Inhibition of Translation in Eukaryotic Systems by Harringtonine

Abstract

The Cephalotaxus alkaloids harringtonine, homoharringtonine and isoharringtonine inhibit protein synthesis in eukaryotic cells. The alkaloids do not inhibit, in model systems, any of the steps of the initiation process but block poly(U)-directed polyphenylalanine synthesis as well as peptide bond formation in the fragment reaction assay, the sparsomycin-induced binding of (C)U-A-C-C-ArH]Leu-Ac, and the enzymic and the non-enzymic binding of Phe-tRNA to ribosomes. These results suggest that the Cephalotaxus alkaloids inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60-S ribosome subunit and therefore block aminoacyltRNA binding and peptide bond formation.However, the Cephalotaxus alkaloids do not inhibit polypeptide synthesis and peptidyl-[3H]puromycin formation in polysomes. Furthermore, these alkaloids strongly inhibit [14C]trichodermin binding to free ribosomes but hardly affect the interaction of the antibiotic with yeast polysomes. These results clearly suggest that the Cephalotaxus alkaloids cannot interact with polysomes and therefore only inhibit the initial cycles of elongation. This explains the polysome run off that has been observed by some workers in the presence of harringtonine.Harringtonine, homoharringtonine and isoharringtonine are alkaloids from Cephalotaxus harringtonia [l]. These three compounds have an antitumour activity and differ only in a side chain whereas the alkaloid lacking the side chain is inactive [2]. The antitumour activity of these alkaloids is due to their inhibitory effect on protein synthesis [2]. It has been proposed that these alkaloids inhibit the initiation of polypeptide synthesis since they induce the breakdown of polysomes to ribosomes in intact cells and cell lysatesWe have sequentially studied the individual steps of translation to elucidate the specific step(s) inhibited by the Cephalotaxus alkaloids. The results presented here show that these compounds do not inhibit initiation but block peptide bond formation and aminoacyltRNA binding.

The Antitumor Activity of Homoharringtonine against Human Mast Cells Harboring the KIT D816V Mutation

“…HHT has been found to inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60S ribosome subunit (41,42). As expected, HHT dramatically decreased KIT level and inhibited the growth of MCL cells, including D816V-bearing cells.…”

Alkaloid homoharringtonine inhibits polypeptide chain elongation on human ribosomes on the step of peptide bond formation

“…It has been initially proposed that these compounds inhibit initiation in intact cells and cell lysates [ 1,2]. Further investigations clearly showed that such an effect was due to the prevention of the elongation cycle in contradiction to the previous proposals [3,4]. The results presented in the above-mentioned works suggested that Cephalotaxus alkaloids inhibited not only peptide bond formation in the fragment reaction assays but also enzymatic and non-enzymatic binding of the PhetRNAPhe to ribosomes.…”

“…According to [3], Cephalotuxus alkaloids prevent not only transpeptidation but also tRNA binding to ribosomes (both nonenzymatic and enzymatic). Probably, the mechanism of the alkaloids' inhibitory action depends on the source of ribosomes.…”

True

“…11,12 In fact, its cytotoxicity has been shown to be proportional to the extent of inhibition of protein synthesis. 13,14 HHT has been shown to induce apoptosis in vitro, however, the main components of HHT-induced apoptotic pathway remain to be identified. 12,15 In the present study we investigated cell death induced by HHT in human leukemic cells.…”