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Betel quid (BQ) chewing, a popular habit in numerous Asian countries including India and Taiwan, has a strong correlation with an increased risk of oral squamous cell carcinoma (OSCC). While substantial efforts have been made to test the cytotoxic, genotoxic and mutagenic effects of BQ extract and its components, the disease mechanisms underlying BQ-induced oral carcinogenesis remain obscure. Here, we show that a neuronal protein, microtubule-associated protein 2 (MAP2), was induced by BQ extract in cultured normal human oral keratinocytes (NHOKs). Subsequent analyses demonstrated that such induction was more eminent and consistent in the high-molecular-weight isoform of MAP2 (hmw-MAP2) than that in its low-molecular-weight counterpart (lmw-MAP2). Furthermore, we analyzed expression of hmw-MAP2 protein in 88 oral specimens consisting of clinicopathologically pre-malignant (leukoplakia) and malignant (OSCC) lesions, along with their adjacent normal mucosa. Immunohistochemistry revealed that, with the exposure to BQ, the hmw-MAP2 was over-expressed in 41.2% (7/17) of OSCC, 11.2% (1/9) of leukoplakia and none (0/19) of normal mucosa. In contrast, expression of the hmw-MAP2 was barely detected in BQ-free OSCC. These results suggest a significant correlation between expression of the hmw-MAP2 and BQ-associated progression of oral carcinogenesis (P = 0.0046). Interestingly, the hmw-MAP2 was found to preferentially express in histopathologically less differentiated OSCC (P = 0.014); the percentages of positive staining in poorly, moderately and well differentiated OSCC were 62.5, 21.4 and 7.1%, respectively. However, BQ chewing appeared to have marginal correlation with such propensity. Finally, we show that the majority of hmw-MAP2-positive poorly differentiated lesions were also histopathologically invasive. Taken together, these findings suggest the possibility that the hmw-MAP2 may be a diagnostic marker for BQ-chewing lesions and a potential therapeutic target. To our knowledge, this study has provided the first clinical implication that closely links a cytoskeletal protein to BQ-associated oral cancer. [ABSTRACT FROM AUTHOR]