Additive antitumor effects of the HER1/HER2 tyrosine kinase inhibitor, lapatimib with antiestrogen, fulvestrant in breast cancer cells *
PURPOSES:Recent preclinical studies suggest that activation of growth factor signaling induces estrogen-independent activation of the estrogen receptor（ER）pathway and causes resistance to endocrine therapy in breast cancer. It has been suggested that inhibitors of growth factor signaling overcome such endocrine resistance. To clarify this hypothesis, the combined antitumor effects of the human epidermal growth factor receptor（HER）1/HER2 tyrosine kinase inhibitor, lapatinib（Lap）with an antiestrogen, and fulvestrant（Ful）were investigated in three breast cancer cell lines KPL-1, KPL-3c and KPL-4. The action mechanisms of the combined effects were also investigated. METHODS:All three cell lines were established in our laboratory. KPL-1 and KPL-3C cells are ER-positive and express low levels of HER1/HER2, and KPL-4 cells are ER-negative and overexpress HER2. Cell growth was measured using a Coulter counter. Cell cycle analysis was measured by propidium iodide staining and flow cytometry（FACS）. Apoptosis was measured by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling（TUNEL）analysis. The protein expression levels of factors related to cell cycle progression and the induction of apoptosis were measured by Western blot analysis. RESULTS:The KPL-4 cell line was the most sensitive to Lap（the 50％ growth inhibitory concentration [IC50], 1.5μM）among the three cell lines tested. Lap induced significant apoptosis（an increase in the sub-G1 fraction by FACS and in TUNEL-positive cells）in KPL-4 cells. The antitumor effects of Lap were modest in KPL-1 and KPL-3C cells（IC50, 8.1μM and 5.0μM, respectively）. Combined treatments with Lap and Ful resulted in an additive antitumor effect associated with an increase in apoptosis. Lap decreased the expression levels of two anti-apoptotic factors, Bcl-2 and surviving, in KPL-4 cells but not in KPL-1 and KPL-3C cells. In contrast, combined treatments with Lap and Ful decreased the expression of Bcl-2 and survivin in KPL-1 and KPL-3C cells. In addition, Lap alone or in combination with Ful increased the expression levels of p21 and/or p27 in the three breast cancer cell lines. CONCLUSION:These findings suggest that Lap has a potent antitumor activity on breast cancer expressing high levels of HER2 and that combined therapy with Lap and Ful additively inhibits the growth of ER-positive breast cancer. (Accepted on April 18，2007)
