Assistant Professor
Ph.D. University of Manitoba, 1994
Research in my lab is focused on the cell cycle, and in particular on the controls governing the decision to transit from G2 into mitosis. DNA damage induces arrest in the G2 phase of the cell cycle, a response conserved from yeast to humans (Taylor and Stark, 2004). By protecting genomic integrity, G2 arrest helps to prevent tumorigenesis.
Regulation of the G2/M Transition by Tumor Suppressor p53.
We discovered that p53 can cause G2 arrest (Agarwal et al., 1995). p53 is an important factor in tumorigenesis and more than half of all human tumors contain inactivating mutations in the p53 gene. The tumor suppressor function of p53 lies in its ability to induce either cell cycle arrest or cell death in response to genotoxic stress. p53 controls multiple points of the cell cycle including the G1/S transition, progression through S phase, and the transit from G2 into mitosis (reviewed in Agarwal et al., 1998b; Taylor and Stark, 2001).
DNA damage triggers a rapid, p53-independent pathway that initiates G2 arrest. p53 maintains the arrest ensuring that cells do not enter mitosis with damaged DNA. We are analyzing the mechanism by which p53 maintains G2 arrest. Part of this mechanism involves transcriptional repression mediated by Rb/E2F complexes which form complexes in response to p53 (Taylor et al., 1999b; Taylor et al., 2001). We are using standard biochemical approaches to analyze events downstream of p53 that allow the Rb/E2F complex to form. Next, we will examine promoter occupancy by Rb/E2F in vivo using chromatin immunoprecipitation. 
Our analysis of the role of p53 in blocking entry into mitosis has led us to a modified view of the mammalian cell cycle. Many years of research have shown that animal cells can exit the cell cycle exclusively from G1, for example during terminal differentiation. Our work suggests that when DNA is damaged, p53 can drive G2 cells out of the cell cycle into a non-proliferative state (Jackson et al., 2005). This transition involves the Rb/E2F-dependent transcriptional repression of many genes that encode proteins required for mitosis. A hallmark of cancer cells is the inability to exit the cell cycle from G1 when extracellular growth factors are depleted, resulting in cells that proliferate without heed to extracellular cues to stop. Since many tumor cells have mutations that inactivate p53 and Rb, this leads to the intriguing idea that tumor cells may also be unable to stably exit the cell cycle from G2 when DNA is damaged.
Identification of novel cell cycle genes.
We used Affymetrix gene arrays to analyze transcriptional repression by the Rb family proteins p130 and p107. We found that most genes that are repressed in a p130/p107-dependent manner encode proteins needed for cell cycle progression and many are needed for mitosis. We also found several genes with unknown functions that are repressed by p130 and p107. We hypothesize that some of these genes represent novel cell cycle genes. We are currently focusing on one gene, cdca8, that has recently been found to encode a member of the chromosomal passenger complex. This complex plays an important role in controlling the subcellular localization and activity of Aurora B kinase. Aurora B is essential for proper attachment of chromosomes to the mitotic spindle without which cells can mis-segregate chromosomes upon division. Aurora B is also important in ensuring that cells divide into two roughly equal halves, with each half recieving a complete copy of the genome. We are currently studying the post-translational modification of the cdca8 gene product. We are also studying the responses of cancer cells to Aurora B kinase inhibitors to determine whether this class of drug may be useful in the treatment of cancer.
Summary
Tumor cells frequently acquire mutations that inactivate tumor suppressors such as p53 and Rb, and activate oncogenes such as Ras. Some of these changes alter the fidelity of cell division leading to further defects in genomic stability. Our studies are focused on the mechanics of cell division in order to gain insight into the defects that occur in cancer leading to genetic alterations at the chromosomal level. These insights will help in the design of novel diagnostic and therapeutic tools.
Kaur, H, Date, D, Stiff, A, and Taylor, WR. (2006). Borealin is phosphorylated during mitosis independently of Aurora B kinase activity (in preparation).
Kosik, A, Katusin, J, Chadee, D and Taylor, WR. (2006). Investigating the role of Aurora Kinases in Ras signaling (in preparation).
Jacob, C, Stiff, A, Jackson, MW, and Taylor, WR. (2006). borealin is repressed in response to p53/Rb signaling. BBA Gene Structure and Expression (submitted).
Stark, GR and Taylor, WR. (2006). Control of the G2/M transition. Molecular Biotechnology. 32:227-48.
Jackson, MW, Agarwal, MK, Yang, J, Bruss, P, Uchiumi, T, Agarwal, ML, Stark, GR and Taylor, WR. (2005). p53/Rb-dependent transcriptional repression during cell cycle exit at G2. J. of Cell Science 118: 1821-1832.
Taylor, WR. (2004) FACS-based detection of phosphorylated histone H3 for the quantitation of mitotic cells. In Schönthal, AH (ed.) Methods Mol Biol, vol 280, Checkpoint Controls and Cancer: Methods and Protocols, Volume 2. pp 293-300. Humana Press, Totowa, NJ.
Clifford, B, Beljin, M, Stark, GR, and Taylor, WR. (2003). G2 arrest in response to topoisomerase II inhibitors: The role of p53. Can. Res. 63: 4074–4081.
Taylor, WR, Schonthal, AH, Galante, J, and Stark GR. (2001). p130/E2F4 binds to and represses the cdc2 promoter in response to p53. J. Biol. Chem. 276: 1998–2006.
Taylor, WR, and Stark GR. Regulation of the G2/M transition by p53. (2001). Oncogene 20: 1803-1815.
Taylor, WR, Agarwal, ML, Agarwal, A, Stacey, DW, and Stark, GR. (1999a). p53 inhibits entry into mitosis when DNA synthesis is blocked. Oncogene 18: 283-296.
Taylor, WR, DePrimo, SE, Agarwal, A, Agarwal, ML, Schonthal, AH, Katula, KS, and Stark GR. (1999b). Mechanisms of G2 arrest in response to overexpression of p53. Mol. Biol. Cell 10: 3607-3622.
Agarwal, ML, Agarwal, A, Chernova, OB, Taylor, WR, Sharma, YK, and Stark, GR. (1998a). A p53-dependent S-phase checkpoint protects cells lacking a G1 checkpoint from DNA damage in response to starvation for pyrimidine nucleotides. Proc. Natl. Acad. Sci. USA 95: 14775-14780.
Huang, A, Fan, H, Taylor, WR, and Wright, JA. (1997). Ribonucleotide reductase R2 gene expression and changes in drug sensitivity and genome stability. Cancer Res. 57: 4876-4881.
Agarwal, ML, Agarwal, A, Taylor, WR, Wang, Z-Q. Wagner, EF, and Stark, GR. (1997). Defective induction but normal activation and function of p53 in mouse cells lacking poly-ADP-ribose polymerase. Oncogene 15: 1035-1043.
Agarwal, ML, Agarwal, A, Taylor, WR, and Stark, GR. (1995). p53 controls both G2/M and G1 cell cycle checkpoints and mediates reversible growth arrest in human fibroblasts. Proc. Natl. Acad. Sci. USA 92: 8493-8497.
Taylor, WR, Chadee, DN, Allis, CD, Wright, JA, and Davie, JR. (1995). Fibroblasts transformed by combinations of ras, myc, and mutant p53 exhibit increased phosphorylation of histone H1 that is independent of metastatic potential. FEBS lett. 377: 51-53.
Taylor, WR, Greenberg, AH, Turley, EA, and Wright, JA. (1993). Cell motility, invasion and malignancy induced by overexpression of K-FGF or bFGF. Exp. Cell Res. 204: 295-301.
Taylor, WR, Egan, SE, Mowat, M, Greenberg, AH, and Wright, JA. (1992). Evidence for synergistic interactions between ras, myc and a mutant form of p53 in cellular transformation and tumor dissemination. Oncogene 7: 1383-1390.