Objective: To study the function and prognostic value of the recently identified tumor suppressor gene LKB1 in human breast cancer.
Methods: Estrogen receptor-positive human breast cancer cells of MCF-7 line and estrogen receptor-negative human breast cancer cells of the lines MDA-MB-435 and MDA-MB-231 were cultured and tested for the LKB1 expression. Plasmid pcDNA4 TOPO HisMAX/LKB1 expressing LKB1 was constructed and transfected into the MDA-MB-435 cells. PCR and Western blotting were used to detect the expressions of LKB1 mRNA and protein. Flow cytometry was used to determine the cell cycle. PCR and Western blotting were used to detect the expressions of LKB1 mRNA and protein in 121 human breast cancer samples. The association with relapse free survival (RFS) and overall survival (OS) was assessed by univariate analysis (log-rank test and Kaplan-Meier method).
Results: Human breast cells of MCF-7 line expressed LKB1 and the cells of MDA-MB-435 and MDA-MB-231 lines did not. Flow cytometry showed that 40.9% of the MDA-MB-435 transfected with pcDNA4 TOPO HisMAX/LKB1 was at the G(1) stage, a proportion significantly higher than that of the control (18.3%). The expressions of cyclin D1 and cyclin E were significantly down-regulated and the expression of p21(WAF1/CIP1) was significantly up-regualed. LKB1 was lowly expressed in 43 out of the 121 cases of breast cancer (35.4%) and highly expressed in 78 cases (64.6%). Low expression of LKB1 was correlated with pathological grade (P = 0.013), cancer size (P = 0.001), status of estrogen receptor (P = 0.048), and status of lymph nodes metastasis (P = 0.003). Univariate analysis showed that low expression of LKB1 was significantly correlated with shorter RFS (P = 0.048) and low OS (P = 0.003).
Conclusion: Reintroducing LKB1 into breast cell of the lines lacking LKB1 expression restores LKB1 activity and induces growth suppression by G(1) cell cycle block. The LKB1 mediated G(1) cell cycle arrest is caused by up-regulation of the expression of p21(WAF1/CIP1) in MDA-MB-435 breast cancer cells.