REGULATED EXPRESSION OF THE RETINOBLASTOMA SUSCEPTIBILITY GENE IN MAMMARY CARCINOMA CELLS RESTORES CYCLIN D1 EXPRESSION AND G1-PHASE CONTROL

The product of the retinoblastoma susceptibility tumour suppressor gene, pRb, is a negative regulator of cell proliferation. In order to investigate the interaction between pRb and the cell cycle machinery in more detail, a functional Rb gene was reintroduced into the Rb-deficient human mammary carcinoma cell line Bt549. Since constitutive high level expression of Rb turned out to be difficult to maintain, the tetracycline-dependent gene expression system was used. A number of clones was generated which all showed low level expression in the noninduced state. Considerable induction rates were obtained. The low level of noninduced Rb expression was sufficient to induce the expression of cyclin D1 the level of which was not further increased by upregulation of Rb expression. Concomittantly, an increase in cell doubling time was observed due to retardation of the cell cycle in the G1-phase. The data suggest that limiting amounts of cyclin D1 determine, at least partly, the extent of growth-repressing properties of pRb. The inducible system allows for maintenance of Rb-reconstituted cells at a low level of expression and for their use in the investigation of downstream functions of pRb.