Aphidicolin, a specific inhibitor of DNA polymerase alpha, inhibits conversion of lanosterol to C-27 sterols in mouse L cells

Aphidicolin synchronization of mouse L cells perturbs the relationship between cell killing and DNA double-strand breakage after X-irradiation

Mouse L cells were synchronized in early S-phase by two 12 h incubations in medium containing aphidicolin (2 micrograms/ml), separated by 8 h in drug-free medium. The relationship between X-ray-induced cell killing and DNA double-strand breakage was then examined for cells that had entered S-phase, G2-phase, mitosis, and G1-phase following release from aphidicolin and was compared to the response of asynchronous cultures. Aphidicolin-synchronized cells showed cycle phase-dependent changes in their dose-responses for both killing and DNA dsb. However, on the basis of the level of DNA dsb per unit length of DNA required to produce a lethal lesion, aphidicolin-synchronized cells were more sensitive to X-rays than were asynchronous cultures. This sensitivity peaked 2 h after release from aphidicolin treatment and then progressively declined towards the asynchronous culture value. It is argued that these results are due to deregulation of the temporal order of DNA replication following aphidicolin treatment, and can be incorporated into the critical DNA target size model (Radford, Hodgson, and Matthews, in preparation) by postulating that the targets for radiation action in mammalian cells are DNA-associated with potentially transcriptionally active proto-oncogenes or constitutive fragile sites.

Inhibition of cholesterol biosynthesis in 3T3 fibroblasts by 2-aza-2,3-dihydrosqualene, a rationally designed 2,3-oxidosqualene cyclase inhibitor

The effect of 2-aza-2,3-dihydrosqualene, a new compound designed to inhibit the 2,3-oxidosqualene-lanosterol cyclase [A. Duriatti et al., Biochem. Pharmac. 34, 2765 (1985)] was studied as inhibitor of cholesterol biosynthesis in Swiss 3T3 fibroblasts. Treatment with the drug of cells which were grown for 2 days in a delipidated medium resulted in a marked decrease of [14C]acetate incorporation into the C27-sterol fraction. An IC50 = 0.3 microM was calculated when the cells were preincubated for a period of 4 hr with 2-aza-2,3-dihydrosqualene. This inhibition was correlated with an intracellular accumulation of 2,3-[14C]oxidosqualene and of 2,3:22,23-[14C]dioxidosqualene, indicating that the cyclase was indeed an intracellular target of the drug. A precursor-product relationship of the accumulated [14C]squalene oxide(s) and the [14C]sterols was demonstrated in chase experiments in the absence of drug. Sterols more polar than cholesterol were also detected in treated fibroblasts and in the cells which underwent chase experiments; they were mainly composed of 24,25-epoxycholesterol. The C27-[14C]sterols of [14C]acetate pulse labeled cells consisted in a mixture of desmosterol and cholesterol; treatment of the cells with 2-aza-2,3-dihydrosqualene resulted in a decreased conversion of desmosterol into cholesterol indicating that the delta 24-sterol reductase might be another target of the drug. 2-Aza-2,3-dihydrosqualene at 1 microM affected normal growth of 3T3 fibroblasts, this effect could be prevented by addition of exogeneous cholesterol (50 microM). Growth arrest of the treated cells was correlated with a decrease in cellular sterol content to less than 40% of controls. About 30% of the C27-sterol fraction, of the treated cells, was desmosterol. Our work demonstrates that 2-aza-2,3-dihydrosqualene is a valuable new inhibitor of cholesterol biosynthesis in mammalian cells.

Biochemical characterization of the cholesterol-dependent growth of the NS-1 mouse myeloma cell line

The biochemical basis for the cholesterol-dependent growth phenotype of the NS-1 myeloma cell line has been investigated. In one series of experiments, the growth response of NS-1 cells to several of the intermediates of cholesterol biosynthesis was studied in serum-free medium. The cholesterol precursors, squalene and lanosterol, were totally ineffective in promoting NS-1 cell growth. In contrast, cholesterol precursors downstream from lanosterol, i.e., desmosterol and 7-dehydrocholesterol, completely replaced cholesterol in supporting NS-1 cell growth. In a second series of experiments, NS-1 cells and NS-1-503 cells (a cholesterol growth-independent variant of NS-1 cells) were labelled with [2-14C]acetate and the distributions of radioactivity between cholesterol and its precursors were determined by thin-layer chromatography using two different solvent systems. The major labelled sterol product (greater than 80%) in NS-1 cells after a 24-h exposure to [2-14C]acetate was lanosterol. In contrast, the major labelled sterol product (greater than 95%) in NS-1-503 cells after a 24-h exposure to [2-14C]acetate was cholesterol. Taken together, these results indicate that NS-1 cells are defective in cholesterol biosynthesis and identify the site of lesion as the demethylation of lanosterol to C-29 sterol intermediates.

Differentiation of neuroblastoma cells induced by an inhibitor of mevalonate synthesis: relation of neurite outgrowth and acetylcholinesterase activity to changes in cell proliferation and blocked isoprenoid synthesis

Mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, stimulates neurite outgrowth and acetylcholinesterase (ACE) activity in C1300 (Neuro-2A) murine neuroblastoma cells. Sprouting of neurites began within 4-8 h, before changes in cell proliferation could be detected by [3H]thymidine incorporation or flow cytometry. In contrast, the increase in ACE activity was temporally correlated with suppression of DNA synthesis, which occurred after 8 h. The activity of the membrane marker enzyme phosphodiesterase I was not stimulated by mevinolin. Suppression of protein synthesis with cycloheximide blocked the induction of ACE activity but only partially inhibited neurite outgrowth in the mevinolin-treated cultures. When mevinolin was removed from the culture medium, most of the cells retracted their neurites within 2 h, but ACE activity did not decline until DNA synthesis began to return to control levels after 10 h. Similarly, retraction of neurites in differentiated cells exposed to colchicine was not accompanied by a decrease in ACE activity. DNA histograms suggested that mevinolin arrests neuroblastoma cells in both the G1 and G2/M compartments of the cell cycle. Other cytostatic drugs that arrest cells at different stages of the cell cycle did not cause Neuro-2A cells to form neurites such as those seen in the mevinolin-treated cultures. When incorporation of [3H]acetate into isoprenoid compounds was studied in cultures containing mevinolin in concentrations ranging from 0.25 microM to 25 microM, the labeling of cholesterol, dolichol, and ubiquinone was suppressed by 90% or more at all concentrations. However, significant growth arrest and cell differentiation were observed only at the highest concentrations of mevinolin. Supplementing the medium with 100 microM mevalonate prevented the cellular response to mevinolin, but additions of cholesterol, dolichol, ubiquinone, or isopentenyl adenine were generally ineffective. The cholesterol content of neuroblastoma cells incubated with 25 microM mevinolin for 24 h was not diminished, and protein glycosylation, measured by [3H]mannose incorporation, was decreased only after 24 h at high mevinolin concentration. These studies suggest that the stimulation of neurite outgrowth and the increase in ACE activity induced by mevinolin are independent phenomena. Whereas neurite outgrowth is not related directly to the effects of mevinolin on cell cycling, the induction of ACE is correlated with the inhibition of cell proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)