Ovarian cancer accounts for more than half of the deaths due to gynecological malignancy . There were an estimated 14,000 deaths in 2010, thus making it the 5th most common cause of cancer death among women in the United States . As most of the ovarian cancer patients are diagnosed in late stage and 80% of the patients recur despite successful surgery and chemotherapy, the 5-year survival rate is only 30% . Hence, specific and sensitive screening programs and identification of targets that are central to ovarian pathogenesis are of paramount value in decreasing the mortality of ovarian cancer.
Epithelial ovarian cancer is a tumor with great diversity. According to World Health Organization (WHO) criteria, ovarian tumors can be classified as benign, low malignant potential (borderline), or malignant . The histologic classification of ovarian carcinomas is based on morphologic criteria and corresponds to the different types of epithelia in the female reproductive system . There are four major histologic subtypes of epithelial ovarian cancer . Serous tumors are the most common type of ovarian neoplasm with epithelial cells resembling those of fallopian tube and comprise about 50% of primary epithelial ovarian tumors. Mucinous tumors represent 12-15% of epithelial ovarian cancers. They are cystic tumors with locules lined with mucin-secreting epithelial cells resembling either endocervical or colonic epithelium. Recent studies have shown that some mucinous ovarian tumors can be misdiagnosed due to metastasis from other organs . Endometrioid and clear cell tumors each account for 10% of epithelial ovarian cancers. These tumors are thought to arise from foci of endometriosis and endometriotic cysts within the ovary [7, 8]. Different tumor subtypes are characterized by dysregulation in specific pathways and have important ramifications in disease prognosis and treatment response [9–11]. Unfortunately, the molecular mechanisms underlying ovarian carcinogenesis and histological differentiation remain elusive.
The cancer stem cell (CSC) model hypothesizes the presence of a cellular hierarchy in the tumors such that a subset of tumor cells have the ability to self-renew and generate the diverse cells that comprise the tumor . CSCs may therefore be responsible for continual sustainment of tumorigenesis, as well as multilineage differentiation into different types of tumors. However, it is difficult to definitively identify cell surface immunophenotypes representing CSCs and their progeny in solid tumors. The cell surface biomarkers described thus far for the same tumor types are found highly variable by different research groups [13–15]. Recently there have been reports showing that differentiated cells can acquire self-renewing capacity [12, 16] and stem-like cancer cells arise de novo from non-stem cells in vitro and in vivo[17, 18], suggesting bidirectional interconversions between stem and non-stem compartments. Perturbation of the cell-state dynamics by genetic or pharmacological methods has the potential to change the proportions of subpopulations of cells. Hence, it is likely that the “stemness” of a tumor and its response to therapeutic manipulation depends on the stochastic state equilibrium in the populations of cancer cells. The sphere assay discovered in early stem cell studies relies on the capability of stem cells to form spheres when cultured in serum-free medium with growth factors to maintain the undifferentiated state . Mammospheres formed by human mammary epithelial cells exhibit characteristics of early progenitor/stem cells and are able to differentiate along all three mammary epithelial lineages and develop complex functional mammary structures. Tumor sphere cells have recently been widely adopted as an in vitro model to study CSCs for human cancers [20–24]. The sphere cells possess self-renewal capacity, with continuous capacity of the dissociated single cells to form secondary spheres. Lower numbers of sphere cells than bulk cancer cells are sufficient to form tumors when transplanted into non-obese diabetic-severe combined immunodeficient (NOD-SCID) mice and show great metastatic capacity [20–24].
Many of the sphere cells and stem cells reported in different systems have been found to be associated with elevated ALDH1A1 enzyme activity as measured by a commercially available kit, Aldefluor® [20, 25, 26]. Positive correlations between ALDH1A1 enzyme activity and expression are apparent , indicating that ALDH1A1 expression or activity may be used with other cell surface markers to identify tumor-initiating cells in hepatocellular, prostate and breast solid carcinomas [28–30]. ALDH1A1 expression has been found to be associated with early metastasis and poor clinical outcome . Aldehyde dehydrogenase (ALDH) proteins are a superfamily of 19 enzymes that are found to protect cells from cytotoxic and carcinogenic aldehydes in various organelles including the nucleus, cytosol, mitochondria, and endoplasmic reticulum [31, 32]. The ALDH enzymes also play a crucial role in epithelial homeostasis. Thus, deregulation of these enzymes is linked to multiple cancers, such as breast, prostate, lung and colon cancers [33–37]. In this study, we aimed to investigate if the expression of ALDH isozymes varied among different histological subtypes of ovarian tumor tissues. Our focus was on ALDH class 1, 3 and 7 isozymes, all of which have been reported to be associated with cancer development [28, 33–35]. Moreover, as a preliminary approach to explore the potential association between these ALDH isozymes and cancer cells in stem-like state, we have also investigated the expression levels of these ALDH isozymes in ovarian cancer cells growing as spheres in serum-free medium.