Transforming growth factor β and dexamethasone cooperatively enhance c-jun gene expression and inhibit the growth of human monocytoid leukemia cells

Glucocorticoids inhibit the proliferation of lymphoid leukemia cells, whereas most myeloid leukemia cells are resistant to glucocorticoids. However, this study showed that glucocorticoids significantly and preferentially inhibited growth of monocytoid leukemia cells in combination with a low concentration of transforming growth factor β (TGFβ). Combined 1α, 25-dihydroxyvitamin D₃ and TGFβ markedly induced monocytic differentiation of U937 cells, whereas dexamethasone (Dex) and TGFβ essentially did not, although both combinations similarly inhibited the growth of U937 cells. The growth inhibition was accompanied by a block in the cell cycle progression from G₁ to S phase (G₁ arrest). Expression of glucocorticoid receptors was not affected by TGFβ, although they are induced during the monocytic differentiation of myelogenous leukemia cells and have increased sensitivity to glucocorticoids. The expression of TGFβ receptors also was not enhanced by Dex. TGFβ significantly stimulated glucocorticoid responsive element-mediated transcription activity. Combined Dex and TGFβ stimulated the expression of c-jun and c-fos early responsive genes in U937 cells, although Dex or TGFβ alone did not. The combination synergistically induced expression of c-jun gene, reaching a maximum level at 24 h. On the other hand, expression of c-fos gene was induced by TGFβ alone and increased additively in combination with Dex. Treatment with antisense oligonucleotide complementary to the first exon of c-jun mRNA reduced the growth-inhibitory effect of Dex and TGFβ in a dose-dependent manner. However, exposure of U937 cells to the sense oligomer of c-jun mRNA or an antisense oligomer of c-fos mRNA did not affect the growth inhibition. These results suggested that the preferential expression of c-jun and stimulation of glucocorticoid responsive element-mediated transactivation are closely associated with the growth arrest of U937 cells incubated with Dex and TGFβ.