Activation of anchorage-independent growth of HT1080 human fibrosarcoma cells by dexamethasone

Anchorage independence is an important hallmark of the transformation that correlates with tumorigenicity. We have isolated a variant clone of HT1080 human fibrosarcoma cells (cl-2) that is specifically defective in anchorage-independent growth. Interestingly, 10(-7) M dexamethasone (DEX) substantially rescued the anchorage-independent growth of cl-2 cells in semisolid culture. DEX also promoted the anchorage-independent growth of parental HT1080 cells. However, the agent had no effect on the anchorage-dependent growth of cl-2 and parental cells in ordinary liquid culture. Cell cycle analysis demonstrated that the population of G0/G1 cells increased, whereas that of S and G2/M cells decreased in growth-arrested cl-2 cells in suspension culture. However, such an effect of anchorage loss on cell cycle progression was alleviated by adding 10(-7) M DEX. In cl-2 cells in semisolid culture, DEX suppressed the expression of P27Kip1, whereas it stimulated the expression of cyclin A and hyperphosphorylated retinoblastoma (Rb) proteins. On the other hand, DEX had no effect on cyclin D1 and P21Cap1 expression. These effects of DEX, except for the suppression of P27Kip1, were blocked by an antimicrofilament drug, cytochalasin D. Our results suggest that the stimulation of anchorage-independent growth by DEX involves at least two regulatory mechanisms, i.e., one that leads to the suppression of P27Kip1 protein without requiring cytoskeletal integrity, and another that requires cytoskeletal integrity, leading to stimulation of cyclin A and hyperphosphorylation of Rb protein.