Cancer stem cell markers in lung adenocarcinoma

In solid tumors, cancer stem cells (CSCs) are defined as cells that have the ability to perpetuate themselves through self-renewal and to generate mature cells of a particular tissue through differentiation [1]. The CSCs hypothesis suggests that there is a small subset of cancer cells that are responsible for tumor initiation and growth, possessing properties such as indefinite self-renewal, slow replication, intrinsic resistance to chemotherapy and radiotherapy, and an ability to give rise to differentiated progeny. This suggests that CSCs resistant to these treatments are involved in cancer recurrence and metastasis. Therefore, there is an urgent need to identify new therapeutic drug targets for successful treatment. Lung cancer is the most common cancer worldwide with a 5-year survival rate of < 15%, the reason seems to be a lack of useful therapeutic targets [2]. As previously reported by Miyata et al. in our group, we reviewed ALDH1 and CD133 [3]. In addition, we investigated CD44standard (CD44st) and CD44variant6 (CD44v6) as CSC markers. ALDH1 proteins (mainly ALDH1A1, ALDH1A2 and ALDH1A3), which are primarily localized in the cytosol of cells from various tissues, include enzymes able to oxidize retinol and aliphatic aldehydes [4]. ALDH enzyme activity has also emerged as a promising marker of CSCs, and indeed as normal stem cells. In addition to being a putative stem-cell marker, ALDH activity is also known to be involved in drug resistance. In order to assess CSC markers in lung cancer specimens, a paraffin-embedded section is prepared, and histological diagnosis is performed by HE staining. The IHC staining is also performed by the LSAB method using the ALDH1A1 monoclonal antibody. The results were semi-quantitatively graded based on the percentage of cells stained and the intensity of staining. Briefly, the staining intensity was rated as weak (1+), moderate (2+), or strong (3+) and multiplied by the percentage of positive cells. ALDH1A1 score = (% of cells of intensity 1×1) + (% of cells of intensity 2×2) + (% of cells of intensity 3×3). The total scores were categorized as follows: 0-100 = Grade 1, 101-200 = Grade 2, 201-300 = Grade 3. Grade 2 or 3 tumors were considered positive for ALDH1A1 [4]. CD133 antigen, also called prominin 1 (PROM1), is a 120 kDa five-transmembrane glycoprotein. The function is currently unknown but it is expressed on the cell surface in many malignancies [4]. There is also considerable evidence here suggesting that CD133 expression in a subpopulation of lung cancer cells also identifies CSCs. The IHC staining of CD133 was performed on 5 micrometer (μm) paraffin sections using monoclonal antibodies. The CD133 expression score was defined as the percentage of cells showing strong expression levels in membrane sections of tumor