPPE is a more potent inhibitor of PMA-induced platelet aggregation (IC50 = 18.8 ± 0.7 × 10-6 m), a weak inhibitor of Mr 47,000 phosphorylation (IC50 = 80–200 × 10-6 m), but is without effect on [3H] phorbol dibutyrate binding. BPD, which lacks the alkylaminoethoxy side chain necessary for binding to the AEBS/DPPE site, is devoid of anti-PMA effects. These results are compared to the inhibition of [3H]histamine binding in rat cortex membranes (K1 value for DPPE = 0.83 ± 0.62 × 10-6 m; K1 value for dec-DPPE = 6.6 ± 3.5 × 10-6 m; BPD is inactive) and growth inhibition of MCF-7 cells (IC50 value for DPPE = 4.5 × 10-6 m; IC50 value for dec-DPPE = 1.5 × 10-5 m; BPD is ineffective at all concentrations tested). Thus, while dec-DPPE is a more potent inhibitor of PKC-mediated phosphorylation, DPPE is a more potent inhibitor of histamine binding and is correspondingly more antiproliferative than dec-DPPE. The results support a relationship between antagonism of histamine binding and growth inhibition but argue against an association between the antiproliferative effects of DPPE and dec-DPPE and inhibition of PKC. The findings for DPPE suggest that platelet response to PMA, antagonized by diphenylmethane-type AEBS-ligands, may be mediated, at least in part, by mechanisms other than activation of protein kinase C-dependent phosphorylation.
Footnotes
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↵1 This work was supported by grants from the Manitoba Health Research Council, The University of Manitoba Faculty Fund, The Medical Research Council of Canada, and by a generous gift from the estate of Morris Katz.
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↵2 To whom requests for reprints should be addressed.
- Received January 25, 1988.
- Revision received April 7, 1988.
- Accepted April 11, 1988.
- ©1988 American Association for Cancer Research.
Volume 48, Issue 14
