Activity and regulation by growth factors of calmodulin dependent protein kinase III (elongation factor 2-kinase) in human breast cancer

Calmodulin-dependent protein kinase III (CaM kinase Ill, elongation factor- 2 kinase) is a unique member of the Ca2+/CaM-dependent protein kinase famil y. Activation of CaM kinase ill leads to the selective phosphorylation of e longation factor 2 (eEF-2) and transient inhibition of protein synthesis. R ecent cloning and sequencing of CaM kinase III revealed that this enzyme re presents a new superfamily of protein kinases. The activity of CaM kinase i ll is selectively activated in proliferating cells; inhibition of the kinas e blocked cells in G(0)/G(1)-S and decreased viability To determine the sig nificance of CaM kinase III in breast cancer, we measured the activity of t he kinase in human breast cancer cell lines as well as in fresh surgical sp ecimens. The specific activity of CaM kinase III in human breast cancer cel l lines was equal to or greater than that seen in a variety of cell lines w ith similar rates of proliferation. The specific activity of CaM kinase III was markedly increased in human breast tumour specimens compared with that of normal adjacent breast tissue. The activity of this enzyme was regulate d by breast cancer mitogens. In serum-deprived MDA-MB-231 cells, the combin ation of insulin-like growth factor I (lGF-I) and epidermal growth factor ( EGF) stimulated cell proliferation and activated CaM kinase III to activiti es observed in the presence of 10% serum. inhibition of enzyme activity blo cked cell proliferation induced by growth factors. In MCF-7 cells separated by fluorescence-activated cell sorting, CaM kinase III was increased in S- phase over that of other phases of the cell cycle. In summary, the activity of Ca2+/CaM-dependent protein kinase ill is controlled by breast cancer mi togens and appears to be constitutively activated in human breast cancer. T hese results suggest that CaM kinase ill may contribute an important link b etween growth factor/receptor interactions, protein synthesis and the induc tion of cellular proliferation in human breast cancer.