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Aldehyde dehydrogenase 1 (ALDH1) is expressed in stem/progenitor cells, including cancer-initiating cells (CIC) of various organs. In the present study, ALDH1 expression was immunohistochemically examined in uterine endometrioid adenocarcinoma. The ALDH1 was expressed in a small portion of tumor cells, and these ALDH1-expressing cells were less mature than ALDH1-non-expressing cells. The ALDH1-expressing (ALDH1-hi) cells were more tumorigenic, resistant to anti-cancer agents and more invasive than ALDH1-lo cells. Culture of the sorted ALDH1-hi cells yielded both ALDH1-hi and ALDH1-lo cells, whereas ALDH1-lo cells yielded ALDH-lo cells alone. Clinically, a high-level of ALDH1 expression in tumor cells was correlated with T category, lymphatic invasion, recurrence and prognosis of patients. Patients with high ALDH1 expression showed poorer prognoses than those with low expression (P = 0.015 for disease-free survival [DFS] and P = 0.010 for overall survival [OS]), and high ALDH1 expression was an independent factor for poor prognosis. Aldehyde dehydrogenase 1 is a candidate for CIC marker for uterine endometrioid adenocarcinoma. (Cancer Sci 2011; 102: 903–908)
Tumors consist of heterogeneous cell populations derived from a single clone. Recently, it has been demonstrated that cells with tumorigenic potential are limited to a small population among tumor cells, called cancer-initiating cells (CIC), in cancers of blood (leukemia), breast, brain and colon.(1–11) Cancer-initiating cells efficiently efflux anti-tumor agents and degrade reactive oxygen species that are related to radiation-induced apoptosis. Furthermore, CIC are in a quiescent state for cell division, and thus escape the attack of various anti-cancer drugs targeting the rapidly dividing tumor cells. These characteristics enable CIC to be resistant to anti-tumor drugs and radiation therapy.(12–15)
Endometrioid adenocarcinoma is one of the most common malignancies of the female genital system.(16,17) Despite the advances in methods for detection and treatment, prognosis of patients with endometrioid adenocarcinoma still remains unfavorable. Therapeutic strategies targeting CIC would be necessary to improve cure rates, but studies on CIC of endometrioid adenocarcinoma are limited. Gotte et al.(18) demonstrated that Musashi-1, highly expressed in neural stem cells, was co-expressed with Notch-1 in a subpopulation of endometrial cells and endometrioid adenocarcinoma cells. Kato et al.(19) demonstrated that the side-population of endometrioid adenocarcinoma, which is considered to contain CIC, possessed higher tumorigenic activities than non side-population cells. To our knowledge, the relationship of stem cell marker expression to prognosis has not been reported in endometrioid adenocarcinoma.
Aldehyde dehydrogenase 1 (ALDH1), a predominant isoform of the ALDH family in mammals, oxidizes retinol to retinoic acid in early stages of stem cell differentiation, and hematopoietic and neural stem cells show high ALDH1 activity.(20–22) Cancer-inducing cells of human multiple myeloma, acute myeloid leukemia and cancers of brain, lung and breast also show high ALDH1 activity.(23–27) The activity of ALDH1 might be a common marker for both normal and malignant stem cell populations. In the present study, ALDH1 expression was immunohistochemically examined in endometrioid adenocarcinoma and its clinical implications were evaluated.
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- Materials and Methods
Normal stem/progenitor cells of various lineages, such as hematopoietic, neural and mesenchymal stem cells, show high ALDH1 activity.(20–22) In addition, CIC have been reported to show high ALDH1 activity: the ALDH1-hi population is tumorigenic and resistant to chemotherapy in cancers of colon, breast, lung, pancreas, bladder, prostate and ovary.(23–27) To our knowledge, the role of ALDH1 in uterine cancer has never been studied. In the present study, we showed that ALDH1-hi endometrioid adenocarcinoma cells to be more tumorigenic, resistant to anti-cancer agents and invasive than ALDH-lo cells. Culture of the sorted ALDH-hi cells yielded both ALDH-hi and ALDH-lo cells, whereas culture of the ALDH-lo cells yielded ALDH-lo cells alone. These findings suggest that the ALDH-hi population possessed the character of CIC in endometrioid adenocarcinoma of uterus, like cancers of other organs.
In clinical specimens, ALDH1-expressing tumor cells were mostly negative for CD9, ER and PgR, indicating that ALDH1 expression was detected in tumor cells of a less mature state. Since CIC are in the immature state, this was consistent with the notion that ALDH1 was expressed in cells with CIC character.
The expression of ALDH1 was limited to a small portion of endometrioid adenocarcinoma cells, which were randomly located in the tumor tissues. Cells with CIC character in squamous cell carcinoma are reported to be located in the outer layer of cancer nests,(29,30) contrasting with the absence of any specific location of CIC in endometrioid adenocarcinoma.
Diffuse expression of ALDH1 was found in some clinical cases of endometrioid adenocarcinoma (Fig. 1d). This appeared to be incompatible with the concept that CIC comprise a small population of cancer cells with multiple differentiations and long-term repopulation capabilities. In several reports, CIC markers were expressed in most tumor cells in clinical samples, such as ALDH1 in breast cancers.(20,31) When most tumor cells possess CIC character, the tumor character might become aggressive. Alternatively, ALDH1 might be a marker of undifferentiated cancer cells but not a CIC marker.
The clinical implication of ALDH1 expression was evaluated in 98 cases of endometrioid adenocarcinoma. The characteristics of patients, such as age and stage, in the current study were similar to those in a previous report,(32) indicating that the results obtained from the current study are commonly applicable to endometrioid adenocarcinoma worldwide. The present study showed that a high level of ALDH1 expression was correlated with T category, lymphatic invasion, resistance to chemotherapy, recurrence, and prognosis of patients. Patients with higher ALDH1 expression showed poorer prognoses than those with lower expression (P = 0.015 for DFS and P = 0.010 for OS), and high ALDH1 expression was an independent poor prognostic factor. These findings were consistent with the previous observation that a high percentage of ALDH1-expressing cells in most types of epithelial tumors, such as breast, lung, pancreatic, bladder, ovarian and prostate, is associated with a poorer outcome of these patients.(23–27) Thus, ALDH1 might be a common marker for CIC among cancers of various organs.
Endometrioid adenocarcinoma is the most common invasive malignancy of the female genital system, and novel therapeutic strategies targeting CIC would be necessary to improve cure rate. Very recently, Yang et al.(33) reported that LIN28 positively and let-7 negatively regulates ALDH1 expression in breast and ovarian cancers, and suggested that targeting ALDH1 expression via a LIN28/let-7 axis by small chemical compounds could be a therapeutic modality. Then, ALDH1 would be an effective target for therapies to CIC not only in breast and ovarian cancers but also in endometrioid adenocarcinoma of the uterus. Further studies on ALDH1 regulation may open a new therapeutic modality for endometrioid adenocarcinoma.