Cell Division Cycle Genes Among the factors that regulate the cell cycle checkpoints, proteins encoded by the cdc2 family of genes and the cyclin proteins appear to play particularly important roles (8,9). Growth factor–stimulated mammalian cells express early-response or delayed-response genes, depending on the chronological sequence of the appearance of specific RNAs. The early-and delayed-response genes act as nuclear transcription factors and stimulate the expression of a cascade of other genes. Early-response genes such as c-Jun and c-Fos enhance the transcription of delayed-response genes such as E2Fs. E2F transcription factors are required for the expression of various cell cycle genes and are functionally regulated by the retinoblastoma (Rb) protein. Binding of Rb to E2F converts E2F from a transcriptional activator to a repressor of transcription. Phosphorylation of Rb inhibits its repressing function and permits E2F-mediated activation of genes required for entry into the S phase. Cdk4-cyclin D, Cdk6-cyclin D, and Cdk2-cyclin E complexes cause phosphorylation of Rb, which remains phosphorylated throughout the S, G2, and M phases of the cell cycle. After completion of mitosis, a decline of the level of Cdk-cyclins leads to dephosphorylation of Rb by phosphatases and, consequently, an inhibition of E2F in the early G1 phase.
Cdks are being evaluated as targets for cancer treatments because they are frequently overactive in cancer disease and Cdk-inhibiting proteins are dysfunctional. The Cdk4 inhibitor P1446A-05, for example, specifically inhibits Cdk4-mediated G1-S phase transition, arresting cell cycling and inhibiting cancer cell growth (10). SNS-032 selectively binds to Cdk2, -7, and -9, preventing their phosphorylation and activation and subsequently preventing cell proliferation.
As cells approach the G1-S phase transition, synthesis of cyclin A is initiated. The Cdk2-cyclin A complex can trigger initiation of DNA synthesis by supporting the prereplication complex. The p34 cdc2 protein and specific cyclins form a complex heterodimer referred to as mitosis-promoting factor (MPF), which catalyzes protein phosphorylation and drives the cell into mitosis. Cdk1 assembles with cyclin A and cyclin B in the G2 phase and promotes the activity of the MPF. Mitosis is initiated by activation of the cdc gene at the G2-M checkpoint (11,12). Once the G2-M checkpoint is passed, the cell undergoes mitosis. In the presence of abnormally replicated chromosomes, progression past the G2-M checkpoint does not occur.
The p53 tumor suppressor gene participates in cell cycle control. Cells exposed to radiation therapy exhibit an S-phase arrest that is accompanied by increased expression of p53. This delay permits the repair of radiation-induced DNA damage. In the presence of p53 mutations, the S-phase arrest that normally follows radiation therapy does not occur (13,14). The wild type p53 gene can be inactivated by the human papillomavirus E6 protein, preventing S-phase arrest in response to DNA damage (15).