STAT3 Knockout Cell Line (A549)

Lung cancer is the second leading cause of cancer death worldwide and is strongly associated with cisplatin resistance. The transcription factor signal transducer and activator of transcription 3 (STAT3) is constitutively activated in cancer cells and coordinates critical cellular processes as survival, self-renewal, and inflammation. In several types of cancer, STAT3 controls the development, immunogenicity, and malignant behavior of tumor cells while it dictates the responsiveness to radio- and chemotherapy. It is known that STAT3 phosphorylation at Ser727 by mechanistic target of rapamycin (mTOR) is necessary for its maximal activation, but the crosstalk between STAT3 and mTOR signaling in cisplatin resistance remains elusive. In this study, using a proteomic approach, we revealed important targets and signaling pathways altered in cisplatin-resistant A549 lung adenocarcinoma cells. STAT3 had increased expression in a resistance context, which can be associated with a poor prognosis. STAT3 knockout (SKO) resulted in a decreased mesenchymal phenotype in A549 cells, observed by clonogenic potential and by the expression of epithelial-mesenchymal transition markers. Importantly, SKO cells did not acquire the mTOR pathway overactivation induced by cisplatin resistance. Consistently, SKO cells were more responsive to mTOR inhibition by rapamycin and presented impairment of the feedback activation loop in Akt. Therefore, rapamycin was even more potent in inhibiting the clonogenic potential in SKO cells and sensitized to cisplatin treatment. Mechanistically, STAT3 partially coordinated the cisplatin resistance phenotype via the mTOR pathway in non-small cell lung cancer. Thus, our findings reveal important targets and highlight the significance of the crosstalk between STAT3 and mTOR signaling in cisplatin resistance. The synergic inhibition of STAT3 and mTOR potentially unveil a potential mechanism of synthetic lethality to be explored for human lung cancer treatment.

In this study, we reveal a novel relationship between RNF213, an E3 ubiquitin ligase associated with Moyamoya disease (MMD) and the ubiquitination of both endogenous and pathogenic substrates, and EGFR, the epithelial growth factor receptor involved in cell growth, angiogenesis, and cancer. RNF213 knockdown or knockout in HeLa and A549 cells markedly reduces EGFR phosphorylation at key tyrosine sites following EGF and TGFα stimulation. In RNF213 knockout cells, HER2 phosphorylation, typically activated through heterodimerization with EGFR, and Src recruitment and/or phosphorylation are also diminished. Mutations in the RNF213 RING, RZ finger, or AAA+ domains, including the prevalent R4810K mutation in MMD, consistently reduce EGFR phosphorylation. In vivo, EGF injections increase EGFR and HER2 phosphorylation in WT but not in RNF213 knockout mice. Despite the reduced phosphorylation levels of these tyrosine kinases in knockout cells, the activation of downstream signals such as AKT, ERK1/2, and STAT3 remains unaffected, although phosphorylation of PLCγ, a key mediator of Ca release, is selectively reduced by RNF213 knockout. These findings demonstrate that RNF213 modulates EGFR-related pathways and specific downstream signal pathways, possibly affecting physiologic and pathogenic angiogenesis, and may have implications for unraveling the etiology of MMD and for developing cancer therapies that target RNF213.