SIRF--a novel regulator element controlling transcription from the p55Cdc/Fizzy promoter during the cell cycle

Context is key: Understanding the regulation, functional control, and activities of the p53 tumour suppressor

The p53 tumour suppressor is considered one of the most critical genes in cancer biology. By upregulating apoptosis, cell cycle arrest, and DNA damage repair in normal cells, p53 prevents the propagation of cells with tumorigenic potential; therefore, mutations in p53 are associated with carcinogenic transformation and can be accompanied by the accumulation of a novel gain-of-function oncogenic protein, mutant p53. Although p53 is most often understood to utilize context-dependent post-translational modifications to achieve regulation of its many target genes, recent research has also sought to define other mechanisms of regulating p53 gene expression prior to translation and to understand how this alternative regulation of p53 may influence target gene expression and cellular outcome. This review attempts to summarize what is known about p53 regulation at the transcriptional, post-transcriptional, and post-translational levels while paying special attention to the ways in which context may influence p53 regulation and subsequent regulation of its target genes.

Cell cycle arrest through indirect transcriptional repression by p53: I have a DREAM

Activation of the p53 tumor suppressor can lead to cell cycle arrest. The key mechanism of p53-mediated arrest is transcriptional downregulation of many cell cycle genes. In recent years it has become evident that p53-dependent repression is controlled by the p53–p21–DREAM–E2F/CHR pathway (p53–DREAM pathway). DREAM is a transcriptional repressor that binds to E2F or CHR promoter sites. Gene regulation and deregulation by DREAM shares many mechanistic characteristics with the retinoblastoma pRB tumor suppressor that acts through E2F elements. However, because of its binding to E2F and CHR elements, DREAM regulates a larger set of target genes leading to regulatory functions distinct from pRB/E2F. The p53–DREAM pathway controls more than 250 mostly cell cycle-associated genes. The functional spectrum of these pathway targets spans from the G₁ phase to the end of mitosis. Consequently, through downregulating the expression of gene products which are essential for progression through the cell cycle, the p53–DREAM pathway participates in the control of all checkpoints from DNA synthesis to cytokinesis including G₁/S, G₂/M and spindle assembly checkpoints. Therefore, defects in the p53–DREAM pathway contribute to a general loss of checkpoint control. Furthermore, deregulation of DREAM target genes promotes chromosomal instability and aneuploidy of cancer cells. Also, DREAM regulation is abrogated by the human papilloma virus HPV E7 protein linking the p53–DREAM pathway to carcinogenesis by HPV. Another feature of the pathway is that it downregulates many genes involved in DNA repair and telomere maintenance as well as Fanconi anemia. Importantly, when DREAM function is lost, CDK inhibitor drugs employed in cancer treatment such as Palbociclib, Abemaciclib and Ribociclib can compensate for defects in early steps in the pathway upstream from cyclin/CDK complexes. In summary, the p53–p21–DREAM–E2F/CHR pathway controls a plethora of cell cycle genes, can contribute to cell cycle arrest and is a target for cancer therapy.

Transcriptional control of mitosis: deregulation and cancer

Research over the past few decades has well established the molecular functioning of mitosis. Deregulation of these functions has also been attributed to the generation of aneuploidy in different tumor types. Numerous studies have given insight into the regulation of mitosis by cell cycle specific proteins. Optimum abundance of these proteins is pivotal to timely execution of mitosis. Aberrant expressions of these mitotic proteins have been reported in different cancer types. Several post-transcriptional mechanisms and their interplay have subsequently been identified that control the level of mitotic proteins. However, to date, infrequent incidences of cancer-associated mutations have been reported for the genes expressing these proteins. Therefore, altered expression of these mitotic regulators in tumor samples can largely be attributed to transcriptional deregulation. This review discusses the biology of transcriptional control for mitosis and evaluates its role in the generation of aneuploidy and tumorigenesis.

The central role of CDE/CHR promoter elements in the regulation of cell cycle-dependent gene transcription

The cell cycle-dependent element (CDE) and the cell cycle genes homology region (CHR) control the transcription of genes with maximum expression in G(2) phase and in mitosis. Promoters of these genes are repressed by proteins binding to CDE/CHR elements in G(0) and G(1) phases. Relief from repression begins in S phase and continues into G(2) phase and mitosis. Generally, CDE sites are located four nucleotides upstream of CHR elements in TATA-less promoters of genes such as Cdc25C, Cdc2 and cyclin A. However, expression of some other genes, such as human cyclin B1 and cyclin B2, has been shown to be controlled only by a CHR lacking a functional CDE. To date, it is not fully understood which proteins bind to and control CDE/CHR-containing promoters. Recently, components of the DREAM complex were shown to be involved in CDE/CHR-dependent transcriptional regulation. In addition, the expression of genes regulated by CDE/CHR elements is mostly achieved through CCAAT-boxes, which bind heterotrimeric NF-Y proteins as well as the histone acetyltransferase p300. Importantly, many CDE/CHR promoters are downregulated by the tumor suppressor p53. In this review, we define criteria for CDE/CHR-regulated promoters and propose to distinguish two classes of CDE/CHR-regulated genes. The regulation through transcription factors potentially binding to the CDE/CHR is discussed, and recently discovered links to central pathways regulated by E2F, the pRB family and p53 are highlighted.

Dual inhibition of Cdc20 by the spindle checkpoint

The metaphase-to-anaphase transition is triggered by the Anaphase-Promoting Complex (APC), an E3 ubiquitin ligase that targets proteins for degradation, leading to sister chromatid separation and mitotic exit. The function of APC is controlled by the spindle checkpoint that delays anaphase onset in the presence of any chromosome that has not established bipolar attachment to the mitotic spindle. In this way, the checkpoint ensures accurate chromosome segregation. The spindle checkpoint is mostly activated from kinetochores that are not attached to microtubules or not under tension that is normally generated from bipolar attachment. These kinetochores recruit several spindle checkpoint proteins to assemble an inhibitory complex composed of checkpoint proteins Mad2, Bub3, and Mad3/BubR1. This complex binds and inhibits Cdc20, an activator and substrate adaptor for APC. In addition, the checkpoint complex promotes Cdc20 degradation, thus lowering Cdc20 protein level upon checkpoint activation. This dual inhibition on Cdc20 likely ensures that the spindle checkpoint is sustained even when the cell contains only a single unattached kinetochore.

Human cyclin B3. mRNA expression during the cell cycle and identification of three novel nonclassical nuclear localization signals

Cyclins form complexes with cyclin-dependent kinases. By controlling activity of the enzymes, cyclins regulate progression through the cell cycle. A- and B-type cyclins were discovered due to their distinct appearance in S and G(2) phases and their rapid proteolytic destruction during mitosis. Transition from G(2) to mitosis is basically controlled by B-type cyclins. In mammals, two cyclin B proteins are well characterized, cyclin B1 and cyclin B2. Recently, a human cyclin B3 gene was described. In contrast to the expression pattern of other B-type cyclins, we find cyclin B3 mRNA expressed not only in S and G(2)/M cells but also in G(0) and G(1). Human cyclin B3 is expressed in different variants. We show that one isoform remains in the cytoplasm, whereas the other variant is translocated to the nucleus. Transport to the nucleus is dependent on three autonomous nonclassical nuclear localization signals that where previously not implicated in nuclear translocation. It had been shown that cyclin B3 coimmunoprecipitates with cdk2; but this complex does not exhibit any kinase activity. Furthermore, a degradation-resistant version of cyclin B3 can arrest cells in G(1) and G(2). Taken together with the finding that cyclin B3 mRNA is not only expressed in G(2)/M but is also detected in significant amounts in resting cells and in G(1) cells. This may suggest a dominant-negative function of human cyclin B3 in competition with activating cyclins in G(0) and the G(1) phase of the cell cycle.