Effects of ATRA on MMP-2 and MMP-9 expression. Cells were incubated with the indicated concentrations of ATRA for 24 h. (a) MMPs expression was determined by zymogram. (b) Densitometric analysis of total expression of MMP-2. Relative MMP-2 level is comparod to negative control. The negative control group contains only serum-free DMEM with 1% DMSO and the positive control group contains 0.2% CHX solution. These data are representative of three independent experiments. *Statistically significant difference from the negative control group (P<0.05). ATRA, all-trans retinoic acid; CHX, cycloheximide; DMEM, Dulbecco's modified Eagle's medium; DMSO, dimethyl sulphoxide; MMP, matrix metalloproteinase.