Basal protein expression is associated with worse outcome and trastuzamab resistance in HER2⁺ invasive breast cancer

Background We investigated the effect of basal protein expression on trastuzamab response in patients with HER2-positive (HER2⁺) breast cancer who received trastuzamab (T) and in HER2⁺ breast cancer cell lines. Patients and Methods Expression of cytokeratin (CK) 5/6, CK14, and epidermal growth factor receptor (EGFR) was evaluated after immunohistochemical staining in paraffin-embedded tissue of 97 patients with stage I to III HER2⁺ breast cancer treated with chemotherapy/T. Groups with and without basal protein expression were compared with respect to clinicopathologic parameters and survival. We treated 4 cell lines (2 basal-HER2 [HCC1569, HCC1954] and 2 nonbasal HER2 [BT474, SKBR3]) each with vehicle, T 20 μg/mL, paclitaxel 0.01 μM (P), and T with P (T + P). Cell viability was assessed and HER2 pathway suppression was compared between groups using immunoblot analysis. Mammosphere formation was used to assess breast cancer stem cell properties. Results EGFR expression was significantly associated with cancer-specific survival (CSS) (P =.05). CK5/6 expression strongly correlated with overall and disease-free survival, and CSS (P =.03, P =.04, and P =.03, respectively). Statistical significance was maintained for EGFR and CK5/6 after adjustment for covariates. CK14 was not associated with survival. All cell lines expressed similar levels of HER2. T and P alone inhibited proliferation of nonbasal cell lines; T + P had an additive cytotoxic effect. Basal cells were resistant to T, P inhibited proliferation, but T + P had no additive cytotoxic effect on cell growth in basal cells. Immunoblot analysis showed a significant decrease in phosphorylated Akt levels after treatment with T or T + P in nonbasal cells but not in basal cells. Akt blockade suppressed growth of basal and nonbasal HER2⁺ cells. Furthermore, basal HER2 cell lines had increased mammosphere formation, which suggests increased stem cell properties compared with nonbasal HER2 cell lines. Conclusion CK5/6 and EGFR expression are predictive of worse prognosis in HER2⁺ breast cancer patients treated with T. Basal HER2 breast cancer cell lines are resistant to trastuzamab, which is mediated through the Akt pathway; AKT inhibition abrogates this resistance. Basal HER2 cell lines also have increased stem cell properties, which might play a role in the resistance pathway.