Characterization and Hormonal Regulation of Casein Kinase II Activity in Heterotransplanted Human Breast Tumors in Nude Mice

Abstract

Cytosolic casein kinase type II activity has been identified in MCF-7 and MDA-MB-231 human breast cancer cells heterotransplanted into athymic nude mice. Sephacryl S-300 chromatography of MCF-7 and MDA-MB-231 tumor cytosols revealed a major peak of casein kinase activity with an estimated molecular weight of 150,000. This peak was further characterized and optimal conditions for breast tumor casein kinase activity were established. Polylysine (10 μg) acted as a potent stimulator with casein as the phosphate acceptor protein. This enzyme used both ATP and GTP as phosphate donors and the Km for GTP was 10 μM. The rate of phosphorylation with increasing concentrations of [γ-^32P]GTP revealed typical Michaelis-Menten kinetics and Vmax was approached at a concentration of 30 μM GTP. MgCl₂ stimulated enzyme activity at concentrations between 10—20 mM. Quercetin, a bioflavonoid, inhibited casein kinase type II activity in a dose dependent manner. MCF-7 (hormone-dependent) human breast cancer cells (2—3×10⁶) were inoculated into the mammary fat pads of nude mice, supplemented with a 0.5 mg estradiol pellet. To determine the influence of various regulatory agents on casein kinase activity in vivo, tumor-bearing mice were treated for five days with estradiol, progesterone, dexamethasone or tamoxifen. Casein kinase type II was partially purified by gel filtration on a Sephacryl S-300 column and assayed in the presence of polylysine and casein. Dexamethasone treatment significantly decreased casein kinase II activity in MCF-7 tumors, which are receptor-positive for estrogen, androgen and glucocorticoid receptors.