Effects of Tamoxifen, Tamoxifen Metabolites, and Nafoxidine on Adenosine 3′,5′-Monophosphate Phosphodiesterase: Correlations with Growth Inhibitory Activities but not Estrogen Receptor Affinities^*

Abstract

Triphenylethylenes [Tamoxifen (TAM), TAM metabolites, and nafoxidine] were found to inhibit Ca2+-calmodulin (CaM)-dependent cAMP phosphodiesterase (PDE) activity of the quail oviduct, whereas 17 beta-estradiol was inactive. The Ca2+-CaM-independent PDE activity was not affected by triphenylethylenes, suggesting that they do not interact directly with the active site of the enzyme. Kinetic analysis indicated that these drugs competitively inhibited the activation of PDE by CaM with the following potencies: N-desmethyltamoxifen, Ki = 3 microM; metabolite Z, trans-4-hydroxytamoxifen, and TAM Ki = 5 microM; nafoxidine, Ki = 8.5 microM; and metabolite Y and cis-4-hydroxytamoxifen, Ki = 50 microM. Injected alone into immature quails, none of these drugs significantly affected oviduct weight. When administrated together with estradiol benzoate, these drugs reduced the trophic effect of estradiol in a dose-dependent relationship, with ID50 values ranging from 0.07 mg/kg for N-desmethyltamoxifen to 2.02 mg/kg for cis-4-hydroxytamoxifen. The order of growth inhibitory potency was not correlated with estrogen receptor affinities, but was the same as that reported for PDE inhibition. This correlation suggests that interaction of antiestrogen with Ca2+-CaM dependent PDE may be one of the mechanisms responsible for the estrogen antagonist activity of these drugs.

17β-Estradiol Affords Protection against 4-Vinylcyclohexene Diepoxide-Induced Ovarian Follicle Loss in Fischer-344 Rats

“…The dose of genistein was chosen to be the same as the E2 dose, because genistein has been found to bind ER␤ as well as stimulate ER (␣ and ␤)-mediated transcription at similar levels as the endogenous ligand E2 (19). The dose of the ER antagonist was selected because 4-hydroxytamoxifen has been shown to inhibit E2-induced uterine weight increases at 2.02 mg/kg (20). Four hours after the final dose, animals were killed by CO 2 inhalation, and ovaries, uterus, and trunk blood were collected.…”

Estrogen stimulated lipogenic activity in the ovary of the freshwater prawn,Macrobrachium Rosenbergü

“…Interestingly, this was not observed in the case of malate dehydrogenase enzyme. This peculiar 5-agonistic and antagonistic nature of action of tamoxifen is also observed in the vertebrate system depending on the specific biochemical parameters, organs and animal systems used (Victoria et al, 1977;Fanidi et al, 1989). Activation of both of these enzymes is mediated through specific protein synthesis since the application of cycloheximide inhibited the 5 stimulated activities of glucose-6-phosphate dehydrogenase and malate dehydrogenase.…”

Effects of anti‐oestrogens and β‐estradiol on calcium uptake by cardiac sarcoplasmic reticulum

“…It has been suggested, for example, that inhibition of PKC by tamoxifen occurs as a result of a tamoxifen‐induced alteration of the interaction of PKC and phospholipids ( O'Brian et al ., 1985 ); (2) anti‐oestrogens could affect proteins by directly interacting with the proteins themselves (see Custódio et al ., 1996 ; Hardy & Valverde, 1994 ; Song et al ., 1996 ; Waldegger et al ., 1996 ). In support of this mechanism, it has been reported that tamoxifen can bind to calmodulin ( Lopes et al ., 1990 ) and could inhibit calmodulin‐dependent enzymes in this manner ( Fanidi et al ., 1989 ; Lopes et al ., 1990 ). In our experiments, we have shown that, at all concentrations between 2 and 10 μ M , tamoxifen inhibits cardiac SR Ca 2+ uptake to a greater extent than does 4‐hydroxytamoxifen (see Figure 2).…”

“…In addition to its effects on cardiac muscle and lens cells, tamoxifen has also been found to block some plasma membrane Cl − , K + and Ca 2+ channels ( Dick et al ., 1999 ; Hardy & Valverde, 1994 ; Morley & Whitfield, 1994 ; Song et al ., 1996 ; Szücs et al ., 1996 ; Valverde et al ., 1995 ; Vandenberg et al ., 1994 ; Voets, 1995 ; Waldegger et al ., 1996 ) and to inhibit cyclic AMP phosphodiesterase activity and other calmodulin‐dependent enzymes through mechanisms that may involve an effect on calmodulin or calmodulin binding sites ( Fanidi et al ., 1989 ; Lopes et al ., 1990 ). Although the effects of 4‐hydroxytamoxifen and clomiphene on cellular function have not been as extensively studied, 4‐hydroxytamoxifen has been reported to block volume‐regulated Cl − channels ( Zhang et al ., 1994 ) and clomiphene has been reported to block gap junctional communication in cultured rat cardiomyocytes ( Verrecchia & Hervé, 1997a , 1997b ).…”