Figure 2. PTEN down regulation and HER2 overexpression generate trastuzumab resistant tumors in NOD/SCID mice.

(A and D) Trastuzumab treatment of primary spheres of MCF7- HER2⁺ and Sum159-HER2⁺ reduced the formation of secondary spheres by 50%, while the secondary spheres from MCF7-PTEN⁻, Sum159-PTEN⁻, MCF7-HER2⁺PTEN⁻ and Sum159- HER2⁺PTEN⁻ cells were not affected by trastuzumab. (B and E) Trastuzumab treatment had no effect on the Aldefluor-positive cell population in MCF7-HER2⁺PTEN⁻ and Sum159- HER2⁺PTEN⁻ cells while it reduced this population by more than 50% in MCF7-HER2⁺ and Sum159-HER2⁺ cells. (C and F) MCF7-HER2⁺PTEN⁻ and Sum159-HER2⁺PTEN⁻ cells compared to MCF7-HER2⁺ and Sum159-HER2⁺ cells generated rapidly growing tumors with (G) metastasis to liver and lung. (H and I) While MCF7-HER2⁺ tumors responded to trastuzumab treatment in mice leading to inhibition of tumor growth and decrease in Aldefluor-positive population in mice, while MCF7-HER2⁺PTEN⁻ tumors were resistant to trastuzumab. Error bars represent the mean and standard deviation of two independent experiments in panels A, B, D and E, and of the 5 tumor sizes from 5 different mice in panels H and I. (*p ≤ 0.01, #p ≤ 0.05).