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

Abstract

Summary Calmodulin-dependent protein kinase III (CaM kinase III, elongation factor-2 kinase) is a unique member of the Ca 2+ /CaMdependent protein kinase family. Activation of CaM kinase III leads to the selective phosphorylation of elongation factor 2 (eEF-2) and transient inhibition of protein synthesis. Recent cloning and sequencing of CaM kinase III revealed that this enzyme represents a new superfamily of protein kinases. The activity of CaM kinase III is selectively activated in proliferating cells; inhibition of the kinase blocked cells in G 0 /G 1 -S and decreased viability. To determine the significance of CaM kinase III in breast cancer, we measured the activity of the kinase in human breast cancer cell lines as well as in fresh surgical specimens. The specific activity of CaM kinase III in human breast cancer cell lines was equal to or greater than that seen in a variety of cell lines with 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 regulated by breast cancer mitogens. In serum-deprived MDA-MB-231 cells, the combination of insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF) stimulated cell proliferation and activated CaM kinase III to activities observed in the presence of 10% serum. Inhibition of enzyme activity blocked 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 Ca 2+ /CaM-dependent protein kinase III is controlled by breast cancer mitogens and appears to be constitutively activated in human breast cancer. These results suggest that CaM kinase III may contribute an important link between growth factor/receptor interactions, protein synthesis and the induction of cellular proliferation in human breast cancer.

Involvement of protein kinase Cδ in contact-dependent inhibition of growth in human and murine fibroblasts

“…We assumed that the decrease in seruminduced DNA-synthesis was due to inhibition of Calmodulin-dependent protein kinase III. It has been reported that Rottlerin is equieective in inhibiting this kinase (Gschwendt et al, 1994b) hence leading to inhibition of proliferation (Palfrey et al, 1987;Parmer et al, 1997). Our hypothesis was supported by the fact that the Calmodulin antagonist Tri¯uoperazine (5 mM) decreased DNA-synthesis of serum-stimulated cells to a similar extent but had no eect on contact-inhibition (data not shown).…”

When translation meets transformation: the mTOR story

“…Recent studies show that a compound that acts as a selective inhibitor of eEF2 kinase (NH125) decreases the viability of a range of cancer cell lines, and this effect is countered by artificial overexpression of eEF2 kinase consistent with the idea that eEF2 kinase plays a role in cell viability (Arora et al, 2003). Earlier data obtained with another compound that inhibits eEF2 kinase, rottlerin, indicated loss of viability and growth, as well as changes in cell morphology (Parmer et al, 1997). However, it is now clear that rottlerin can inhibit other kinases in addition to eEF2 kinase (Davies et al, 2000) and caution must therefore be exercised in interpreting data obtained with such reagents.…”

“…There are data indicating that eEF2 kinase activity is elevated in certain cancer cells (e.g., breast cancer; Parmer et al, 1999) and that its activity is enhanced in proliferating cells (Bagaglio et al, 1993;Bagaglio and Hait, 1994;Cheng et al, 1995;Parmer et al, 1997). This appears surprising since eEF2 kinase is a negative regulator of protein synthesis, and thus presumably of cell growth/proliferation.…”

A Matter of Life or Death (or Both): Understanding Autophagy in Cancer

“…eEF-2 kinase is a structurally unique enzyme (21) whose activity is increased in cancer (22). An early clue to the importance of eEF-2 kinase in cell survival came from an unlikely source (hibernation experiments in squirrels).…”

“…A target that is activated rather than inhibited (e.g., mTOR, Akt, and PI3K) to initiate our sustain autophagy would be preferable, because it is far easier to block a target than to activate one. In that regard, eEF-2 kinase becomes increasingly attractive because it is overexpressed in many forms of cancer (22,43) and it is activated during autophagy (36); its activity terminates protein elongation and conserves energy (44); and its unique structure makes this kinase amenable to selective inhibition (21,43). Conditional inactivation of eEF-2 kinase by genetic (conditional knockout) or pharmacologic approaches in cancer models would therefore help address the proper role of autophagy in cancer treatment.…”