Colchicine inhibits hepatic cholesterol synthesis and microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA reductase) activity

The effect of colchicine on cholesterol processing by the progesterone-producing cells of the luteinized ovary

Previously, we indicated that luteal cells from colchicine-treated superovulatory (luteinized) rats show decreased capacity for progesterone production. The current study investigates the possibility that colchicine exerts this effect by interfering with the mechanism by which cholesterol is processed and/or synthesized by luteal cells. To this end, animals were treated with saline or colchicine after which the luteinized ovary or isolated luteal cells were assayed for their cholesterol content, their ability to synthesize cholesterol endogenously, or their ability to utilize lipoprotein-delivered cholesterol for the production of progesterone. The results show that animals treated with colchicine show a number of changes in luteal cell cholesterol metabolism: namely a 60% decline in stored cholesterol, a 3-fold rise in the activity of the cholesterol synthesizing enzyme HMG CoA reductase (although no change occurs in other cholesterol metabolizing enzymes), and a 3-fold rise in the capacity of the cells to incorporate precursor [14C]acetate into cholesterol. On the other hand, cells of animals treated with colchicine or cells treated with colchicine under in vitro circumstances are unable to fully utilize cholesterol provided by high density lipoproteins (HDL): this occurs despite the fact that the binding of HDL particles to luteal cells is quite normal after colchicine treatment. These findings are consistent with the view that a primary effect of colchicine on luteal cell progesterone production is in preventing the normal uptake of HDL-cholesterol.

Effects of antimicrotubule agents on phospholipid metabolism in rat hepatic subcellular membranes

Treatment of animals with antimicrotubule drugs has been shown to cause a perplexing variety of cellular changes which, theoretically, could be the result of changes in endomembrane biosynthesis, composition or flow. In the current study we have focused on this possibility by identifying antimicrotubule drug-induced changes in the phospholipid metabolism of hepatic subcellular membranes. Young adult rats were pretreated with radiolabeled [32 P]orthophosphate for 12 hr, and subsequently given saline, colchicine (2.5 mg/kg body wt) or vinblastine (20 mg/kg body wt) for 4 additional hr. Afterwards, the livers were homogenized, and separate microsomal and Golgi membrane fractions were prepared and subjected to phospholipid extraction and identification using two-dimensional thin-layer chromatography. The results show that colchicine and vinblastine given in vivo caused specific, rapid and in some cases, dramatic changes in phospholipid turnover in different membrane fractions of rat liver. The drugs specifically increased labeling of phosphatidylinositol-4-monophosphate and phosphatidylinositol-4,5-biphosphate and decreased the radioactivity associated with phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol in all fractions examined. In contrast, the antimicrotubule drugs produced a differential effect on the labeling pattern of sphingomyelin and lysophosphatidylcholine, i.e. they stimulated labeling of these phospholipids in microsomes, produced no changes in heavy Golgi fractions, and markedly increased their labeling in light Golgi fractions. These data suggest that antimicrotubule drugs restrict the incorporation of certain precursor phospholipids into forming membranes but do not affect the subsequent metabolism of these phospholipids. At the same time, the drugs appear to retard the flow of membranes from one cellular compartment to another.

An effect of colchicine on galactosyl- and sialyltransferases of rat liver Golgi membranes

Colchicine inhibited the activity of the galactosyl- and sialyltransferases of rat liver Golgi membranes. The sialyltransferase was more sensitive to the drug than galactosyltransferase since it was inhibited to a greater extent and at lower concentrations of colchicine than the galactosyltransferase. Two soluble enzymes, i.e. that from rat serum and that isolated from bovine milk, were not inhibited by colchicine. Even with very high concentrations of colchicine a marked stimulation of activity was observed. The data suggest that the inhibition observed in the Golgi membranes is in some way related to the arrangement of the enzymes in the lipid bilayer. In support of this hypothesis, the milk galactosyltransferase became very sensitive to colchicine after incorporation of the enzyme into lipid vesicles. The incorporation of colchicine into Golgi membranes was shown to decrease the order parameter as determined by electron spin resonance which reflects an increased fluidity of the Golgi membranes. A change in fluidity may be responsible for the inhibition of enzyme activity at least in part.

Effects of microtubule-disruptive and membrane-stabilizing agents on low density lipoprotein metabolism by cultured human fibroblasts

The cellular mechanisms involved in the uptake and metabolism of low density lipoprotein (LDL) by cultured normal human fibroblasts have been investigated with the aid of drugs known to disrupt cytoplasmic microtubules or to inhibit membrane fusion. Two drugs which disrupt microtubules by differing mechanisms, colchicine and vinblastine, each reduced the high affinity surface binding of 125I-labelled LDL by fibroblasts. Associated reductions of the endocytosis and degradation of the lipoprotein could be attributed almost entirely to this effect. In contrast, lumicolchicine, an analogue of colchicine without microtubule-disruptive activity, had little or no effect on 125I-labelled LDL metabolism. Each of two groups of membrane-stabilizing agents, the phenothiazines and the tertiary amine local anaesthetics, directly inhibited both the internalization of 125I-labelled LDL following high affinity binding to cell surface receptors and the catabolism of the lipoprotein subsequent to endocytosis, supporting previous morphological evidence for the importance of membrane fusion in these processes.

A sensitive enzymatic method for determination of free and esterified tissue cholesterol

An enzymatic assay currently in use in clinical laboratories for quantitating total serum cholesterol has been modified for the determination of microgram quantities of tissue cholesterol. Two types of assays were developed. In the first, more generally useful method, lipids extracted with chloroform/methanol were solubilized by the addition of detergent and assayed for free and total cholesterol using laboratory prepared mixtures of reagents. Although different detergents were effective, it appears that Triton X-100 may be most universally applicable. In the second type of assay, commercially available reagent mixtures were employed for the determination of total cholesterol only. Results of both types of assays compare favorably with determination using the colorimetric assay on 3beta-hydroxysterols recovered from digitonides.