Changes in the organization of non-epithelial intermediate filaments induced by triethyl lead chloride

Leukamenin E, an ent-kaurane diterpenoid, is a novel and potential keratin intermediate filament inhibitor

Many ent-kaurane diterpenoids exhibit notable antitumor activity in vitro and in vivo, and some have been used as cancer therapeutic agents in China. In this study, we identified a novel molecular target of leukamenin E, an ent-kaurane diterpenoid, using an available whole-cell model in combination with immunofluorescence imaging and mass spectrometry (MS). The cytoskeleton-disrupting drugs cytochalasin B and colchicine caused the depolymerization of microfilaments and the collapse of microtubules and vimentin filaments, respectively, but had little effects on HepG2 and NCI-H1299 cells spreading as well as keratin filament (KF) reassembly, indicating that KFs are involved in cell spreading. Leukamenin E blocked HepG2 and NCI-H1299 cells adhesion/spreading and KF reassembly at subtoxic concentrations, indicating that leukamenin E may target KFs. Moreover, leukamenin E, at 3 μM for 24 h or 10 μM for 3 h, induced massive KF depolymerization in well-spread HepG2 and NCI-H1299 cells treated with/without cytochalasin B and colchicine. MS analysis indicated that leukamenin E could covalently modify amino acid residue(s) in a synthetic peptide based on keratin 1 and keratin 10 sequences, suggesting that covalent modification of the synthetic peptide by leukamenin E caused assembly inhibition or disrupted KF polymerization in HepG2 and NCI-H1299 cells. In addition, acridine orange/ethidium bromide staining and western blotting confirmed that there was no correlation between the KF-disrupting effects and apoptosis or keratin expression. Thus, we propose that leukamenin E is a novel inhibitor of KF assembly, and as such, can serve as a chemical probe of KF functions and a potential molecular target for ent-kaurane diterpenoid-based therapeutics.

Zinc alters actin filaments in Madin-Darby canine kidney cells

Exposure of semiconfluent cultures of Madin-Darby canine kidney cells to 10 microM zinc leads to a change in the organization of the actin filament system. Most of the stress fibers at the basal end of the cell are lost and the actin associated with the lateral membrane and junctional regions appears to retract into the cytoplasm. In addition, at the base of the cell in regions of cell-substratum contact, dense, actin-rich plaques appear. These alterations in actin filaments are associated with a change in cell shape. Microtubules were unaffected by exposure to 10 microM zinc. At zinc concentrations greater than or equal to 50 microM the microtubules depolymerized. Exposure to cadmium alters the actin filaments as well but the effect is different from the change seen with zinc. When the cells are exposed simultaneously to zinc and cadmium the cells appear the same as they would if exposed to zinc alone. Exposure of MDCK cells to either metal, individually or in combination, results in a significant and similar increase in F-actin content as determined spectrofluorometrically. The changes in organization and amount of F-actin are associated with a reduction in the ability of the cells to remain attached to the substrate, a toxic effect of these metals with regard to epithelial function. The results indicate that zinc, an essential metal, and cadmium, a highly toxic metal, interact with the actin cytoskeleton in intact cells.

Cytokeratin reorganization induced by adenosine diphosphate in kidney epithelial cells

Exogenous adenosine diphosphate (ADP), the most potent mitogen for nontransformed African green monkey kidney epithelial cells of the BSC-1 line, rapidly alters the appearance of the cell monolayer. Examination of the cells with indirect immunofluorescence using monoclonal antibodies reveals a considerable reorganization of cytokeratin filaments without a major change in the pattern of microtubules or microfilaments. In untreated confluent cells, cytokeratin filaments are predominantly confined to a star-like spot in the perinuclear area, but these can be seen to begin to spread within 2 min after addition of ADP. The effect is particularly notable using anti-cytokeratin 8 antibodies. At 6 h this process is complete and produces a well-developed filamentous network throughout the cell. By 12 h, the network appears to collapse, so that the filaments again form a spot in the perinuclear area, a process that is complete by 24 h. Immunoblotting of total cellular proteins reveals no apparent alterations in the amounts of several species of cytokeratins, including cytokeratin 8 and 18, at 3 or 24 h after exposure to ADP. Other purine and pyrimidine nucleotides which do not stimulate DNA synthesis in these cells fail to alter cytokeratin organization, and there is no apparent alteration in the distribution of vimentin, another intermediate filament protein. The rapid ADP-induced cytokeratin reorganization appears to coincide with the induction of early growth-response gene transcription in these cells and may be correlated with the capacity of ADP to subsequently initiate DNA synthesis. This dramatic and reversible cytokeratin reorganization immediately after exposure to ADP may be an important step in the mitogenic signal transduction pathway.

Cellular intermediate filament networks and their derangement in alcoholic hepatitis

Intermediate filaments are major components of most eukaryotic cells that form from the polymerization of protein subunits that are expressed in tissue and development specific fashions. The interactions of intermediate filaments with a myriad of other cellular proteins and structures give rise to a complex overall cellular architecture that is likely responsible for cellular well-being. The mature 10-nm filaments are relatively stable cellular structures, but the intermediate filaments undergo major morphological and biochemical changes, especially during mitosis, differentiation, and in response to certain drugs. Evidence exists that hepatocyte intermediate filaments (keratin filaments) are deranged in alcoholic hepatitis, an inflammatory liver disease of alcoholics and heavy spree drinkers. The classical and characteristic pathological hepatocyte inclusion bodies of alcoholic hepatitis, Mallory bodies, are composed in part of normal keratins that likely derive from the pre-existing hepatocyte intermediate filament network. It is unclear if intermediate filament network derangement in alcoholic hepatitis is directly caused by the actions of ethanol or its metabolites on intermediate filaments or their associated structures, or whether alcohol causes a cellular insult or injury elsewhere and a subsequent response (e.g., immune) causes intermediate filament network derangement. The precise mechanisms responsible for intermediate filament derangement remain to be elucidated; however, experimental data exist that support and refute several hypotheses. Hopefully, further studies will help determine a better overall understanding of the abnormalities of intermediate filaments and their relationship to the pathophysiology of alcoholic hepatitis and other diseases.

The interaction of triethyl lead with tubulin and microtubules

The impact of triethyl lead chloride was studied on: (i) the in vitro assembly and disassembly of microtubules from porcine brain by turbidometry and electron microscopy, (ii) the microtubule system of living mammalian cells using immunofluorescence microscopy, (iii) cell motility and chemotaxis employing the methods of phagokinetic track formation and the Boyden chamber assay, respectively, and (iv) thiol groups of the protein tubulin by their titration in the presence and absence of the organic lead compound. Triethyl lead chloride inhibited microtubule assembly and depolymerized preformed microtubules in vitro and in living cells. Random motility of cells was not markedly inhibited by triethyl lead chloride, whereas chemotaxis (directed cellular movement) was strongly inhibited. Triethyl lead chloride was found to interact with 2 thiol groups of the tubulin dimer. The interaction of triethyl lead chloride with the tubulin/microtubule system in vivo likely causes aneuploidy and is at least partly responsible for the cytotoxicity of the drug.

Metabolic inhibitors and intermediate filament organization in human fibroblasts

A number of metabolic inhibitors including the mRNA transcription inhibitor actinomycin D; the protein synthesis inhibitors emetine, cycloheximide, and puromycin; the energy metabolism inhibitors sodium azide and oligomycin; the amino acid analog L-azetidine-2-carboxylic acid; sodium fluoride; and acrylamide each cause the collapse of vimentin filament organization while leaving microtubule organization apparently unaffected in the human fibroblastic cell line MCH23. The protein kinase inhibitor N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H8) caused a partial collapse of vimentin organization but its effect was more difficult to discern, since it also induced a dramatic change in cellular morphology. Each of these drugs produced a significant inhibition of protein synthesis at concentrations that affected vimentin organization. The mechanisms by which these drugs affect intermediate filament organization are unclear, but our results demonstrate that intermediate filament organization in MCH23 cells is affected by a wide range of drugs and that such drugs cannot be used without great caution as reagents for the study of intermediate filament organization and function.