MxiI inhibits the proliferation of U87 glioma cells through down-regulation of cyclin B1 gene expression

Mxil is a Mad family member that plays a role in cell proliferation and differentiation, To test the role of Mxil on tumorigenesis of glioma cells we transfected a CMV-driven MXII cDNA in U87 human glioblastoma cells. Two clones were isolated expressing MXII levels 18- and 3.5-fold higher than wild-type U87 cells (clone U87.Mxi1.14 and U87.Mxi1.22, respectively). In vivo, U87.Mxi1.14 cells were not tumorigenic in nude mice and delayed development of tumours was observed with U87,Mxi1.22 cells. in vitro, the proliferation rate was partially and strongly inhibited in U87.Mxi1.22 and U87,Mxi1.14 cells respectively, The cell cycle analysis revealed a relevant accumulation of U87.Mxi1.14 cells in the G(2)/M phase. Interestingly, the expression of cyclin B I was inhibited to about 60% in U87.Mxi1.14 cells, This inhibition occurs at the transcriptional level and depends, at least in part, on the E-box present on the cyclin B I promoter, Consistent with this, the endogenous Mxi1 binds this E-box in vitro. Thus, our findings indicate that Mxi1 can act as a tumour suppressor in human glioblastomas through a molecular mechanism involving the transcriptional down-regulation of cyclin B I gene expression.