Protein synthesis in rabbit reticulocytes. XIII. Lack of mRNA (poly r (A)) binding activity in highly purified EIF-1

Selective inactivation of rabbit reticulocyte initiation factor eIF-3 by helenalin

Helenalin, a sesquiterpene lactone which reacts primarily with exposed sulfhydryl groups, was shown to be an effective inhibitor of protein synthesis in rabbit reticulocyte lysates. Optimal inhibition required a 30 min preincubation in the absence of any added thiol compound. Beta-Mercaptoethanol was more effective than reduced glutathione in protecting enzyme sulfhydryl groups from inactivation by helenalin. Using partially fractionated systems, it was possible to show that helenalin had no effect on the elongation reactions or on the formation of the ternary initiation complex. However, the conversion of the ternary complex to the 48 S initiation complex was strongly inhibited. In this assay, only the initiation factor(s) were sensitive to helenalin. Using an assay system which requires all the initiation factors for optimal activity it was possible to show that the 0-40% ammonium sulfate cut of initiation factors (containing eIF-3 and eIF-4B) was sensitive to helenalin, while the 40-50% ammonium sulfate cut (containing eIF-2 and eIF-5) was not. Both ammonium sulfate cuts were equally sensitive to inhibition by the sulfhydryl reagent N-ethylmaleimide. Three purified rabbit reticulocyte initiation factors were then tested in the same assay system. Only eIF-3 showed appreciable sensitivity to helenalin, while eIF-2, eIF-3 and eIF-4B were all sensitive to inactivation by N-ethylmaleimide. These data suggest that helenalin may possess a relatively high degree of specificity as a sulfhydryl reagent.

Investigation of sesquiterpene lactones as protein synthesis inhibitors of P-388 lymphocytic leukemia cells

The mode of action of helenalin and bis(helenalinyl) malonate as protein synthesis inhibitors of P-388 lymphocytic leukemia cells was investigated. The initial characterizations were carried out in crude lysates of the P-388 cells. In the lysate, there was a 4 min lag after the addition of drug before inhibition of protein synthesis occurred. Both drugs allowed run-off of preformed polysomes, but did significantly inhibit the formation of the 80 S initiation complex suggesting a preferential inhibition of one or more initiation reactions. The effect of these drugs on inhibition of both elongation and initiation reactions was further investigated using more fractionated systems prepared from P-388 cells. Poly(U)-directed polyphenylalanine synthesis was marginally inhibited by both drugs, but the degree of inhibition was not sufficient to explain the inhibition observed in either the lysate or in whole cell preparations of P-388. The formation of the ternary initiation complex was not significantly inhibited by either drug, but the conversion of this complex to the 48 and 80 S initiation complexes was inhibited. The inhibition of 48 S initiation complex formation by both drugs was sufficient to explain their inhibition of protein synthesis in whole cells.

Antitumor agents LVI: the protein synthesis inhibition by genkwadaphnin and yuanhuacine of P-388 lymphocytic leukemia cells

Two natural products isolated from the plant Daphne genkwa have been shown to possess antileukemic activity in mice. Genkwadaphnin and yuanhuacine were observed to inhibit DNA and protein synthesis in P-388 leukemic cells. A detailed study of the effects of these two diterpene esters on protein synthesis of leukemic cells was undertaken. The major effects of genkwadaphnin and yuanhuacine on protein synthesis were blockage of the elongation process and interference with the peptidyl transferase reaction. The latter reaction was suppressed at concentrations of the diterpene esters which were commensurate with concentrations that inhibited whole cell in vitro protein synthesis in P-388 cells.

Antitumor agents XLVIII: Structure-activity relationships of quassinoids as in vitro protein synthesis inhibitors of P-388 lymphocytic leukemia tumor cell metabolism

A series of brusatol, bisbrusatol, and bruceantin esters were examined for their ability to inhibit protein synthesis in P-388 lymphocytic leukemia cells. Compounds which produced high T/C % values (170-272) resulted in ID50 of 5.4-15.5 microM for inhibition of whole cell protein synthesis, ID50 of 1.3-13 microM for inhibition of endogenous protein synthesis in cell homogenates, and ID50 of 1.9-6 microM for inhibition of polyuridine directed polyphenylalanine synthesis using "runoff" ribosomes and a "pH 5" enzyme preparation. The polyuridine directed polyphenylalanine synthesis requires neither initiation nor termination factors, suggesting that quassinoids are exclusively elongation inhibitors. Bruceantin, brusatol, and bisbrusatolyl malonate allowed a runoff of the polyribosomes to 80S free ribosomes. However, formation of the ternary complex and 80S initiation complex were not inhibited by the quassinoids. Thus, these agents do not affect the individual steps leading to the formation of a stable 80S initiation complex in P-388 cells. Brusatol, bruceantin, and bisbrusatolyl malonate inhibited the formation of the first peptide bond between puromycin and [3H]methionyl-transfer RNA bound to the initiation complex, indicating peptidyl transferase activity is inhibited by the quassinoids in P-388 cells. These studies also suggest that the free 80S ribosome is the site of binding by the quassinoid. Ribosomes actively conducting protein synthesis will continue protein synthesis and terminate before the quassinoids bind. This proves quassinoids are elongation inhibitors of tumor cells. A strong correlation was observed between potent antileukemic activity and the ability to inhibit protein synthesis in P-388 lymphocytic leukemia cells.

Antitumor agents XLVII: The effects of bisbrusatolyl malonate on P-388 lymphocytic leukemia cell metabolism

Bisbrusatolyl malonate, which was shown previously to be active against P-388 lymphocytic leukemia cell growth, was investigated for inhibitory effects on nucleic acid and protein synthesis. DNA and RNA synthesis as well as protein synthesis were markedly inhibited at 10,25, and 50 mu mole final concentrations in vitro. The major sites of inhibition of nucleic acid synthesis appeared to be DNA polymerase, messenger and transfer RNA polymerases, orotidine-5'-monophosphate decarboxylase, phosphoribosyl pyrophosphate amino transferase, and dihydrofolate reductase. Moderate inhibition of nucleotide kinase activities and oxidative phosphorylation processes occurred after drug treatment. Cyclic adenosine monophosphate levels were reduced. Protein synthesis was inhibited during the elongation step of peptide synthesis. The data suggested that bisbrusatolyl malonate interfered with the peptide bond formation. However, the ongoing polypeptide synthesis must be completed before the drug can bind to the ribosome effectively.

Mechanism of eukaryotic protein synthesis inhibition by brusatol

The mechanism by which brusatol inhibits protein synthesis in rabbit reticulocytes has been investigated. When added to reticulocyte lysates, brusatol inhibits endogenous protein synthesis only after a lag of 2-4 min at 30 degrees C. During this period 80 S ribosomes accumulate. Brusatol is equally effective in inhibiting endogenous protein synthesis in lysates and poly(U)-directed polyphenylalanine synthesis with runoff ribosomes. In fractionated reticulocyte systems, brusatol does not inhibit formation of the ternary, 40 S, and 80 S initiation complexes, but does inhibit the reaction of puromycin with initiation complexes containing [35S]Met-tRNAf. These data suggest that brusatol inhibits the peptidyl transferase elongation reaction of protein synthesis, but can do so only after one round of protein synthesis has been completed. Thus, the mechanism of action of brusatol in the rabbit reticulocyte system is very similar to the effects previously reported for bruceantin in a yeast system.