Ever since Bishop and his co-workers discovered the c-myc gene in the late 1970s (Bishop 1982), voluminous literature has documented its central role in proliferation and malignant transformation of human and animal cells (Amati et al. 1998, Bouchard et al. 1998, Dang et al. 1999). Most, if not all, types of human malignancy have been reported to have amplification and/or overexpression of this gene, although the frequency of these alterations varies greatly among different reports (Nesbit et al. 1999). In 1992, researchers started to realize that aberrant expression of c-myc could cause apoptosis (Evan et al. 1992, Shi et al. 1992), although the phenomenon had actually been observed much earlier (Wurm et al. 1986). Studies in recent years have further shown that the c-myc gene regulates growth, both in the sense of cell size and in the context of tissue differentiation (Gandarillas & Watt 1997, Iritani & Eisenman 1999, Johnston et al. 1999, Schmidt 1999, Schuhmacher et al. 1999). Thus, it is now known that the c-myc gene participates in most aspects of cellular function, including replication, growth, metabolism, differentiation, and apoptosis (Packham & Cleveland 1995, Hoffman & Liebermann 1998, Dang 1999, Dang et al. 1999, Elend & Eilers 1999, Prendergast 1999). How the c-Myc protein may be specifically directed to perform one, but not the others, of these functions is still obscure, despite the fact that the relevant literature has been accumulating at a fast pace in the past two decades. This review focuses on the profound roles of c-Myc in breast cancer and in the actions of the hormones that are eitologically related to breast cancer.

In this study, we analysed gene amplification, RNA expression and protein expression of the c-myc gene on archival tissue specimens of high-grade human breast cancer, using fluorescent in situ hybridisation (FISH), nonradioactive in situ hybridisation and immunohistochemistry. The specific question that we addressed was whether expression of c-Myc mRNA and protein were correlated with its gene copy amplification, as determined by FISH. Although c-Myc is one of the most commonly amplified oncogenes in human breast cancer, few studies have utilised in situ approaches to directly analyse the gene copy amplification, RNA transcription and protein expression on human breast tumour tissue sections. We now report that by using the sensitive FISH technique, a high proportion (70%) of high-grade breast carcinoma were amplified for the c-myc gene, irrespective of status of the oestrogen receptor. However, the level of amplification was low, ranging between one and four copies of gene gains, and the majority (84%) of the cases with this gene amplification gained only one to two copies. Approximately 92% of the cases were positive for cmyc RNA transcription, and essentially all demonstrated c-myc protein expression. In fact, a wide range of expression levels were detected. Statistically significant correlations were identified among the gene amplification indices, the RNA expression scores and protein expression scores. c-myc gene amplification, as detected by FISH, was significantly associated with expression of its mRNA, as measured by the intensity of in situ hybridisation in invasive cells (P ¼ 0.0067), and by the percentage of invasive cells positive for mRNA expression (P ¼ 0.0006). c-myc gene amplification was also correlated with the percentage of tumour cells which expressed high levels of its protein, as detected by immunohistochemistry in invasive cells (P ¼ 0.0016). Thus, although multiple mechanisms are known to regulate normal and aberrent expression of c-myc, in this study, where in situ methodologies were used to evaluate highgrade human breast cancers, gene amplification of c-myc appears to play a key role in regulating expression of its mRNA and protein.

C-myc is an oncogene that functions both in the stimulation of cell proliferation and in and apoptosis. C-myc elicits its oncogenic activity by causing immortalization, and to a lesser extent the transformation of cells, in addition to several other mechanisms. C-myc may also enhance or reduce the sensitivity of cancer cells to chemotherapy, but how this dual function is controlled is largely unclear. Cyclin D1 (D1) is another oncogene that drives cell cycle progression; it acts as a growth factor sensor to integrate extracellular signals with the cell cycle machinery, though it may also promote apoptosis. C-Myc collaborates with TGFalpha, epidermal growth factor receptor, Ras, PI3K/Akt, and NF-kappaB. in part via coordination in regulation of D1 expression, because D1 is a common downstream effector of these growth pathways. Coordination of c-Myc with D1 or its upstream activators not only accelerates tumor formation, but also may drive tumor progression to a more aggressive phenotype. Because c-Myc may effect immortalization while D1 or its upstream activators elicit transformation, targeting c-myc and D1 may be a good strategy for cancer prevention. Moreover, since D1 imposes chemoresistance on cancer cells, targeting D1 may also be a good strategy for cancer chemotherapy, whereas practicioners should be cautious to downregulate c-myc for chemotherapy, since c-Myc may elicit apoptosis.