Inhibition of EGFR Signaling and Activation of Mitochondrial Apoptosis Contribute to Tanshinone IIA-Mediated Tumor Suppression in Non-Small Cell Lung Cancer Cells

BACKGROUND: Deregulation of epidermal growth factor receptor (EGFR) signaling plays a critical role in non-small cell lung cancer (NSCLC) tumorigenesis. The natural product Tanshinone IIA (Tan IIA) exhibits significant anti-tumor effect in various human cancers, however, the mechanism remains elusive. METHODS: The inhibitory effect of Tan IIA NSCLC cells was determined by MTS and soft agar assays. The activation of EGFR signaling and the protein level of myeloid cell leukemia 1 (Mcl-1) were examined by immunoblot (IB), immunohistochemical staining (IHC), and ubiquitination analysis. The in vivo anti-tumor effect was validated by the xenograft mouse model. RESULTS: Tan IIA inhibits NSCLC cells through suppression of EGFR signaling. Tan IIA decreases cell viability and colony formation in EGFR wild type and activating mutant cell lines. The IB data further confirmed that Tan IIA suppresses EGFR phosphorylation time- and dose-dependently. Tan IIA destabilizes Mcl-1 and shortens the half-life. Ubiquitination analysis showed that treatment with Tan IIA promotes Mcl-1 ubiquitination and degradation. Further study showed that the downregulation of EGFR-Akt signaling is required for Tan IIA-induced Mcl-1 reduction. Ectopic overexpression of constitutively activated Akt1 compromised these antitumor efficacies in Tan IIA-treated NSCLC cells. Finally, Tan IIA inhibited the in vivo tumor growth. CONCLUSION: Our data indicate that Tan IIA acts as an EGFR signaling inhibitor, and targeting EGFR-Akt-Mcl1 axis could provide a new option for NSCLC treatment.