Cyclin b1 promoter activity and functional cdk1 complex formation in G1 phase of human breast cancer cells.

Overexpression of cyclin B has been detected in various human breast cancer cell lines, breast tumor tissues, and immortalized but nontransformed breast cells. The cause of this overexpression has not been thoroughly investigated, nor is it known if cyclin B protein forms a functional complex with its partner, cdk1, at inappropriate cell cycle periods. In this study we examined the pattern of cyclin B1 promoter activity in three breast cancer cell lines, BT-549, MDA-MB-157, T-47D, and the immortalized breast cell line MCF-10F. Using cells stably transfected with a cyclin B1 promoter-luciferase reporter, luciferase activity was measured throughout the cell cycle in lovastatin synchronized cells and in G1 and S/G2 phases of asynchronized cells by flow cytometry. Results demonstrate that the cyclin B1 promoter activity increases, as expected, during the S/G2 period in all the cell lines. However, some promoter activity can be detected in G1 phase of the different cell line with BT-549 displaying the more altered pattern. Functional cyclin B1-cdk 1 protein complex was detected in G1 phase of BT-549 and T-47D cell lines. These results suggest that in a subset of transformed breast cancer cells altered cyclin B1 promoter activity may contribute to the misexpression of cyclin B protein.