Persistent Aurora (Aur) A and B over-expression, centrosome amplification (CA), chromosomal instability (CIN) and aneuploidy invariably occur together in a vast majority of human neoplasms. These molecular changes are frequently found (>80%) in human sporadic ductal carcinoma in situ (DCIS) and in primary invasive ductal breast cancers (IDBC). In solely 17β-estradiol (E₂)-induced mammary tumors in ACI rats, Li et al. have previously shown AurA protein over-expression, CA, CIN and aneuploidy (Li et al., 2004). AurA and B, mitotic kinases involved in controlling proper cell division, are modulated by cell cycle progression. Their over-expression is implicated in the deregulation of chromosomal duplication/segregation, and cytokinesis leading to CA, CIN and aneuploidy. To determine whether the over-expression of Aur kinases is a common feature of E₂-induced oncogenesis, AurA and B expression were examined in the Syrian hamster tumors of the kidney. Western blot analysis of the E₂-induced Syrian hamster tumors of the kidney revealed that both AurA and B protein expression was markedly increased when compared to cholesterol-treated controls. Moreover, immunohistochemistry revealed that this increase in AurA and B expression was specifically localized to E₂-induced tumor cells. Using an in vitro kinase assay, a significant increase in AurA kinase activity was detected in these tumors of the kidney and a significant increase in AurA mRNA levels was detected as measured by real-time PCR. The over-expression of both kinases was markedly reduced in E₂-induced tumor-bearing hamsters subjected to either a 10-day E₂-withdrawal period or treated for a similar period with Tamoxifen citrate (Tx) plus E₂, when compared to tumors of the kidney from hamsters treated with E₂ alone. These results indicate that both AurA and B are under estrogen control mediated by estrogen receptor α (ERα). Additionally, examination of centrin and γ-tubulin expression in Syrian hamster tumors of the kidney indicates an increase in centrosome number and size, a characteristic of CA. The AurA expression was co-localized to isolated amplified tumor centrosomes, identified by their high levels of centrin and γ-tubulin expression. CIN and aneuploidy in Syrian hamster tumors of the kidney has previously been demonstrated (Papa et al., 2003). In order to assess, during E₂-induced oncogenesis, whether the over-expression of AurA and B contributes to the deregulation of the centrosome cycle via their specific protein substrates, we analyzed the protein expression of centrin, histone H3, PP1 and TPX2, all of which were significantly over-expressed in Syrian hamster tumors of the kidney and in ACI rat mammary gland tumors. Additionally, the expression of MDM2, which regulates the tumor suppressor gene p53, and p53 protein, an AurA substrate, were examined in Syrian hamster tumors of the kidney and in ACI rat mammary gland tumors. MDM2 expression was significantly increased in both Syrian hamster tumors of the kidney and in ACI rat mammary gland tumors at the protein and mRNA level, and estrogen modulation studies showed that MDM2 protein expression is either directly or indirectly controlled by E₂. Binding studies show that MDM2 is bound to p53wt. MDM2-p53wt binding was inhibited in Syrian hamster tumors of the kidney after treatment with the small molecule inhibitor RITA (Reactivation of p53 and Induction of Tumor cell Apoptosis). In E₂-induced Syrian hamster tumors of the kidney, Western blot analysis revealed that RITA treatment led to a significant increase in p53wt protein expression when compared to untreated tumor samples. In addition, RITA treatment led to the increased expression of Bax, a downstream target gene of p53, which promotes apoptosis. These data suggest that Aur kinase over-expression, induced by estrogens, may interfere with bipolar spindle formation and chromosome segregation, leading to CA, CIN and aneuploidy, thus supporting the concept that AurA over-expression and CA are causative events, induced by estrogen, that lead to tumor development. The overexpression of MDM2, in tandem with AurA, indicates a close relationship between these two entities in fostering CA, CIN, aneuploidy and tumor development, defining events of E₂-driven oncogenesis.