## 1 Introduction  
Lung cancer is one of the prevalent cancers in the world and non-small cell lung carcinoma represents 80% of lung cancers [1]. Current standard therapies only contribute to 15% of the survival rate within overall 5-year survival, because this cancer is a systemic disease [2]. Cell cycle in cancer-genesis is one of the key pathways to control cancer cell progression [3]. It is regulated by the coordinated activation of a cyclin-dependent kinase (CDK)/cyclin complex. Especially, Cyclin E binds CDK2 to activate the cyclinE/CDK2 complex at early phase (G1/S) of the cell cycle, which can be controlled by the phosphorylation of tumor suppressor proteins [4]. Both cyclin kinase inhibitors p21 and p27 are able to regulate CDK by binding to CDK and proliferating cell nuclear antigen (PCNA). Then, p21 and p27 lead to cancer-cell-cycle arrest and the blockade of DNA synthesis to inhibit cancer cell proliferation [5]. On the other hand, AKT, known as protein kinase B, also plays a key role in cancer cell proliferation, survival and metabolism [6]. The hyperactivation of AKT has been found to contribute to the misregulation of cell cycle progression in several types of human cancers [7]. The phosphorylated AKT (P-AKT) has been reported to promote cancer progression and cancer drug resistance [8]. Thus, the improvement of p27 and p21 expression and deduction of P-AKT could have potential prognostic and therapeutic implications in human cancers [9].  
Catechins are a group of common polyphenols present in tea, red wine, cocoa and other leaf or fruit drinks [10]. A number of studies have confirmed that catechin extract possesses health benefits through the prevention of various chronic diseases. For example, catechin-rich tea extract has shown significant effects in animal models on anti-mutation, immune regulation, and cardiovascular system health, especially in delaying tumor onset [11]. The mechanisms of the health benefits have been reported to be associated with the great antioxidant activity of catechins [12]. Basically, catechins are polyphenol-based secondary metabolites with a structure of two linked phenols benzopyran derivatives [13]. Different from most other catechins, which naturally contain one or more glucoside groups, catechin is the most basic catechin compound without glycosylation, resulting in higher bioavailability and absorption efficiency in the body [14]. Epigallocatechin-3-gallate, a major compound in the catechins group, or natural catechins extract, was usually used to examine the anti-cancer activity of catechins. For example, epigallocatechin-3-gallate in green tea was selected to investigate the expression of the cancer cell cycle regulatory gene [15]. Plum catechins extract was applied to induce apoptosis in human hepatoma HepG2 cells [16]. However, the mechanism of the anti-cancer activity of the most basic catechin has not been studied. In order to understand the action site inhibiting cancer cell proliferation, pure catechin was used in this study. The inhibitory effects on A549 cancer cells' proliferation, and the p21/ p27 expression of A549 cells were determined. The levels of Cyclin E 1 and P-AKT in the cancer cell after being treated by catechin were also analyzed. Therefore, this study could reveal the mechanism of the anti-cancer activity of the basic catechin compound without interference from other catechins or co-existing extracted compounds. The results would be helpful in understanding the potential and the mechanisms of catechin compounds from natural beverages and foods on cancer prevention.