In the last 2 decades, there have been great progress in the study of carcinogenesis of type I endometrial cancer [45
]. However, studies on type II carcinogenesis have been remarkably limited until recently. Type II endometrial cancer represents only about 15% of the endometrial cancers but accounts for approximately 80% of endometrial cancer-related deaths [46
]. In contrast, type I cancers constitute up to 85% of all endometrial cancers which are typically with a good prognosis. Therefore, the development of molecular markers specific for the different histologic types would provide useful adjuncts to the morphologic assessment of endometrial lesions and thus facilitate patient care. Meanwhile, it is clinically important if the molecular markers actually play a role in carcinogenesis so that an effective therapeutic intervention may be administered.
In this report, we examined the expression of Nrf2 in a large number of endometrial tissue samples. We found that Nrf2 is an excellent cytoplasmic marker by immunohistochemistry to differentiate ESC from other endometrial cancers. Nrf2 was expressed in a significantly higher proportion of ESC (68%), as compared with endometrioid carcinomas (6%) (p
< 0.001), clear cell carcinoma (13%) (p
= 0.001), and other endometrial cancers ( and ). Mucinous carcinoma showed negative staining in 7 of the 8 cases tested. Non-neoplastic endometrial tissues were mostly negative. Therefore, assimilation of entire data indicates that Nrf2 expression pattern appears to be completely different between ESC and other endometrial cancers. Nrf2 seems to be a highly sensitive and specific cytoplasmic marker for the serous type of endometrial cancers. Positive staining, particularly in more than 50% of the neoplastic cells with at least moderate staining intensity, may lead us toward the diagnosis of an endometrial serous neoplasm, which has a serious impact in clinic [47
Furthermore, Nrf2 protein expression was dramatically different among the endometrial precursor lesions. EmGD including both serous and clear cell types are recognized precancers of ESC and CCC, respectively [38
]. Serous EIC, although develops earlier than ESC, is considered an early form of ESC due to its high association of extra uterine disease [47
]. AEH and/or EIN are well recognized type I pre-cancer [50
]. Nrf2 was negative in such lesions (), whereas it was positive in a substantial number of precursor lesions of type II cancer, which illustrated an interesting contrast. The expression rates of Nrf2 were dramatically increased in both serous EmGD (40%) and EIC (44%) as compared with lesions of AEH/EIN (0%) or corresponding clear cell precursors (10% and 25%, respectively) (). In our opinion, the congruence in the expression rates between serous EmGD and serous EIC suggests that endometrial neoplasia with serous differentiation is not only different from those with endometrioid as previously known [45
] but also different from those with clear cell differentiation. Moreover, the high level of Nrf2 expression in serous EmGD is a solid line of evidence linking this precancer to the full-blown ESC.
More evidence has indicated a positive role of Nrf2 in carcinogenesis and chemoresistance. The first evidence came from the finding that Nrf2 was elevated during hepatocarcinogenesis [32
]. Its elevation was then found in many cancers including lung, ovary, and breast when the Nrf2 negative regulator Keap1 lost function [23
]. Using genetic manipulation, Zhang's group and others have demonstrated a strong positive correlation between Nrf2 levels and resistance to chemotherapeutic drugs such as cisplatin in both endometrial and ovarian cancer cells [23
]. Furthermore, we have recently shown that reduced levels of Nrf2 (either directly or via manipulation of its negative regulator) significantly sensitized SPEC-2 cells and its xenografts to chemotherapeutic drugs [28
]. Collectively, these findings may explain why Nrf2 is associated with tumor progression and the aggressiveness of ESC. Further studies are required to investigate how the level of Nrf2 is regulated in ESC and its precancers.
It is interesting to note that Nrf2 expression in CCC and its precursors was closer to that in EEC, rather than that in ESC. There is, thus far, no molecular study on endometrial cancer that included a sufficient number of clear cell carcinoma has been published. In various studies, a small number of CCCs were analyzed, mostly together with ESC. Thus, it has been speculated that CCC is similar to ESC. However, evidence supporting that they have different molecular pathogenesis pathways does exist. Compared to ESC, alterations of p53, PTEN, K-Ras, and EGFR were significantly different in CCC [53
]. Therefore, it is not clear if pathogenesis of CCC is similar to ESC. Significantly different Nrf2 expression between CCC and ESC and their precursors is supportive of a different pathogenetic mechanism in these type II endometrial cancers.
In summary, our results in this study indicate that expression of Nrf2 is closely associated with endometrial serous cancer, while it seems not related to type I endometrial cancer. Importantly, strong and diffuse Nrf2 cytoplasmic expression is highly sensitive for ESC including its early and pre-cancers. As a result, Nrf2 may serve as a useful diagnostic marker in the assessment of endometrial cancers and their precursor lesions, particularly when the amount of available tissue material is limited and morphology is ambiguous. Alteration of Nrf2 expression may represent one of the early molecular events in the neoplastic transformation of endometrial serous cancers. Furthermore, overexpression of Nrf2 may be partially responsible to the aggressive biological behavior and dismal clinical outcome of ESC due to its known effect of increased resistance to chemotherapeutic drugs. The findings may also provide an opportunity for therapeutic intervention against chemoresistancevia applications of either Nrf2 inhibitors or gene knockdown approaches for ESC.