Acetyltanshinone IIA reduces the synthesis of cell cycle-related proteins by degrading p70S6K and subsequently inhibits drug-resistant lung cancer cell growth

Targeted therapies using tyrosine kinase inhibitors (TKIs) against epidermal growth factor receptor (EGFR) have improved the outcomes of patients with non-small cell lung cancer (NSCLC). However, due to genetic mutations of EGFR or activation of other oncogenic pathways, cancer cells can develop resistance to TKIs, resulting in usually temporary and reversible therapeutic effects. Therefore, new anticancer agents are urgently needed to treat drug-resistant NSCLC. In this study, we found that acetyltanshinone IIA (ATA) displayed much stronger potency than erlotinib in inhibiting the growth of drug-resistant NSCLC cells and their-derived xenograft tumors. Our analyses revealed that ATA achieved this effect by the following mechanisms. First, ATA could bind p70S6K at its ATP-binding pocket to prevent phosphorylation, and second by increasing the ubiquitination of p70S6K to cause its degradation. Since phosphorylation of S6 ribosome protein (S6RP) by p70S6K can induce protein synthesis at the ribosome, the dramatic reduction of p70S6K after ATA treatment led to great reductions of new protein synthesis on several cell cycle-related proteins including cyclin D3, aurora kinase A, polo-like kinase, cyclin B1, survivin; and reduced the levels of EGFR and MET. In addition, ATA treatment increased the levels of p53 and p21 proteins, which blocked cell cycle progression in the G1/S phase. Taken together, as ATA can effectively block multiple signaling pathways essential for protein synthesis and cell proliferation, ATA can potentially be developed into a multi-target anti-cancer agent to treat TKI-resistant NSCLC.