Regulation of the p14ARF-Mdm2-p53 pathway: An overview in breast cancer
Introduction
Carcinogenesis occurs when abnormal cells begin to proliferate and have uncontrollable growth. Breast cancer is one of the most common malignancies in women in the United States (Greenlee et al., 2001, Cianfrocca and Goldstein, 2004). Though the mortality rate due to breast cancer is decreasing all over the world, a universal cure has yet to be found (Tripathy, 2005, Althuis et al., 2005). Breast cancer begins in the mammary ducts and progresses in four stages. In stage 0, a tumor is noninvasive and cell growth does not affect the functions of neighboring tissues. In stage 1, the tumor becomes invasive and begins to affect neighboring tissues, but has not reached the lymph nodes. In stage 2, the abnormal cells have reached individual lymph nodes. In stage 3, the cells in the individual lymph nodes begin to clump together and cause inflammation. In stage 4, the cancer spreads to other organs such as the brain, lung, or liver (Mettlin, 1999).
Section snippets
The cell cycle
The cell cycle is a structured set of biochemical events that duplicate the chromosomes of eukaryotes (Pardee et al., 1978, Morgan, 1995). Progression in the cell cycle is primarily regulated by cyclin-dependent kinases (Cdks) (Eastman, 2004). Innate mechanisms are involved in determining whether cells should proceed through the steps of the cell cycle.
The normal cell cycle divides cells mitotically using different phases. During the first gap (G1) phase of the cycle, cells respond to signals
Abnormalities in the cell cycle regulators
Specific regulatory genes function at various checkpoints throughout the cell cycle to check for damage and abnormalities. DNA damage checkpoints are at the G1/S boundary, S/G2 boundary, and also the G2/M boundary phase (Fig. 1). If damage is detected, cell growth cycle arrest and apoptosis occur. Normally, a growth factor binds to the cell surface and induces intracellular signals to initiate replication of the DNA and cell division. However, when abnormally active intracellular molecules bind
Tumor suppressor genes
Tumor suppressor genes prevent the action of oncoproteins. The three most widely studied, p14ARF, Rb, and p53, are cell cycle modulators. They function to control cell growth and at various checkpoints throughout the cell cycle to ensure that no abnormalities occur (Vestey et al., 2004, Pinto et al., 2005). However, inactivation and suppression by factors upstream and downstream of these gene products can cause development and growth of cancer.
p53
p53 is a 53-kDa nuclear phosphoprotein with 393-amino-acid residues (Hainaut et al., 1997, Whitfield, 1990, Sunpaweravong and Sunpaweravong, 2005, Hall and McCluggage, 2006). It functions to control the DNA damage and other cellular stresses that occur within a cell by inducing growth cycle arrest or apoptosis (Pinto et al., 2005, Pinto-de-Sousa et al., 2004, Eissa et al., 1990). p53 remains inactive unless cells are stressed or damaged. Any DNA damage, even a small break in the double-helical
Mutated p53 due to inactivation of upstream gene regulators and cofactors
Revelation of the importance of p53 in human carcinomas led to an extensive search for regulators of its expression. One such regulator is ATM, which is mutated in ataxia–telangectasia (A–T) (Carvajal et al., 2005). Normally, ATM activates p53 by phosphorylation of Ser15 to cause growth arrest or induce apoptosis. However, suppression of its action, mutation, or absence of ATM leaves p53 inactive and growth continues. Another protein, Chk2, normally activates p53 by phosphorylation of Ser20
MDM2 inactivates p53
Mouse double minute 2 (Mdm2), Hdm2 in humans, is a 491-amino-acid residue phosphoprotein located in the nucleolus, has a short half of 5 to 30 min, and is commonly overexpressed in about one third of human breast carcinomas (Sun et al., 1998, Jiang et al., 1997, Oliner et al., 1992, Chene, 2003). It normally induces G1 arrest in the cell cycle and also associates with and inactivates the tumor suppressor, p53 (Sun et al., 1998, Serrano et al., 1997). This dual role of Mdm2 helps to limit the
Mdm2 antagonists
Nutlins, Nutley inhibitors, are small-molecule inhibitors that selectively inhibit binding of Mdm2 to p53 (Vassilev et al., 2004, Tovar et al., 2006). However, nutlins do not function when p53 is inactivated by sequence mutation and are active when wild-type p53 is present (Galbiati et al., 2001). Therefore, these nutlins prevent proliferation of cancer cells and can provide potential anti-cancer benefits (Sigal and Rotter, 2000). The drug, leptomycin B, averts the shuttling of Mdm2 and p53
p14ARF
P14ARF, P19ARF in the mouse, is a 15-kDa protein that blocks the G1 and G2 phases in the cell cycle and inhibits growth of abnormal cells by indirectly activating p53 (Silva et al., 2003). Its expression in the cell cycle results from cellular stress, serum starvation or viral oncogenes. It also inhibits ribosomal RNA processing and interacts with topoisomerase I (Tago et al., 2005). The p14ARF gene is located predominantly in the nucleolus and on chromosome 9p21 in humans. This INK4a/ARF locus
Activators and inactivators of p14ARF
ARF is activated by various oncogenes, including cMyc, E1A, E2f1, Ras, and v-Abl (Weber et al., 2002, Anim et al., 2005). P14ARF can be inactivated as a consequence of hypermethylation of the promoter region of its gene and overexpression of several ARF repressors including Twist, Tbx-2 and Bmi-1 (Anim et al., 2005, Jing and Ming-Hua, 2003, Jacobs et al., 1999).
P14ARF→Mdm2→p53 pathway
The p14ARF-Mdm2-p53 pathway, commonly referred to as the p53 pathway, is disrupted in the development of cancer and is the focus of many investigations. P14ARF inhibits the activity of Mdm2, and this inhibition indirectly also hinders degradation of p53 (Tao and Levine, 1999, Kuo et al., 2003) (Fig. 4). Mdm2 shuttles p53 into the cytoplasm for degradation, but p53 attenuates Mdm2 activity if p53 is activated by maintaining an autoregulatory feedback loop where p53 acts as a transcription factor
Suppressors/Activators of P14ARF-Mdm2-p53 pathway
Several genes have been implicated as potential suppressors or activators of the pathway (Table 1).
Concluding remarks
Breast cancer continues to be diagnosed in one in eight American women and persists as the second leading cause of cancer death in women (Simstein et al., 2003, Seltzer, 2000). Further effective treatments for breast cancer may be developed if full knowledge of the mechanisms of action of the various proteins involved in tumor suppressor pathways, including the p53 pathway (Fig. 5), is revealed (Luka and Bartek, 2004). Improved prognosis should similarly ensue. Signaling proteins, including
Acknowledgments
This work was supported by the National Institutes of Health Grants R01HL070885 (to DKA) and R01HL073349 (to DKA), and Carpenter Chair (to RFM) of Creighton University.
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