Detection of Brk expression in non-small cell lung cancer: clinicopathological relevance

Breast tumor kinase (Brk), also known as protein tyrosine kinase 6, is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk upregulation and oncogenic properties have been found in several malignant tumors, including breast, colon carcinomas, and melanomas, but the expression of Brk and its clinical significance in non-small cell lung cancer (NSCLC) remains unclear. In the current study, we examined the expression of Brk and its correlation with clinicopathological features involving p53, ki67, and E-cadherin status in NSCLC tissue using immunohistochemistry. We also used immunocytochemistry and immunofluorescent staining to examine the Brk expression and its subcellular localization in NSCLC cell lines, including LTE and H460. We further confirmed cytoplasmic and nucleus expression of Brk in LTE and H460 cells using Western blotting. The Brk expression in NSCLC cells was mainly found in cytoplasm (59/122, 48.4%) with some nucleus staining (17/122, 13.9%) with a total positive rate of 53.3% (65/122). Cytoplasmic Brk expression in NSCLC was higher than that in normal lung tissues (24/122, 19.7%) (P < 0.05). Increased cytoplasmic Brk expression in NSCLC was associated with large tumor size (≥3 cm), lymph node metastasis, and advanced tumor–node–metastasis (TNM) stages (III and IV) (P < 0.05). Moreover, increased cytoplasmic Brk expression was positively associated with Ki67 status in NSCLC (P < 0.05). Reduced E-cadherin expression was also found to be associated with lymph node metastasis and advanced TNM stages (III and IV) in NSCLC (P < 0.05). Brk expression was not associated with E-cadherin expression and P53 status in NSCLC (P > 0.05). The present findings indicate an increase of cytoplasmic Brk expression in NSCLC which may play a role in tumor development, including tumor expansion and lymph node metastasis in which Ki67, but not E-cadherin, and P53 status may be involved.