Microtubules and protein secretion in rat lacrimal glands. Relationship between colchicine binding and its inhibitory effect on the intracellular transport of proteins

Regulatory pathways in lacrimal gland epithelium

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

Pancreatic tau related maps: biochemical and immunofluorescence analysis in a tumoral cell line

In the present study, we report the existence of four tau-related microtubule-associated proteins (MAPs) of 48, 50, 55 and 58 kDa in a pancreatic exocrine cell line (AR4-2J). Using immunofluorescence, we demonstrate that these tau-related MAPs are associated with microtubules in AR4-2J cells. That colocalization is particularly striking on microtubules bundles in cellular extensions and is the first evidence for tau-related MAPs colocalization with microtubules in non-neuronal cells. As it has been often discussed for neuronal tau, the localization of tau-related proteins in AR4-2J cells suggests that these proteins may be involved in microtubule bundling.

Are microtubules essential for the secretory process in rat parotid gland?

The role of microtubules in the exocrine secretory process is not yet well established, and their disruption by anti-microtubule drugs leads to variable effects on intracellular transit and protein secretion. We investigated the involvement of microtubules in the regulated secretory process of rat parotid glands using microscopic techniques and pulse-chase experiments. We showed that 10 microM colchicine or nocodazole destroys the microtubule network in parotid acinar cells but only weakly reduces the release of newly synthesized proteins. The half-effect was obtained with 0.22 microM colchicine. Moreover, this small reduction was found to be independent of the nature of the drug (colchicine, colcemid, or nocodazole) and of the nature of the stimulation (beta-adrenergic or cholinergic pathways). Using nocodazole, we have been able to determine that the steps affected by the drug are very early events in the secretory pathway. Finally, we showed by kinetic analysis that microtubule disruption slows protein release only moderately but does not reduce the total amount of secreted protein. We conclude from this study that microtubule integrity is not essential for protein secretion in rat parotid gland.

Microtubules and the organization of the Golgi complex

Electron microscopic and cytochemical studies indicate that microtubules play an important role in the organization of the Golgi complex in mammalian cells. During interphase microtubules form a radiating pattern in the cytoplasm, originating from the pericentriolar region (microtubule-organizing centre). The stacks of Golgi cisternae and the associated secretory vesicles and lysosomes are arranged in a circumscribed juxtanuclear area, usually centered around the centrioles, and show a defined orientation in relation to the rough endoplasmic reticulum. Exposure of cells to drugs such as colchicine, vinblastine and nocodazole leads to disassembly of microtubules and disorganization of the Golgi complex, most typically a dispersion of its stacks of cisternae throughout the cytoplasm. These alterations are accompanied by disturbances in the intracellular transport, processing and release of secretory products as well as inhibition of endocytosis. The observations suggest that microtubules are partly responsible for the maintenance and functioning of the Golgi complex, possibly by arranging its stacks of cisternae three-dimensionally within the cell and in relation to other organelles and ensuring a normal flow of material into and away from them. During mitosis, microtubules disassemble (prophase) and a mitotic spindle is built up (metaphase) to take care of the subsequent separation of the chromosomes (anaphase). The breaking up of the microtubular cytoskeleton is followed by vesiculation of the rough endoplasmic reticulum and partial atrophy, as well as dispersion of the stacks of Golgi cisternae. After completion of the nuclear division (telophase), the radiating microtubule pattern is re-established and the rough endoplasmic reticulum and the Golgi complex resume their normal interphase structure. This sequence of events is believed to fulfil the double function to provide tubulin units and space for construction of the mitotic spindle and to guarantee an approximately equal distribution of the rough endoplasmic reticulum and the Golgi complex on the two daughter cells.

The effects of cytotropic compounds on the resialylation of human asialotransferrin type 3 in the rat

Effects of chloroquine, colchicine, leupeptin, taxol and vinblastine on the resialylation and degradation of human [125I]asialotransferrin type 3 were studied in rats. An improved experimental technique was applied that permitted the quantification of resialylated ligand produced by individual animals over 3 h by using deconvolution. All three microtubule inhibitors increased the proportion of the dose undergoing resialylation by 35-39%. In addition, colchicine, and, especially, vinblastine enhanced the overall recovery of the dose as protein-bound 125I. The dose recovery was also augmented by leupeptin without any concomitant change in resialylation. Chloroquine suppressed resialylation and this effect could only be partially lifted by the administration of colchicine. The blood of colchicine-treated rats possessed no resialylating activity toward the ligand even when supplemented with additional alkaloid in vitro. The observations support the view that the respective fractions of the ligand destined for resialylation and degradation can, to a certain extent, be varied independently of each other. The effects of short-term starvation (20 h) and refeeding (4 h) on these processes are also presented.

Comparative effects of cytochalasin D on the protein discharge induced by alpha- and beta-adrenergic or cholinergic agonists in rat exorbital lacrimal glands

Cytochalasin D altered the kinetics of peroxidase and radiolabeled protein discharge from rat exorbital lacrimal glands in vitro, in response to various secretagogues. The changes were different with each inducer. The discharge due to isoproterenol was immediately inhibited by 95%; the discharge evoked by noradrenaline via alpha-adrenergic receptors was progressively reduced and was inhibited by 50% after 30 min, whereas that evoked by carbachol was not influenced during the initial discharge period and was diminished by only 30% after 30 min. When calcium was removed from the incubation medium, the secretory responses were lowered and the inhibitory effect of cytochalasin D was still observed. The rate of protein discharge inhibition was related to the dose and was maximal with 2 X 10(-6) M cytochalasin D when the discharge resulted from cholinergic, alpha- or beta-adrenergic or dibutyryl cAMP stimulation. Cytochalasin D did not impair cellular energetics nor other stimulations induced through muscarinic or adrenergic receptors. Cytochalasin D effects could be related to interaction with actin, leading to the inhibition of the release of proteins into the incubation medium following the activation of the adrenergic system.

Vinblastine-induced formation of annulated lamellae in SV3T3 cells

SV40-transformed 3T3 cells formed microtubular crystals in the cytoplasm when treated with vinblastine sulfate. Annulated lamellae, an unusual intracellular organelle found in some actively proliferating cell types, appeared concurrently with the formation of microtubular crystals. Although crystals were also seen in non-transformed 3T3 cells treated with vinblastine, annulated lamellae were not found.

Protein secretion induced by isoproterenol or pentoxifylline in lacrimal gland: Ca2+ effects

In exorbital lacrimal glands, pentoxifylline (a methylxanthine) induces labeled protein secretion in a dose-related manner: the half-maximal and maximal stimulations are at 4 and 10 mM, respectively. In the presence of papaverine (10(-5) M), a phosphodiesterase inhibitor, labeled protein discharge is strongly stimulated by isoproterenol, via beta-adrenergic receptors: the maximal response is at 10(-6) M. l-Propranolol specifically inhibits the secretory stimulation to isoproterenol in a dose-related manner: for 5 X 10(-6) M isoproterenol in the presence of 10(-5) M papaverine, the half-maximal and maximal inhibitions are at 3 X 10(-7) and 10(-5) M, respectively. The beta-adrenergic response is mimicked by the adenosine 3',5'-cyclic monophosphate (cAMP) analogue dibutyryl cAMP (DBcAMP) at a 10(-3) M concentration. The time course of labeled protein secretion induced by pentoxifylline, DBcAMP, and isoproterenol shows a latency. In the presence or absence of extracellular calcium, pentoxifylline and isoproterenol immediately increase the cAMP intracellular level. Extracellular calcium omission increases the observed latency and also affects the maximal rate of protein secretion. As opposed to the cholinergic agonist, pentoxifylline has only a slight but sustained effect on 45Ca efflux, whereas isoproterenol has none. These data suggest that labeled protein secretion, such as that of peroxidase, can also be stimulated in rat exorbital lacrimal gland, through beta-adrenergic receptors; in the stimulation evoked by a beta-adrenergic agonist, DBcAMP, or methylxanthine, calcium could play a key role.

Inhibitory effect of colchicine on translocation of alkaline phosphatase to the plasma membrane concomitant to its induction in rat liver

A single injection of colchicine (1--3 mg/kg body weight) caused a remarkable induction of hepatic alkaline phosphatase, which increased linearly in the homogenate starting at 5--6 h and reached a maximum level (14-fold of the control activity) at 20--22 h after the drug treatment. In the plasma membrane, however, the increase in specific activity and the recovery of alkaline phosphatase were greatly inhibited up to 12 h after the treatment. Such an inhibitory effect of colchicine was confirmed by a combination experiment of the drug treatment with bile duct ligation; in the plasma membrane elevation of the enzyme induced by bile duct ligation was also greatly retarded by colchicine. The subcellular distribution of the enzyme activity in livers was determined among the four groups of rats with or without bile duct ligation and/or colchicine administration taken at 8 h after each treatment. In the control and the bile duct-ligated livers, the highest specific activity was observed in the plasma membrane fraction, while in the colchicine-treated livers, with or without bile duct ligation, the highest activity was found in the Golgi fractions. These results indicate that the Golgi membranes enriched with the induced enzyme were blocked by the drug to prevent migration toward the plasma membranes enriched with the induced enzyme were blocked by the drug to prevent migration toward the plasma membrane, thus demonstrating involvement of the Golgi complex in the translocation route of newly synthesized alklaine phosphatase to the plasma membrane.

Nonelectrolyte fluxes across gastric mucosa in relation to gastric stimulation. Is gastric juice secreted by osmosis or exocytosis?

The effects of histamine and thiocyanate, added to the serosal bathing solution, on unidirectional fluxes of some nonelectrolytes (thiourea, methylated thiourea derivates, mannitol), and on H+, pepsinogen and mucous secretion were investigated in frog (Rana esculenta) fundic gastric mucosa. Histamine (10(-4) M) increases significantly the outfluxes (serosa to mucosa fluxes) of only thiourea and its derivates (but not mannitol) and the stimulation is the greater the more lipidsoluble the nonelectrolyte is. Influxes (mucosa to serosa fluxes) of the same molecules are not affected. In parallel histamine stimulates H+-secretion but does not modify pepsinogen and mucous secretion. SCN- (10(-2) M) inhibits the histamine effect on thiourea outfluxes and on H+-secretion, while pepsinogen and mucous secretion are not affected. Colchicine (10(-4) M) pretreatment inhibits the histamine effect on outfluxes and H+-secretion. It is concluded that: (1) histamine induces a secretion of nonelectrolytes towards the lumen; (2) such secretion is correlated with the hormone-induced secretion of HCl and fluid; (3) this process is mediated by an exocytotic mechanism.

A possible complementary role of actin microfilaments and microtubules in triacylglycerol secretion by isolated rat hepatocytes

Incubation of isolated rat hepatocytes with phalloidin, cytochalasins (which, respectively, stabilize and destabilize actin microfilaments), or colchicine (which inhibits polymerization of microtubules), resulted in a dose-dependent inhibition of triacylglycerol secretion (an index of very low density lipoprotein secretion). Upon removal of drugs from incubation media, the inhibitory effect of cytochalasin D on triacylglycerol secretion was reversible, while such was not the case of phalloidin. When used at maximal concentrations, the combined presence of phalloidin + colchicine or cytochalasin D + colchicine had additive inhibitory effects upon hepatic triacylglycerol secretion, which was virtually blocked; this was not the case for phalloidin + cytochalasin D. These experiments support the concept that microfilaments and microtubules may have complementary functions for the hepatic secretion of very low density lipoproteins.