## 4 Discussion  
Although it is well known that MUC1 is highly expressed in invasive pancreatic ductal carcinoma, its role in the progression of the disease is unknown. This study revealed that MUC1 is a key molecule for cell proliferation and invasion in PDACs, and that TJP2, a target of MUC1 and miR-345, contributes to PDAC cell proliferation. Additionally, miR-345 can suppress MUC1 and TJP2 expression to promote apoptosis and increase ROS in the cells. MUCs are high molecular weight glycoproteins that play an important role in carcinogenesis and tumor invasion. Some MUCs--MUC1, MUC2, MUC4, and MUC5 AC--are thought to be useful for the early detection and malignant evaluation of pancreas bile duct neoplasms [3; 5; 6; 7; 16; 17]. Epigenetic regulation, such as DNA methylation and histone modification mechanisms, is also considered an important mechanism in carcinogenesis [11]. This study found that MUC1 is a key molecule for cell proliferation and invasion of PDAC by regulating apoptosis and ROS production. The significance of MUC1 in PDAC has been reported that its increased expression in cancer is an indicator of malignancy, and that it is involved in cell proliferation, angiogenesis, changes in life expectancy due to methylation, and enhanced efficacy of drugs, making it valuable as a diagnostic marker and therapeutic target [11; 12; 18; 19; 20]. However, no treatment has been established that directly targets MUC1. The reason is that MUC1 is likely to be important for the maintenance of normal cellular functions. Therefore, miRNAs and their target molecules that regulate the expression of MUC1 while avoiding direct targeting of MUC1 may be of value as alternative diagnostic markers and therapeutic targets for MUC1. We emphasize that finding miRNAs and their candidate targets involved in the expression of MUC1 may also contribute to the diagnosis and treatment of PDAC from this point of view. In various cancers, miRNAs play important roles in functions related to cancer progression, such as cell proliferation, invasion, epithelial-mesenchymal transition, metabolism, immune response, and metastasis. Several miRNAs have been reported to be involved in molecular biological functions in PDAC cells. The expression of miRNAs in PDAC cells was comprehensively analyzed by array analysis, and the involvement of a significant number of miRNAs was presumed [21; 22; 23; 24; 25; 26; 27]. In particular, miR-100/miR-125b is induced by TGF-\(\beta\) and produces various physiological effects [28]. Several miRNAs have been implicated in lymph node metastasis [26]. MiR-34a targets TP53 and functions in a suppressive manner against PDA [29]. Array analyses revealed that miR-196a-2 may be a poor prognostic factor for PDA and miR-192 is involved in cancer growth [21; 27]. It has been shown that miR-454 acts on SDF-1 and is involved in PDA proliferation [23]. MiR-7-5p, let-7d, and miR-135b were revealed to inhibit cancer stem cell growth [30]. MiR-141 was shown to inhibit PDA proliferation and colony formation by targeting YAP-1 [31]. In this study, Panc0203 and Panc1, human PDA cell lines, were used to examine the expression of cancer-related miRNAs that have been previously studied [32; 33; 34; 35; 36; 37]. We examined miR-345, which has relatively low expression levels and partial gene sequence complementarity with MUC1 (Figure S2). miR-345 has been reported to suppress cell function in various cancers [38; 39; 40; 41]. In our previous experiments, we found that miR-345 expression was maintained at low levels in tumor cell lines, such as bladder and prostate cancers (not published). This study found that miR-345 was also expressed at low levels in PDA cell lines, and its overexpression suppressed cell proliferation and invasive ability. TJP2 has been reported to be involved in congenital or juvenile hepatobiliary carcinogenesis due to a defect in the protein caused by a genetic abnormality [42; 43]. In the present study, TJP2 was suppressed by miR-345, while suppression of TJP2 expression also reduced MUC1 expression.