Glucocorticoids and the plasma membrane

Dexamethasone modifies the susceptibility to serum cytotoxicity and increases the metastatic efficiency of a colon carcinoma cell line

Metastatic inefficiency is a phenomenon by which a majority of tumor cells is lost in the blood stream during the metastatic process. We investigated the effects of dexamethasone (DEX), a synthesized glucocorticoid, on the serum susceptibility of a colon carcinoma cell line, HT-29, with respect to metastatic inefficiency. The susceptibility to serum cytotoxicity of these carcinoma cells is possibly an important factor in metastatic inefficiency. In this study, we used glucocorticoid because it modifies the function of the plasma membrane and has been shown to enhance the hematogenous metastasis of some tumor cells. Using HT-29 cells that had been treated with DEX in vitro, the following factors were evaluated: the metastasis of intrasplenic injected cells; in vitro and in vivo proliferation; motility; the production of matrix metalloproteinases (MMPs); and the expression of the membrane complement regulatory proteins CD46, CD55, and CD59. The number of viable cells in the liver after an intraportal injection of tumor cells was determined by the expression of human beta-globin mRNA that is aberrantly expressed in HT-29 cells. In addition, we investigated 100% serum-induced proliferation, susceptibility, and apoptosis. Treatment with DEX was found to accelerate liver metastasis; here, the number of metastatic colonies and the weight of the liver were both significantly increased in DEX-treated HT-29 (HT-29DEX) cells. In contrast, there was no difference in terms of cell motility; the production of MMPs; or the expression of CD46, CD55, or CD59 between the HT-29 and HT-29DEX cells. The HT-29DEX cells exhibited enhanced proliferation in the serum, as well as resistance to cytotoxicity when exposed to 100% serum. In addition, DEX slightly inhibited serum-induced apoptosis. Finally, the expression of colon cancer-derived beta-globin mRNA was detectable 24 h after intravenous injection, but only in samples obtained from the HT-29DEX-, but not in those from the HT-29-inoculated mice. These results indicate that DEX reduced the metastatic inefficiency of the HT-29 cells, resulting in a hematogenous metastasis-prone phenotype. It is thus expected that the acquisition of resistance against serum cytotoxicity is among the mechanisms that contribute to the efficiency of hematogeneous metastasis.

The effects of dexamethasone on palate mesenchymal cell phospholipase activity

Corticosteroids will induce cleft palate in mice. One suggested mechanism for this effect is through inhibition of phospholipase activity. This hypothesis was tested by measuring the effects of dexamethasone, a synthetic corticosteroid, on phospholipase activity in cultures of palate mesenchymal cells. Palate mesenchymal cells were prelabeled with [3H]arachidonic acid. The cells were subsequently treated with various concentrations of dexamethasone. Concurrently, cultures of M-MSV-transformed 3T3 cells were prepared identically. After treatment, phospholipase activity was stimulated by the addition of serum or epidermal growth factor (EGF), and radioactivity released into the medium was taken as a measure of phospholipase activity. Dexamethasone (1 X 10(-5) or 1 X 10(-4) M) could inhibit serum-stimulated phospholipase activity in transformed 3T3 cells after 1 to 24 hr of treatment. However, no inhibition of activity was measured in palate mesenchymal cells following this period of treatment. Not until 120 hr of treatment with dexamethasone (1 X 10(-4) M) was any significant inhibition of serum-stimulated phospholipase activity observed in palate mesenchymal cells. When EGF was used to stimulate phospholipase activity, dexamethasone (1 X 10(-5) M) caused an increase in phospholipase activity in palate mesenchymal cells. These observations suggested that phospholipase in transformed 3T3 cells was sensitive to inhibition by dexamethasone. However, palate mesenchymal cell phospholipase is only minimally sensitive to dexamethasone, and in certain instances can be enhanced. These results cannot support the hypothesis that corticosteroids mediate their teratogenic effect via inhibition of phospholipase activity.

Dexamethasone increases the synthesis and secretion of a partially active fibronectin in rat hepatoma cells

After treatment with dexamethasone, rat hepatoma-tissue culture cells show a markedly enhanced adhesion to the substratum and increased cell-to-cell interaction. In addition, there is a profound change in the production of secretory glycoproteins. Although the relative synthesis and secretion of a gelatin-binding, fibronectinlike glycoprotein is increased threefold, we do not think this protein is responsible for the improved adhesion properties of the cells because the hepatoma cells do not bind normal fibronectin and because the HTC-produced fibronectin is neither bound by fibroblasts nor has it any affinity for ganglioside-containing phospholipid vesicles. Therefore, these hepatoma cells represent a unique system for studying the regulation of fibronectin synthesis by glucocorticoids. Furthermore, analyses of primary fetal rat hepatocytes have shown that these cells, unlike normal adult hepatocytes, synthesize and secrete fibronectin, which is structurally related to the HTC-cell protein. The comparison of this protein with fibronectin from normal cells will allow a structural characterization of the functional defect in the fibronectin synthesized by transformed cells.

An electrophoretic characterization of the glucocorticoid response of the Fu5-5 rat hepatoma cell line

In this study we have further characterized the response of liver-derived cells to glucocorticoid treatment using the rat hepatoma, Fu5-5. Using two-dimensional electrophoresis we have examined changes in the synthetic rates of cytosol proteins following glucocorticoid administration by using [35S]methionine. We have also demonstrated changes in the incorporation of 32P by several cytosol proteins after hormone treatment. One of these changes occurs within 1 h of hormone treatment. We were unable to detect any changes in the nuclear protein content of Fu5-5 cells after glucocorticoid treatment. We then compared the glucocorticoid-regulated changes in cytosol protein synthesis in Fu5-5 cells with those of the H35 rat hepatoma, monolayer cultures of rat hepatocytes and the response of rat liver in vivo. This examination revealed that although nearly all of the eighteen hormonally responsive proteins appeared to be present in all of the cells types, only three were responsive to glucocorticoid in more than one system. Moreover, the response patterns were often reversed in different cell types, the same protein being induced in one cell type and repressed in another.

Paradoxical effects of glucocorticoids on regulation of plasminogen activator activity of rat hepatoma cells

Incubation of rat hepatoma cells with cAMP derivatives stimulates cell-associated plasminogen activator activity 8- to 22-fold and extracellular plasminogen activator activity 30- to 1300-fold. This time- and concentration-dependent increase is enhanced by phosphodiesterase inhibitors. Dexamethasone, a synthetic glucocorticoid, decreases the plasminogen activator activity of these cells, probably through induction of an inhibitor. Paradoxically, dexamethasone, added simultaneously with cAMP derivatives causes a further 4-fold enhancement of the cAMP-mediated stimulation of plasminogen activator activity. Dexamethasone also alters the time course of cAMP-mediated enhancement of plasminogen activator activity: increased protease activity is detected at 4 hr in cells incubated with 8-bromoadenosine-3':5'-cyclic monophosphoric acid and 1-methyl-3-isobutylxanthine but not until 12 hr in cells incubated with dexamethasone as well. Glucocorticoids thus exert two separate and opposite effects on plasminogen activator activity: induction of an inhibitor and amplification of cyclic nucleotide action. Although permissive and synergistic effects of dexamethasone on cyclic nucleotide action have been reported previously, glucocorticoid regulation of plasminogen activator activity is unique in that the amplification of cyclic nucleotide effects by dexamethasone opposes its regulatory action toward a specific enzyme.