Letter: Role of microtubules in lipoprotein secretion by the liver

Colchicine's effects on lipoprotein particle concentrations in adults with metabolic syndrome: A secondary analysis of a randomized controlled trial

BACKGROUND

Colchicine has received renewed interest for its potential beneficial effects in secondary prevention of cardiovascular disease. This was presumed to be primarily because of its anti-inflammatory effects; however, limited data exist regarding colchicine's impact on other cardiovascular risk factors.

OBJECTIVE

The aim of this study was to examine if colchicine's anti-inflammatory actions would lead to reduced circulating concentrations of oxidized low-density lipoprotein (oxLDL) in metabolically unhealthy individuals. We also examined if colchicine would improve concentrations of other atherogenic lipoprotein subfractions.

METHODS

This is a secondary analysis of a double-blind, randomized, placebo-controlled pilot study in which 40 adults with metabolic syndrome were randomized to colchicine 0.6 mg or placebo twice daily for 3 months. Blood samples were collected in the fasted state. OxLDL was measured using enzyme-linked immunosorbent assay. Nuclear magnetic resonance spectroscopy was used to measure other lipoprotein particle subfraction concentrations.

RESULTS

Compared with placebo, colchicine reduced markers of inflammation, including C-reactive protein, erythrocyte sedimentation rate, and GlycA (P < .01). Concentrations of oxLDL (P = .019) and small LDL (P = .022) appeared significantly increased in the colchicine arm. Colchicine had no significant effect on other lipoprotein subfractions or lipoprotein particle sizes (all P > .05).

CONCLUSION

Although colchicine may have benefit in secondary prevention of cardiovascular disease in at-risk individuals, we found no evidence that these effects are because of improvements in circulating atherogenic lipoprotein particle concentrations. Further studies are needed to confirm whether colchicine increases circulating oxLDL and small LDL levels in adults with metabolic syndrome. If true, additional research is warranted to elucidate the mechanisms underlying these associations.

Ouabain-induced cytoplasmic vesicles and their role in cell volume maintenance

Cellular swelling is controlled by an active mechanism of cell volume regulation driven by a Na(+)/K(+)-dependent ATPase and by aquaporins which translocate water along the osmotic gradient. Na(+)/K(+)-pump may be blocked by ouabain, a digitalic derivative, by inhibition of ATP, or by drastic ion alterations of extracellular fluid. However, it has been observed that some tissues are still able to control their volume despite the presence of ouabain, suggesting the existence of other mechanisms of cell volume control. In 1977, by correlating electron microscopy observation with ion and water composition of liver slices incubated in different metabolic conditions in the presence or absence of ouabain, we observed that hepatocytes were able to control their volume extruding water and recovering ion composition in the presence of ouabain. In particular, hepatocytes were able to sequester ions and water in intracellular vesicles and then secrete them at the bile canaliculus pole. We named this "vesicular mechanism of cell volume control." Afterward, this mechanism has been confirmed by us and other laboratories in several mammalian tissues. This review summarizes evidences regarding this mechanism, problems that are still pending, and questions that need to be answered. Finally, we shortly review the importance of cell volume control in some human pathological conditions.

Effects of medium calcium, and agents affecting cytoskeletal function, on cellular volume and morphology in liver tissue in vitro

The possible role of an exocytotic, vesicular mechanism in cellular volume regulation under iso-osmotic conditions has been studied in slices of rat liver. The effects of incubation conditions and agents affecting the actin cytoskeleton were examined for changes of water, ionic composition, and ultrastructure. Slices were pre-incubated at 1°C in an iso-osmotic buffered medium to induce swelling. Upon restoration to 37°C in the same medium, tissue lost water. The Na+-K+ adenosine triphosphatase (ATPase) inhibitor ouabain inhibited water extrusion of about 50%, an effect that was accompanied by the formation of characteristic vesicles in the cytoplasmic region between the Golgi apparatus and the bile canaliculi. Water extrusion in the presence of ouabain was partially inhibited by trifluoroperazine and completely inhibited when the medium was free of Ca2+. In the presence of ouabain, brefeldin A caused a small reduction of water extrusion, whereas phalloidin and cytochalasins A, D, or E caused a marked inhibition. In these conditions there was a marked increase in size and number of cytoplasmic vesicles and a more widespread distribution of them within the cells, lacking the more specific orientation to the Golgi and canalicular regions that was seen in the presence of ouabain alone. Water extrusion was inhibited by phalloidin and cytochalasins in the absence of ouabain. In conclusion, our results are consistent with the hypothesis that iso-osmotic expulsion of water from hepatocytes can proceed partly through an accumulation of water in cytoplasmic vesicles, followed by exocytosis. This mechanism does not depend on Na+-K+ ATPase activity.

Lipid aldehyde-mediated cross-linking of apolipoprotein B-100 inhibits secretion from HepG2 cells

Hepatic oxidative stress and lipid peroxidation are common features of several prevalent disease states, including alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD), a common component of the metabolic syndrome. These conditions are characterized in part by excessive accumulation of lipids within hepatocytes, which can lead to autocatalytic degradation of cellular lipids giving rise to electrophilic end products of lipid peroxidation. The pathobiology of reactive lipid aldehydes remains poorly understood. We therefore sought to investigate the effects of 4-hydroxynonenal (4-HNE) and 4-oxononenal (4-ONE) on the transport and secretion of very low-density lipoprotein using HepG2 cells as a model hepatocyte system. Physiologically relevant concentrations of 4-HNE and 4-ONE rapidly disrupted cellular microtubules in a concentration-dependent manner. Interestingly, 4-ONE reduced apolipoprotein B-100 (ApoB) secretion while 4-HNE did not significantly impair secretion. Both 4-HNE and 4-ONE formed adducts with ApoB protein, but 4-HNE adducts were detectable as mono-adducts, while 4-ONE adducts were present as protein-protein cross-links. These results demonstrate that reactive aldehydes generated by lipid peroxidation can differ in their biological effects, and that these differences can be mechanistically explained by the structures of the protein adducts formed.

Alpha-substituted 1-aryl-3-dimethylaminopropanone hydrochlorides: potent cytotoxins towards human WiDr colon cancer cells

A series of 1-aryl-2-dimethylaminomethyl-2-propenone hydrochlorides 1 were prepared which possessed IC(50) values of less than 10 microM when examined towards human WiDr colon cancer cells. The related 1-aryl-2-dimethylaminomethyl-3-hydroxypropanone hydrochlorides 2, formed by hydration of the analogs in series 1, also had IC(50) values in the low micromolar range. On the other hand, conversion of 2-dimethylaminomethyl-1-(4-nitrophenyl)-2-propenone hydrochloride 1c into the corresponding 2-mercaptoethanol of adduct 3c led to a 37-fold reduction in potency. Two thirds of the compounds prepared in this study were more potent than a reference drug cisplatin while one third of these molecules displayed greater cytotoxicity to the WiDr cells than human CRL-2522 fibroblasts. A stability study of the 4-nitrophenyl analog in each of the series 1-3 in deuterium oxide was undertaken. In the case of 1c, replacement of the dimethylamino hydrochloride group by a hydroxy function was noted while in series 2, the loss of both water and dimethylamine hydrochloride gave rise to a mixture of two enones. The mercaptoethanol adduct 3c underwent deamination. The data obtained provide guidelines for amplifying the project in the future.

Modification of the composition of polycystin-1 multiprotein complexes by calcium and tyrosine phosphorylation

Mutations in the PKD1 gene are responsible for >85% of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1, polycystin-1, is a large, modular membrane protein, with putative ligand-binding motifs in the extracelluar N-terminal portion, 9-11 transmembrane domains and an intracellular C-terminal portion with phosphorylation sites. A role for polycystin-1 as a cell surface receptor involved in cell-matrix and cell-cell interactions has been proposed. In this study, we have analyzed polycystin-1 and associated protein distribution in normal human epithelial cells and examined the role of cell-matrix versus cell-cell interactions in regulation of the assembly of polycystin-1 multiprotein complexes. Immunocytochemistry, sucrose density gradient sedimentation, co-immunoprecipitation analyses and in vitro binding assays have shown that polycystin-1 associates with the focal adhesion proteins talin, vinculin, p130Cas, FAK, alpha-actinin, paxillin and pp60c-src in subconfluent normal human fetal collecting tubule (HFCT) epithelia when cell-matrix interactions predominate. Polycystin-1 also forms higher S value complexes with the cell-cell adherens junction proteins E-cadherin, beta- and gamma-catenins in confluent cultures when cell-cell interactions are predominant. Polycystin-1 multiprotein complexes can be disrupted by cytochalasin D but not by colchicine, suggesting involvement of the actin cytoskeleton. Although inhibition of tyrosine phosphorylation by tyrphostin inhibits polycystin-1-FAK interactions, E-cadherin interactions are enhanced. High calcium treatment also increases polycystin-1-E-cadherin interactions.

Determination of the structure of chiral molecules using ab initio vibrational circular dichroism spectroscopy

We discuss the theoretical prediction of vibrational circular dichroism (VCD) spectra using ab initio density functional theory (DFT) and the application of this methodology to the determination of the absolute configurations and conformations of chiral molecules.

PCB 118 induces ultrastructural alterations in the rat liver

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that bioaccumulate in the food chain and thus pose a health risk to humans and other animals. In this study, PCB 118 was added to the diets of Sprague Dawley rats for 13 weeks in concentrations of 2, 20, 200, 2000 p.p.b. to the females and 10, 100, 1000 and 10000 p.p.b. to the males. The chemical was dissolved in corn oil; animals that served as the control received corn oil in the diets devoid of PCB. Use of transmission electron microscopy and stereology revealed significant (P<0.05) elevation in the mean volume fraction of smooth reticulum profiles (20 p.p.b.), peroxisomes (200, 2000 p.p.b.) and lipid droplets (2000 p.p.b.) in the females. Hepatocytes from the males exhibited a significant increase in the mean volume fraction of lipid droplets at 10000 p.p. b. (P<0.05). Interactions between large quantity of estrogen and the PCB probably would account for more profound alterations in the liver of female Sprague-Dawley rats than in the males.

MOLECULAR AND CELLULAR ASPECTS OF THE ARBUSCULAR MYCORRHIZAL SYMBIOSIS

Arbuscular mycorrhizae are symbiotic associations formed between a wide range of plant species including angiosperms, gymnosperms, pteridophytes, and some bryophytes, and a limited range of fungi belonging to a single order, the Glomales. The symbiosis develops in the plant roots where the fungus colonizes the apoplast and cells of the cortex to access carbon supplied by the plant. The fungal contribution to the symbiosis is complex, but a major aspect includes the transfer of mineral nutrients, particularly phosphate from the soil to the plant. Development of this highly compatible association requires the coordinate molecular and cellular differentiation of both symbionts to form specialized interfaces over which bi-directional nutrient transfer occurs. Recent insights into the molecular events underlying these aspects of the symbiosis are discussed.

Redistribution and fate of colchicine-induced alkaline phosphatase in rat hepatocytes: possible formation of autophagosomes whose membrane is derived from excess plasma membrane

The redistribution and fate of colchicine-induced alkaline phosphatase (ALPase) in rat hepatocytes were investigated by electron microscopic enzyme cytochemistry and biochemistry. ALPase activity markedly increased in rat hepatocytes after colchicine treatment (2.0 mg/kg body weight, intraperitoneal injection). At 20-24 h after colchicine treatment, the liver showed the highest activity of ALPase. Thereafter, ALPase activity decreased and returned to normal levels at 48 h. In normal hepatocytes from control rats, ALPase activity was seen only on the bile canalicular membrane. However, at 20-24 h after colchicine treatment, colchicine-induced ALPase was redistributed in the sinusoidal and lateral (basolateral) membranes as well as in the bile canalicular membrane. At 30-36 h after colchicine treatment, ALPase activity on the basolateral membrane gradually decreased. In contrast, ALPase in the bile canalicular membrane increased along with the enlargement of bile canaliculi, suggesting that ALPase in the basolateral membrane had been transported to the bile canalicular membrane. Furthermore, ALPase-positive vesicles, cisternae and autophagosome-like structures were frequently seen in the cytoplasm. ALPase was also positive in some lysosomal membranes. ALPase in hepatocytes at 48 h after colchicine treatment returned to almost the same location as in control hepatocytes. Altogether, it is suggested that excessively induced ALPase is at least partially retrieved by invagination of the bile canalicular membrane and then transported to lysosomes for degradation. In addition, this study indicates that excess plasma membrane might be a possible origin of autophagosomal membrane.

The AQP2 water channel: effect of vasopressin treatment, microtubule disruption, and distribution in neonatal rats

Aquaporin 2 is a collecting duct water channel that is located in apical vesicles and in the apical plasma membrane of collecting duct principal cells. It shares 42% identity with the proximal tubule/thin descending limb water channel, CHIP28. The present study was aimed at addressing three questions concerning the location and behavior of the AQP2 protein under different conditions. First, does the AQP2 channel relocate to the apical membrane after vasopressin treatment? Our results show that AQP2 is diffusely distributed in cytoplasmic vesicles in collecting duct principal cells of homozygous Brattleboro rats that lack vasopressin. In rats injected with exogenous vasopressin, however, AQP2 became concentrated in the apical plasma membrane of principal cells, as determined by immunofluorescence and immunogold electron microscopy. This behavior is consistent with the idea that AQP2 is the vasopressin-sensitive water channel. Second, is the cellular location of AQP2 modified by microtubule disruption? In normal rats, AQP2 has a mainly apical and subapical location in principal cells, but in colchicine-treated rats, it is distributed on vesicles that are scattered throughout the entire cytoplasm. This is consistent with the dependence on microtubules of apical protein targeting in many cell types, and explains the inhibitory effect of microtubule disruption on the hydroosmotic response to vasopressin in sensitive epithelia, including the collecting duct. Third, is AQP2 present in neonatal rat kidneys? We show that AQP2 is abundant in principal cells from neonatal rats at all days after birth. The detection of AQP2 in early neonatal kidneys indicates that a lack of this protein is not responsible for the relatively weak urinary concentrating response to vasopressin seen in neonatal rats.

Serial quantitative image analysis and confocal microscopy of hepatic uptake, intracellular distribution and biliary secretion of a fluorescent bile acid analog in rat hepatocyte doublets

To characterize the poorly understood mechanisms of intracellular transport of bile acids, fluorescein isothiocyanate-glycocholate was synthesized and its ring-OH-linked structure established by fast atom bombardment, mass spectroscopy and 13C nuclear magnetic resonance. Biliary secretion of fluorescein isothiocyanate-glycocholate and [14C]-labeled glycocholate in rats was similar, in contrast to the biliary secretion of sodium fluorescein and methylamine-conjugated fluorescein isothiocyanate, which are non-bile acid organic anions. After incubation of cultured hepatocyte doublets with fluorescein isothiocyanate-glycocholate, serial quantitative image analysis of fluorescence was performed in the cellular cytoplasm, perinuclear zone and bile canaliculus. Uptake of fluorescein isothiocyanate-glycocholate into the cytoplasm was inhibited by removal of sodium from the medium and by addition of glycocholate or taurocholate. After preincubation with colchicine, but not lumicolchicine, the proportion of perinuclear to cytoplasmic fluorescein isothiocyanate-glycocholate increased during incubation with fluorescein isothiocyanate-glycocholate. Neither fluorescein isothiocyanate-glycocholate uptake nor canalicular secretion was affected. By confocal microscopy the perinuclear zone containing fluorescein isothiocyanate-glycocholate was identified as the Golgi apparatus by fluorescent colocalization with C6-NBD-ceramide, which specifically identifies the Golgi apparatus. We conclude that colchicine inhibits fluorescein isothiocyanate-glycocholate transfer from the Golgi apparatus to the bile canaliculus. These results suggest that intracellular bile acid transport involves microtubule-dependent vesicular movement from the Golgi apparatus to the bile canaliculus. The role of this process in normal physiology is uncertain.

Modulation of biliary lipid secretion by forskolin and cyclic AMP analogues

Exposure of isolated perfused rat livers to either 100 microM-forskolin, a potent activator of adenylate cyclase, or to 0.5 mM-concentrations of the cAMP analogues chlorophenylthio cAMP (CPTcAMP), dibutyryl cAMP (dbcAMP) and 8-bromo cAMP (8BrcAMP), to provoke increases in intracellular concentrations of cAMP, resulted in marked changes in bile volume and composition. Bile flow reached a peak after 10 min, before declining towards control levels, and an increase in several secretory parameters was also observed at this time. At 20 min, a substantial decrease in the output of both phospholipid and cholesterol was evident, and this suppression of secretion was maintained throughout the remainder of the experiment. The order of effectiveness of the cAMP-elevating agents at decreasing biliary lipid output was CPTcAMP greater than forskolin greater than dbcAMP greater than 8BrcAMP. Biliary output of bile acids was essentially unaltered compared with controls; similarly, no decrease in the secretion of protein and triacylglycerols into the perfusion medium was observed. This suggests that the elevation of intracellular levels of cAMP may cause a selective inhibition of biliary lipid output rather than a more general inhibition of hepatic secretion.

Colchicine-induced redistribution of an apical membrane glycoprotein (gp330) in proximal tubules

Factors governing the selective, polarized insertion of membrane proteins are poorly understood, but some studies have suggested that microtubules are involved in the generation and maintenance of cell polarity. We have examined by immunocytochemistry the effect of the microtubule-disrupting agent, colchicine, on the cellular distribution of an endogenous glycoprotein, gp330, which is normally inserted only into the apical plasma membrane of proximal tubule epithelial cells. In control rats, gp330 was localized in the brush border and in apical invaginations and vesicles. Six hours after injection of colchicine, however, vesicles containing gp330 were dispersed throughout the entire cytoplasm of the cell. Many vesicles were packed into basolateral infoldings, close to the plasma membrane, but there was no significant insertion of gp330 into the basolateral membrane. When rabbit anti-gp330 antiserum was injected intravenously into colchicine-treated rats, immune complexes appeared in the glomerular basement membrane but could not be detected in peritubular basement membranes. This supports the conclusion that colchicine treatment does not result in the insertion of gp330 into the basolateral plasma membrane of proximal tubule cells. Our results indicate that although microtubules are involved in the accumulation of gp330-containing vesicles at the apical pole of the cell, other factors must be required for fusion with the plasma membrane to occur.

Ampicillin inhibits the movement of biliary secretory vesicles in rat hepatocytes

A number of biliary secretory processes are inhibited by administration of ampicillin to isolated perfused rat livers. Reduction in output was observed for phospholipid, cholesterol, the endogenous protein rat serum albumin and the exogenous protein bovine serum albumin, whilst secretin of bile salts was virtually unaffected. All of the affected materials are secreted by processes involving vesicles which are brought to the appropriate pole of the hepatocyte, and the observed inhibitory effects of ampicillin may, therefore, possibly be due to a blockage in the transport of these substances. The effects of ampicillin were much less marked on materials secreted at the sinusoidal pole of the cell.

Sodium valproate inhibits the movement of secretory vesicles in rat hepatocytes

Sodium valproate (VPA), a simple 8-carbon branched chain fatty acid, is an effective anti-epileptic drug with an occasional serious side effect of liver damage, including the accumulation of triacylglycerols within hepatocytes, and reductions in serum protein concentrations. By investigating the effects of VPA, using biliary fistula rats and isolated perfused rat livers, we have shown that secretion of triacylglycerols and rat serum albumin at the sinusoidal pole of hepatocytes, and of phospholipids, lysosomal contents, and IgA at their biliary pole, are all reduced, to somewhat different extents, by acute VPA administration. In addition, the vesicular transcytosis of exogenous protein (i.e. bovine serum albumin) from the perfusion fluid into bile is also decreased by VPA administration. To determine whether the phenomena were specific to VPA, a control series of experiments was also performed using octanoate (a straight-chain analogue of VPA). With the biliary fistula rats, octanoate did not show inhibition of secretion as compared with the saline controls; with the isolated perfused livers, however, octanoate did show such an inhibition. These phenomena suggest that VPA inhibition of secretion may be a factor in its hepatotoxicity, as the effects are apparent in both the whole animal and the isolated perfused liver, whereas octanoate is not hepatotoxic in the whole animal. Since when octanoate is administered to the isolated liver it causes an inhibition in secretion similar to that caused by VPA, it may be that the large dose of this compound reaching the liver affects a key step in liver metabolism or vesicle transport under these circumstances. Since octanoate does not normally reach the liver in such amounts, as it will normally be metabolized by other tissues, it is not hepatotoxic in the whole animal as is VPA.

Microtubular protein impairment by pentanal and hexanal

Pentanal and hexanal are some of the aldehydes produced by lipid peroxidation, that causes damage to several subcellular structures. Lipoperoxidation products may directly attack cytoskeleton structures, the integrity of which is required for secretion mechanisms, e.g. 4-hydroxy-alkenals after microtubular integrity and function. Purified microtubular protein incubated with pentanal and hexanal at different concentrations revealed a tubulin-aldehyde interaction affecting the polymerization reaction and the colchicine-binding activity. These reactions apparently do not involve sulphydryl groups, and the addition of mercaptoethanol does not protect microtubules from the action of aldehydes, the effect of which is however more homogeneous, as only small differences can be noticed among the various aldehyde concentrations used.

The role of microtubules in chlorinated alkane-induced fatty liver

The effect of carbon tetrachloride (CCl4) and methylene chloride (MC) on the hepatic microtubular system and its relationship to the development of fatty liver were examined. Hepatic microtubule content was assessed using a specific colchicine-binding assay. These studies demonstrated that these 2 chlorinated methanes had no early effect on hepatic microtubule content. A significant decrease in microtubules was not observed until after the development of the fatty liver. In vitro tubulin polymerization studies demonstrated that neither CCl4 nor MC had a significant effect on lag-time before tubulin assembly, rate of assembly or the maximum amount of polymerization achieved. It is concluded that an impairment in hepatic microtubule assembly is not responsible for the development of the chlorinated alkane-induced fatty liver.

The effect of colchicine on the development of lithocholic acid-induced cholestasis. A study of the role of microtubules in intracellular cholesterol transport

The pathogenesis of lithocholic acid (LCA-Na)-induced cholestasis involves a rapid accumulation of cholesterol in the bile canalicular membrane. Since microtubules play an important role in the intracellular transport of many materials, including cholesterol, the present study was undertaken to assess the extent to which they participate in the development of LCA-Na-induced cholestasis. Rats were pretreated with either colchicine (0.2 mumol/100 g body wt.) or saline solution 90 min before injection with LCA-Na (12 mumol/100 g body wt.). Colchicine, although not increasing bile flow by itself, significantly reduced the cholestasis caused by LCA-Na (57-32% reduction in bile flow) without affecting its metabolism into less toxic bile acids or its distribution in blood, liver or bile. Bile canalicular membranes isolated from animals treated with a combination of colchicine and LCA-Na contained less cholesterol than those treated with LCA-Na alone. However, membranes obtained from rats treated with colchicine alone contained much less cholesterol than did controls. It was found that the total amount of cholesterol accumulated within the bile canalicular membrane following LCA-Na treatment (LCA-Na + colchicine versus colchicine alone compared with LCA-Na versus controls) was unchanged by colchicine treatment. In view of these findings it is suggested that the total amount of cholesterol present within the bile canalicular membrane determines the extent of LCA-Na-induced cholestasis, LCA-Na probably moves cholesterol to the bile canalicular membrane via a microtubule independent pathway, and microtubules are unlikely to function in the transcellular transport of LCA-Na.

4-Hydroxy-alkenals interaction with purified microtubular protein

4-Hydroxy-alkenals effect on microtubular system has been investigated, comparing the activity of both biogenic and non-biogenic aldehydes. As these aldehydes react essentially with sulphydryl groups, their action on titratable thiol groups microtubular protein was studied. These compounds evidenced an inhibitory effect on this parameter and on tubulin polymerization, where sulphydryls are essential. 4-Hydroxy-alkenals inhibit tubulin polymerization in a dose-dependent manner (0.1-1 mM), with the exception of 4-hydroxy-octenal, that shows an inhibitory action only in concentrations from 0.5 mM up. Its behaviour is very anomalous: in fact the tubulin-colchicine binding, is stimulated rather than inhibited, whereas such binding is inhibited by the other tested aldehydes. Our present results give then a support for an interaction between 4-hydroxy-alkenals and microtubular protein.

Transcytosis and paracellular movements of horseradish peroxidase across liver parenchymal tissue from blood to bile. Effects of alpha-naphthylisothiocyanate and colchicine

The pathways for the entry of horseradish peroxidase (HRP) into bile have been investigated using the isolated perfused rat liver operating under one-pass conditions. Following a 1 min one-pass infusion of HRP, two peaks of HRP activity were noted in the bile. The first, at 5-7 min post-infusion, correlated with the biliary secretion of the [3H]methoxyinulin which was infused simultaneously with the HRP. The second peak of HRP activity occurred at 20-25 min, and correlated with the biliary secretion of 125I-IgA, which was also infused simultaneously with the HRP. If the isolated livers were perfused with a medium containing 2.5 microM-colchicine, the biliary secretion of IgA and the second secretion peak of HRP were inhibited by 60%. If rats were pretreated for 12h with alpha-naphthylisothiocyanate (25mg/100g body wt.) prior to liver isolation, the biliary secretion of [3H]methoxyinulin and the first secretion peak of HRP were increased. Taken together, these results suggest that HRP enters the bile via two routes. The faster route, which was increased by alpha-naphthylisothiocyanate and correlated with [3H]methoxyinulin entry into bile, was probably paracellular, involving diffusion across tight junctions. The slower route, which was inhibited by colchicine and correlated with the secretion of IgA, was probably due to transcytosis, possibly within IgA and other transport vesicles.

The effects of colchicine on secretion into bile of bile salts, phospholipids, cholesterol and plasma membrane enzymes: bile salts are secreted unaccompanied by phospholipids and cholesterol

Colchicine, a drug which interferes with microtubular function, has no effect on the secretion of taurodehydrocholate into bile; it is therefore suggested that bile salts are unlikely to be packaged in vesicles during cellular transit from sinusoidal to canalicular membranes. Colchicine greatly reduces the secretion of phospholipid and cholesterol into bile; it is suggested that this is due to an interruption in the supply of vesicles bringing lipids to repair the canalicular membrane during bile salt output. In the absence of the protective effect of a continuous supply of repair vesicles, micelleforming bile salts damage the canalicular membrane; the increased concentration of plasma membrane enzymes in bile and the increased aspartate aminotransferase activity in plasma and bile are evidence of this damage. Damage to the canalicular membrane may also be an explanation for the reduction in taurocholate transport and the taurocholate-induced cholestasis which are seen with colchicine-treated livers. Such membrane damage is not observed in colchicine-treated livers during the secretion of the non-micelle forming bile salt, taurodehydrocholate.

Abnormal secretion of proteins into bile from colchicine-treated isolated perfused rat livers

The microtubule poison, colchicine, caused an abnormal output of a variety of proteins into rat bile. After 3 h of exposure to the drug, livers were isolated and perfused with media of defined protein composition. There was no essential change in permeability of the hepatobiliary system to proteins (e.g. bovine serum albumin) entering bile from the perfusion fluid. The rat (serum) albumin and fibrinogen that were secreted into bile from colchicine-treated livers were probably derived from the hepatocytes. Disruption of the microtubular system reduces the secretion of proteins at the sinusoidal face of the hepatocyte and results in an accumulation of secretory vesicles in the cytoplasm. It is suggested that under these conditions some of the vesicles discharge their contents into the bile canaliculus.

An electron microscopic study of the effects of vinblastine sulphate on the ultrastructure of the kidney, trachea, liver, peripheral nerve and small intestine of the rat

The effects of vinblastine in high doses on the kidney, liver, trachea, peripheral nerve and small intestine of rats have been studied by scanning and transmission electron microscopy. Dramatic structural changes were seen in glomerular visceral epithelial cells, hepatocytes and small intestinal epithelial cells which appear to be due to the effect of vinblastine on microtubules and possibly also on other proteins and cell membranes. Ciliary microtubules appeared resistant to vinblastine effects.

In vivo effects of colchicine on milk fat globule membrane

Milk secretion in lactating goats was suppressed reversibly by infusing colchicine (2.5 to 5 mg) into one half of the udder via the teat canal. Fat globules were isolated from milks before, during and after (96 h post-infusion) this suppression. Protein, phospholipid, cholesterol (free and esterified), 5'-nucleotidase activity and peptide patterns by gel electrophoresis of these globule samples were determined. Association of [14C]colchicine with milk fat globules in vivo and in vitro also was investigated. Amounts of protein, phospholipid and free cholesterol per g of globule and 5'-nucleotidase per mg of globule protein fall following colchicine infusion. The nature of these changes suggests that the supply of membrane for milk secretion is restricted as a result of the drug treatment. Patterns of globule peptides by gel electrophoresis were qualitatively similar during the experimental period. However, a major globule glycoprotein, Mr = 52 000, showed a significant (3-fold) increase relative to the other principal peptide bands during the period of reduced milk flow. Analysis of milks for radioactivity following infusion of [14C]colchicine revealed that a portion of activity returning in milk is associated with fat globules. This activity peaked at 72 h post-infusion. Evaluation of [14C]colchicine binding to milk fat globules in vitro yielded evidence that the drug binds to the cytoplasmic, but not the exterior surface of the globule membrane. Colchicine's inhibition of milk synthesis and secretion is discussed.

The effects of colchicine on the distribution of glycoprotein-containing vesicles in epithelial cells of the murine colon

In murine colonic epithelial cells, cell-coat glycoproteins are transported to the cell surface in vesicles that originate at the Golgi apparatus. To determine the role of microtubules in the movement of these vesicles the antimicrotubule agent colchicine was injected into mice at several time intervals prior to sacrifice. In the mice that were treated with colchicine for 4.5 h it was observed that the polarity of the cells was disturbed. The Golgi apparatus and nucleus often appeared interchanged in their positions. The glycoprotein-containing vesicles, normally located apically, were sparse in that location, but abundant near the lateral plasma membranes of the cells at the level of the nucleus and Golgi apparatus. Straining by the periodic acid-chromic acid-silver methenamine technique for glycoproteins clearly revealed the reduction of vesicles apically and accumulation of vesicles laterally. The mechanism responsible for the movement of the vesicles to this location is unclear. It is suggested that the accumulation of vesicles in the lateral region may reflect some hindrance in the fusion of the vesicles with the lateral cell membranes.

Studies on vinblastine-induced autophagocytosis in mouse liver. III. A quantitative study

The microtubule inhibitor vinblastine (25 mg/kg, i.p.) induces autophagocytosis in mouse hepatocytes. The formation of autophagic vacuoles, their contents, and other cellular changes after vinblastine injection in hepatocytes, were studied by light and electron microscopic morphometric analysis. The volume density of autophagic vacuoles increased significantly during the experimental period (24 h). This increase was due to the significant increase in their number, which was approximately 5-fold 4 h, 12 h and 24 h after vinblastine injection. The mean volume of the autophagic vacuoles increased significantly 1 h after vinblastine injection, at which time the formation of new autophagic vacuoles was at its greatest. There was an accumulation of single membrane-limited, obviously older autophagic vacuoles in the cytoplasm. Their volume density was at its maximum 12 h after injection, suggesting a retarded turnover of autophagic vacuoles. The segregation of cytoplasmic components into autophagic vacuoles may not be selective after vinblastine injection. The injurious effects of vinblastine were evident both in light and electron microscopic studies. In the parenchymal cells the Golgi cisternae were dilated and disorganized and the volume density of the Golgi apparatus was significantly decreased 12 h after vinblastine injection. The volume density of lysosomes was increased during the 12 h after vinblastine injection. Vesicles containing very low density lipoprotein particles accumulated in the cytoplasm so that their volume density was significantly increased during the entire experimental period. Vinblastine apparently interfered with the transport and secretion of the very low density lipoproteins from the parenchymal cells.

Effects of colchicine and vinblastine on output of proteins into bile

The microtubule poisons colchicine and vinblastine caused a reduction in the biliary output of total protein, compared with controls, in bile-fistula rats. The various protein components of bile showed patterns of change in output differing from one another. Alkaline phosphodiesterase I, a 'plasma-membrane' enzyme, showed a decline in output during the first hour after drug administration. Immunoglobulin A output did not decline until after the first hour. In contrast with these reductions, the biliary output of (rat) plasma albumin into bile was increased. At no time was there any evidence (as measured by lactate dehydrogenase release into bile) for any cytolytic damage. These results are discussed in relation to current theories on the output of proteins into bile and the repair of the plasma membrane in the bile canaliculus.

Microtubules and protein secretion in rat lacrimal glands: localization of short-term effects of colchicine on the secretory process

The pathway and kinetics of the secretory protein transport in rat lacrimal exorbital gland have been established by an in vitro time-course radioautographic study of pulse-labeled protein secretion. The colchicine-sensitive steps have been localized by using the drug at various times with respect to the pulse labeling of proteins. Colchicine (10 microM) does not block any step of the secretory protein transport, but when introduced before the pulse it decreases the transfer of labeled proteins from the rough endoplasmic reticulum to the Golgi area, suppressing their temporary accumulation in the Golgi area before any alteration of this organelle is detectable. Moreover, colchicine inhibits protein release only from the secretory granules formed in its presence because the peroxidase discharge is diminished 1 h after colchicine addition, and the secretion of newly synthesized proteins is strongly inhibited only when colchicine is introduced before secretory granule formation. Morphometric studies show that there is a great increase of secondary lysosomes, related to crinophagy, as early as 40-50 min after colchicine is added. However, changes in lysosomal enzymatic activities remained biochemically undetectable. We conclude that: (a) the labile microtubular system does not seem indispensable for protein transport in the rough endoplasmic reticulum-Golgi area but may facilitate this step, perhaps by maintaining the spatial organization of this area; and (b) in the lacrimal gland, colchicine inhibits protein release not by acting on the steps of secretion following the secretory granule formation, but by acting chiefly on the steps preceding secretory granule formation, perhaps by making the secretory granules formed in its presence incapable of discharging their content.

Milk yield and secretion composition following intramammary infusion of colchicine

Twenty-five lactating dairy cows were in a split block design to investigate effects of intramammary colchicine infusion on milk yield and composition. Right udder halves of 15 cows were infused with colchicine while left udder halves received no infusion. Ten cows received no infusion in either right or left udder halves. At 24 h postintramammary colchicine infusion, milk yield decreased 38.5% in treated halves and 9.3% in control halves. Udder halves of uninfused cows had an 8.7% reduction. Milk from treated halves compared to untreated halves had elevated somatic cells, serum albumin, and pH. Citrate and the molar ratio of citrate to lactoferrin were lower. Lactoferrin and immunoglobulin G did not differ between infused and uninfused halves; however, they increased in treated halves postinfusion as compared to preinfusion. Serum albumin and citrate increased, and the index of selective accumulation of immunoglobulin G decreased in left halves. Control cows did not differ.

Ultrastructural alterations in the liver of rats fed photomirex (8-monohydromirex)

Alterations in the liver of rats fed photomirex or mirex for 28 d have been studied by electron microscopy. A dose of 0.05 or 0.5 ppm photomirex produced selective ultrastructural damage to the mitochondria of hepatocytes. The parenchymal cells had a proliferation of smooth-surfaced endoplasmic reticulum (SER) with a concomitant depletion of glycogen, a reduction of rough-surfaced endoplasmic reticulum (RER) profiles, an abnormal accumulation of lipid droplets, and pleomorphic nuclei at the 5 ppm photomirex level. In the animals receiving 50 ppm photomirex there were degenerating foci in the organ. There was an increase of electron-dense bodies in the hepatocytes from the 5 ppm mirex group but these cells, in the animals receiving 50 ppm mirex, had SER proliferation and glycogen depletion. It is concluded that photomirex causes more severe ultrastructural changes in the livers of rats than mirex.

Ouabain-resistant mechanism of volume control and the ultrastructural organization of liver slices recovering from swelling in vitro

We have studied the net extrusion of water by liver slices recovering from swelling at 1 degree C and have attempted to relate this to ultrastructural alterations. Special attention was paid to the ouabain-resistant extrusion of water. The restoration of many details of intracellular architecture was dissociated from the net loss of water, since an osmotic stimulus (produced by 5% inulin) caused a passive withdrawal of water with little recovery of structure. Also, a similar recovery of structure was produced during active extrusion of water in the absence and presence of ouabain, even though ouabain reduced the water extrusion by 50%. The time-course of water extrusion in the presence of ouabain was correlated with the formation of cytoplasmic vesicles. Incubation without K+ in the medium had similar effects to those caused by ouabain. Colchicine had little effect on water extrusion in presence or absence of ouabain except at concentrations which reduced tissue ATP levels and caused much necrosis. Cytochalasin B alone had little effect on water extrusion, but led to the accumulation of many vesicles in the cytoplasm and appeared to abolish the access of such vesicles to the canaliculi. In the presence of ouabain, cytochalasin B had a similar effect on ultrastructure, and totally prevented the ouabain-resistant water extrusion. Ni2+ had rather similar effects to cytochalasin B both in the presence and absence of ouabain, although to a smaller degree. The results support our previous suggestion that the ouabain-resistant water extrusion proceeds by secretion of water into cytoplasmic vesicles, followed by the exocytotic expulsion of the vesicular contents into the bile canaliculi. Microfilaments appear to play an important role in the process.

The role of microfilaments and microtubules in taurocholate uptake by isolated rat liver cells

The role of microfilaments and microtubules on bile salt transport was studied by investigating the influence of a microfilament and a microtubule inhibitor, cytochalasin B and colchicine, respectively, on taurocholate uptake by isolated hepatocytes in vitro. Hepatocytes were prepared by the enzyme perfusion method and [14C]taurocholate uptake velocity was determined by a filtration assay. Taurocholate uptake obeyed Michaelis-Menten kinetics, maximal uptake velocity and apparent half-saturation constants averaging 0.87 +/0 SD 0.05 nmol . s-1 . 10(-6) cells and 10.9 +/- 1.8 muM, respectively. Cytochalasin B (4.2-420 muM) inhibited taurocholate uptake in a competitive fashion; Ki being 33 +/- 7 muM. At concentrations above 100 muM the compound decreased 36Cl membrane potential and intracellular K+ concentration. Other parameters of cell viability were not affected by cytochalasin B. Colchicine (0.1-1.0 mM), by contrast, inhibited taurocholate uptake non-competitively, Ki being 0.47 +/- 0.07 mM. The inhibition brought about by colchicine was considerably smaller than that induced by cytochalasin B. None of the parameters of cell viability tested was affected by colchicine. These results suggest that microfilaments may be involved in the carrier-mediated hepatocellular transport of bile salts. This could, at least in part, account for cytochalasin B-induced cholestasis. The contribution of the microtubular system, if any, is less important quantitatively. The mechanisms whereby these two components of the cytoskeleton partake in bile salt transport remain to be elucidated.

Role of microtubules in milk secretion--action of colchicine on microtubules and exocytosis of secretory vesicles in rat mammary epithelial cells

Effect of colchicine on microtubules was studied in mammary epithelial cells treated both in vivo and in vitro with the alkaloid. Three hours after the intramammary infusion of colchicine, secretory activity of mammary epithelia ceased, milk constituents accumulated and were randomly distributed within the cytoplasm, sometimes leaking into the perialveolar connective tissue, and autophagic vacuoles were prevalent. It appeared that an accelerated involutionary process was occurring. No microtubules were observed after this treatment. In vitro treated cells appeared to be less affected by the alkaloid. Although numerous casein-containing secretory vesicles accumulated in the cytoplasm, lipid droplet accumulation was less, and fewer autophagic vacuoles were observed, although lysosomes were commonly observed. Occasionally, obliquely sectioned microtubules were found in cells treated with low concentrations of colchicine but were absent at higher colchicine concentrations; however, paracrystalline inclusions (tubulin aggregates) were observed in some cells at all concentrations of the drug. These observations provide evidence that drugs which interfere with microtubule integrity reduce the secretory activity in mammary epithelia. This evidence is consistent with the concept of an association of the microtubular system and the secretory process.