The proto-oncogene c-MET encodes the tyrosine kinase receptor for hepatocyte growth factor (HGF), a pleiotropic cytokine controlling growth, survival, motility, invasive migration, and differentiation of epithelial cells. Like several other epithelial neoplasms, thyroid carcinomas have been found to overexpress c-MET at both the mRNA and protein level. The biological relevance of Met overexpression to thyroid carcinoma natural history, however, remains to be elucidated. Therefore, we analyzed Met expression and response to HGF in two cell lines established from human thyroid carcinomas. In both lines we observed that the overexpressed and constitutively tyrosine phosphorylated HGF receptor maintained biochemical responsiveness to the ligand. Both cell lines were also found to respond to HGF by consistently increasing their motility and invading in vitro reconstituted basal membranes. Conversely, no effect of HGF could be observed in proliferation and survival assays. These data show that overexpression of Met specifically confers to transformed thyroid cells a motile-invasive phenotype that is dependent on exogenous HGF stimulation.

Early heart development is known to be sensitive to retinoid concentrations. Although the influence of retinoids on cardiac morphogenesis has been described previously, the effect of retinoids on cardiomyocyte differentiation during development has not been characterized. We quantified the effects of the retinoic acids all-trans RA and 13-cis RA on α-actin and α-actinin at the subcellular level in cultures of chick embryo cardiomyocytes obtained from Hamburger and Hamilton’s (HH) stage 22, 32 and 40 embryos. The retinoids increased the concentration of α-actin and α-actinin in the cytoplasmic and cytoskeletal fractions of cells at all three stages of development. The effect was greatest in cardiomyocytes treated for 24 h with all-trans RA and in cells from HH22 embryos. The greatest increases in α-actin concentration occurred in the cytoskeletal fraction of HH22 cells cultured for 24 h with all-trans or 13-cis RA, whereas the greatest increases in α-actinin were found in the cytoplasmic fraction of HH22 cells exposed to retinoids for 24 h. We conclude that retinoic acid plays a role in the reorganization of the pattern of sarcomeric protein expression during cardiomyocyte differentiation.

Synchronized CHO-K1 cells and their dibutyryl c-AMP treated counterparts have been characterized by means of static and flow fluorescence cytometry at the level of nuclear DNA and cytoplasmic microtubules. In order to confirm earlier findings on synchronized population, Carnoy fixed and hydrolyzed, several new findings are here reported at the level of single intact cell. The fluorescence intensity of DAPI-stained glutaraldehyde fixed 2C cells correlates well with the average absorbance of the corresponding Feulgen-stained cells, thereby appearing also to be a measure of chromatin condensation during the G1 phase. In the early part of G1, the drastic alteration in anti-beta tubulin immunostaining is shown to parallel microtubule depolymerization induced by calcium or colcemide. The known 1-2 h lengthening of the G1 period after reverse-transformation appears to correlate with a similar delay in the abrupt chromatin decondensation. The above results are discussed in terms of the role of microtubules and nuclear morphometry (and their coupling) in the control of cell cycle progression of transformed vs. fibroblast-like cells.

Several studies indicate that the cytoskeleton may be involved in modulating the cellular response to environmental signals. We have studied the role of the cytoskeleton in regulating glycosaminoglycan (GAG) synthesis and secretion, hyaluronate (HA) endocytosis, the activities of hexoglycosidases, protein synthesis and secretion. Fibroblasts were treated with colchicine (1-8 microM) and nocodazole (1 or 4 microM) to alter microtubules or cytochalasin B (0.5-4 microM) to alter microfilaments. Colchicine inhibited GAG synthesis and secretion in a concentration-dependent manner. It reduced protein and sulphated GAG secretion, while HA secretion was not affected. Concentration-dependent disruption of microtubules from the periphery toward the cellular centre with nocodazole inhibited only the secretion of GAG. Centrosomal microtubles appeared to be required to promote GAG synthesis; intact microtubules promoted the transport of secretory products, intercompatmental transport of lysosomal enzymes and lysosome maturation, but not protein synthesis and HA secretion. Cytochalasin B treatment inhibited, in a concentration-dependent manner, the synthesis and secretion of GAGs and proteins, and the endocytosis of HA. Intact microfilament meshworks appeared to be required to promote synthesis and secretion of proteins and proteoglycans and to contribute to the transmembrane control of receptor-mediated endocytosis. Drug treatment of concanavalin A (Con A)-stimulated fibroblasts inhibited the stimulation of GAG synthesis. It is probable that this effect may result, in part, from drug-induced effects on Con A-mediated endocytosis.

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