Prevention of colchicine toxicity to cultured rat hepatocytes by glucagon, hydrocortisone and insulin

More Types of Inclusion Found in Colchicine-Treated, Cultured Rat Hepatocytes

Primary monolayer cultures of hepatocytes provide a convenient model system for the study of hepatic parenchymal cell function. Recently it has been shown that colchicine toxicity to cultured rat hepatocytes can be prevented by the addition of certain hormones (glucagon, hydrocortisone and insulin) to the growth medium. Using transmission electron microscopy (TEM), we have characterized the structural properties of cultured hepatocytes treated with colchicine with and without hormone supplement.

Mechanism of action of antimitotic drugs: a new hypothesis based on the role of cellular calcium

The antimitotic drugs such as colchicine, podophyllotoxin, etc. are currently believed to exert their cytotoxic and antimitotic effects due to binding of the drug-tubulin complex to the growing ends of microtubules (MTs), leading to an "end-capping or poisoning" effect. However, to account for a number of apparently puzzling observations regarding antimitotic drugs (which cannot be readily explained by the current model) and the mitotic process, a new hypothesis regarding the mechanism of action of antimitotic drugs is proposed. The key observations in this context are as follows: (i) The antimitotic drugs bind specifically to free tubulin. (ii) Cell growth by these drugs is specifically blocked in metaphase, and interphase microtubules do not seem to play any role in the drugs' cytotoxic or antimitotic effects. (iii) Tubulin is specifically associated with a number of membranous organelles (viz. mitochondria, plasma membranes, endoplasmic reticulum) which are responsible for intracellular Ca+2 homeostasis. (iv) Fluorescent derivatives of antimitotic drugs also bind to the above membranous organelles and not to MTs. (v) Ca+2 plays a central role in the control of MT assembly/disassembly in vivo and a Ca+2 pulse is necessary for the metaphase to anaphase transition. (vi) Cellular mutants which exhibit specific resistance to various antimitotic drugs are altered in either tubulin(s) or mitochondrial matrix proteins. To account for these observations, it is suggested that free tubulin present in the above membranous organelles serves as the cellular receptor for these drugs and this binding interferes with the Ca+2 regulatory/signalling mechanism essential for anaphase chromosome movement. The effect of these drugs on interphase MTs appears to be a secondary consequence of this alteration in Ca+2 regulation. The observed changes in mitochondrial matrix proteins in many of the mutants resistant to antimitotic drugs further indicate that mitochondria should play an important role in Ca+2 homeostasis, as it relates to mitosis. The possible mechanisms by which these drugs may interfere with the Ca+2 regulation and some implications of this hypothesis are discussed.

Effect of epidermal growth factor on cultured adult rat hepatocytes

When adult rat hepatocytes were cultured in plastic Petri dishes in a medium containing insulin and glucagon, supplementation with epidermal growth factor (EGF) had a pronounced effect on their viability, morphology, and biochemical integrity. Transmission and scanning electron microscopic studies showed that after 1 week cells denied EGF accumulated numerous non-electron-dense bodies and filamentous whorls, had irregular nuclei, and exhibited atypical cell surfaces. In contrast, cells grown for 2-3 weeks in the presence of EGF had well-preserved cellular organelles and remained as an epithelial-like monolayer. After 3 weeks EGF-exposed cultures were still inducible for liver-specific tyrosine aminotransferase, and both rat albumin and rat transferrin were recoverable from the culture medium. Virtually no viable cells were present at 3 weeks in EGF-deprived cultures.

Methylprednisolone inhibits pemphigus acantholysis in skin cultures

Glucocorticosteroids are used to treat patients with pemphigus, but the mechanism of action is unknown. We studied the effect of methylprednisolone on acantholysis induced in vitro by incubation of normal skin with plasma from a patient with pemphigus. Normal human breast skin was maintained in organ cultures for several days in Ham F-10 medium. Plasma from a patient with active pemphigus vulgaris caused suprabasilar epidermal acantholysis when added to this culture system. In control cultures (F-10 medium and fetal bovine serum), no acantholysis occurred. Acantholysis was prevented when breast skin was preincubated for 24 h in a 0.25 mM solution of methylprednisolone in F-10 medium and fetal bovine serum, suggesting that methylprednisolone directly inhibits acantholysis. No suppression of acantholysis occurred when the methylprednisolone was added to the culture system simultaneously with the pemphigus plasma, suggesting a time requirement for alteration of cellular events. The inhibition of acantholysis was not caused by cell death since methylprednisolone did not alter keratinocyte viability as determined by exclusion of trypan blue dye when keratinocytes were exposed to pemphigus plasma. Similarly, the inhibition of acantholysis was not due to dissolution, alteration, or coating of pemphigus antigen on epidermal cells, since the intercellular antibodies in the plasma bound as well to methylprednisolone-treated epidermis as to untreated epidermis.

Disappearance of visible bile canaliculi caused by vinblastine in primary cultures of rat hepatocytes

Treatment of cultured hepatocytes with vinblastine resulted in the inhibition of the reformation of biliary spaces (at times earlier than 16 h) or in the disappearance of preformed bile canaliculi (at times later than 24 h) detectable on both the light and electron microscopical level. Concomitantly the preferential localization of leucine aminopeptidase around the lucid biliary spaces was lost without any change in the specific activity of this enzyme. Despite these alterations the performance of the cultured cells (e.g., urea synthesis) was not impaired by the exposure to the drug. The effect of vinblastine was mimicked by colchicine, but not by lumicolchicine, indicating that microtubules might play a role in the structural organization of the biliary pole.

Studies of formation and efflux of methotrexate polyglutamates with cultured hepatic cells

Methotrexate polyglutamates are extensively synthesized when cultured hepatocytes and H35 hepatoma cells are exposed to micromolar concentrations of methotrexate. The predominant species found within the cell have from two to four additional gamma-linked glutamate residues. When either cell type containing a mixture of methotrexate and its polyglutamate derivatives is exposed to medium lacking methotrexate, there is a rapid release of methotrexate. This release has a T1/2 of 2 to 4 min and is apparently complete within 30 to 60 min. Methotrexate polyglutamates leave the cells much more slowly and appear to do so by two mechanisms. Although cleavage to methotrexate and subsequent efflux appears to be quantitatively the more important pathway, there is also a slow, finite loss of intact methotrexate polyglutamates from cells which exclude trypan blue. The T1/2 for the loss of methotrexate polyglutamates by both cell types, when placed in medium lacking methotrexate, is approximately 6 to 8 hr. These results, together with those of an earlier study (Galivan, J. (1980) Mol. Pharmacol. 17:105-110), suggest that the polyglutamate derivatives are forms of methotrexate which are as cytotoxic as methotrexate but which offer a potentially greater capacity for cellular destruction because they are retained longer in the tissue.

Factors controlling the concentrations of methotrexate in cultured hepatic cells

The polyglutamate metabolites of methotrexate are as inhibitory to the target enzyme dihydrofolate reductase as is methotrexate. Because of their greater retention they have a longer half-life within the cells and thus a greater potential for cytotoxicity. These metabolites have been found in numerous cells and tissues and are extensively synthesized in cultured hepatic cells. Uptake of methotrexate by primary cultures of rat hepatocytes occurs by a pathway which is independent of the folate coenzymes but appears to be related in some way to cholic acid and organic anion uptake. The evidence for the commonality of these pathways is (a) an instability of both uptake systems in the absence of hormones in the culture medium, (b) nearly equal inhibition of uptake by PCMS and NEM, and (c) cross competition of cholic acid and methotrexate for entry into the cells. Cholic acid and BSP can also selectively inhibit methotrexate polyglutamate formation in hepatocytes. Methotrexate entry into H35 hepatoma cells is mediated by the transport system which is shared by folate coenzymes and is not inhibited by cholic acid, BSP or sulfhydryl reagents. At concentrations of cholic acid or BSP which inhibit methotrexate polyglutamate formation in hepatocytes there is little or no loss of polyglutamate formation in H35 cells, possibly because BSP and cholic acid are taken up less by H35 cells than by hepatocytes.