The effect of the anticancer drugs tamoxifen and hydroxytamoxifen on the calcium pump of isolated sarcoplasmic reticulum vesicles

Abstract

The interactions of tamoxifen (TAM) and its active metabolite 4-hydroxytamoxifen (OHTAM) with the sarcoplasmic reticulum (SR) Ca(2+)-pump were investigated. The turnover of the Ca(2+)-ATPase is strongly inhibited by both drugs at low concentrations that do not significantly perturb the lipid organization of SR membranes. Moreover, TAM decreases Ca(2+) accumulation by SR Ca(2+)-ATPase and increases in parallel the ATP hydrolysis, decreasing the energetic efficiency of the Ca(2+)-pump (Ca (2+)ATP coupling ratio) by about 70% at 30 muM. This uncoupling of ATP hydrolysis from Ca(2+) accumulation is a putative consequence of structural defects induced on membranes, since the ATP hydrolysis at low residual Ca(2+) (Ca(2+) not supplemented) is also stimulated. On the other hand, OHTAM decreases the Ca(2+) uptake to a greater extent than TAM but, unlike TAM, it inhibits ATP hydrolysis. Thus, the Ca (2+)ATP ratio is decreased by about 47% at 30 muM OHTAM; this effect is not a consequence of membrane disruption, since the ATP-splitting activity decreases in parallel to Ca(2+) accumulation and no significant effect is detected for ATP hydrolysis at low residual Ca(2+). The inhibition of the Ca(2+)-pump by OHTAM is putatively related to a direct interaction with the regulatory sites of the enzyme or interactive perturbations at the lipid-protein interface. The effect may result from a decrease of efficiency in the energy transmission and transduction between the ATP use at the catalytic site and the channeling process involved in Ca(2+) translocation. Therefore, the effects of the drugs on the Ca(2+)-pump are different and rule out an unitary mechanism of action on the basis of bilayer structure perturbations.