PTEN, PIK3CA, p-AKT, and p-p70S6K status: association with trastuzumab response and survival in patients with HER2-positive metastatic breast cancer

Abstract

Phosphatase and tensin homolog (PTEN) is a key modulator of trastuzumab sensitivity in HER2-overexpressing breast cancer. Because PTEN opposes the downstream signaling of phosphoinositide 3-kinase (PI3K), we investigated the role of PTEN and other components of the PI3K pathway in trastuzumab resistance. We analyzed the status of PTEN, p-AKT-Ser473, and p-p70S6K-Thr389 using immunohistochemistry. PIK3CA mutation status was analyzed by direct sequencing. Primary tumor tissue was available from 137 patients with HER2-overexpressing metastatic breast cancer who had received trastuzumab-based chemotherapy. We observed that each of the four biomarkers alone did not significantly correlate with trastuzumab response, whereas PTEN loss alone significantly correlated with shorter survival times (P = 0.023). PI3K pathway activation, defined as PTEN loss and/or PIK3CA mutation, was associated with a poor response to trastuzumab (P = 0.047) and a shorter survival time (P = 0.015). PTEN loss was significantly associated with a poor response to trastuzumab (P = 0.028) and shorter survival time (P = 0.008) in patients who had received first-line trastuzumab and in patients with estrogen receptor- (P = 0.029) and progesterone receptor-negative tumors (P = 0.033). p-AKT-Ser473 and p-p70S6K-Thr389 each had a limited correlation with trastuzumab response. When these markers were combined with PTEN loss, an increased correlation with patient outcome was observed. In conclusion, PI3K pathway activation plays a pivotal role in trastuzumab resistance. Our findings may facilitate the evaluation of tumor response to trastuzumab-based and targeted therapies.

Figure 1

Activation of the PI3K pathway was more significantly associated with trastuzumab resistance than PTEN status alone. A: Heterogeneous expression of the PTEN protein in invasive ductal carcinoma lesions examined by IHC in HER2-positive MBC tumors. B: Representative PTEN IHC staining (IRS = 0, 3, 12). Arrows point to fibroblasts and endothelial cells that showed high expression of PTEN, which served as an internal positive control. C: Poor response (P = 0.081) and significantly shorter OS (P = 0.023) after trastuzumab-based therapy in patients with PTEN loss alone (PTEN loss defined as IRS ≤3). D: Significantly poorer response (P = 0.047) and shorter OS (P = 0.015) after trastuzumab-based therapy in patients with PI3K pathway activation (defined as PTEN loss and/or PIK3CA mutation). Scale bar = 50 μm.

Figure 2

Combing either p-AKT+ or p-p70S6K+ with PTEN loss correlated with trastuzumab resistance. A: Representative IHC staining patterns of p-AKT and p-p70S6K in HER2-positive MBC tumors. B: Combining p-AKT+ and PTEN loss was a more powerful marker of trastuzumab resistance than PTEN loss alone. Patients with PTEN-loss/p-AKT+ tumors showed worse responses and significantly shorter survival times (P = 0.016) after trastuzumab-based therapy. C: Combining p-p70S6K+ and PTEN loss was a more powerful marker of trastuzumab resistance than PTEN loss alone. Patients with PTEN-loss/p-p70S6K+ tumors showed worse responses and significantly shorter OS (P = 0.008) after trastuzumab-based therapy. Scale bar = 50 μm.