Introduction Activation of epidermal growth factor receptor

Introduction Activation of epidermal growth factor receptor (EGFR)-receptor tyrosine kinases (RTKs) is a key factor in the progression of non-small cell lung cancer (NSCLC). EGFR-RTKs cause proliferation, invasion, metastasis, and angiogenesis, as well as suppress apoptosis, in lung adenocarcinoma (ADC). Mutations in EGFR are detected in about 10%–17% of human ADCs in the United States and Europe and in about 30%–65% of lung cancers in Asia [1,2]. Pharmacologic inhibitors of EGFR-TK, including gefitinib and erlotinib, have notable therapeutic effects in patients with NSCLCs [3,4]. Strategies targeting EGFR-TKIs have been the first therapeutic choice for patients with advanced lung ADC with EGFR mutations. Unfortunately, drug resistance is inevitable in almost all of these patients after a median treatment period of 10–16 months [5]. Furthermore, patients with EGFR gene T790M-mutations are resistant to erlotinib and gefitinib. Although another second-generation EGFR-TKI, afatinib, can circumvent drug resistance to some degree, it has a very narrow therapeutic window due to intolerability. More recently, the third generation anti-EGFR agent osimertinib has been approved to treat NSCLC with the EGFR T790M mutation. However, acquired resistance to osimertinib has already been reported [6]. Therefore, developing new therapeutic strategies to overcome resistance to conventional EGFR inhibitors is urgently required. Deubiquitinating MRS 1220 synthesis (DUBs) deconjugate ubiquitin from substrates and negatively regulate ubiquitination. As a newly discovered member of DUBs and a key subunit of the human Spt-Ada-Gcn5-acetyltransferase (hSAGA) transcriptional coactivator complex, ubiquitin-specific protease 22 (USP22) interacts with the hSAGA complex to hydrolyze ubiquitins conjugated to histones H2A and H2B, thus activating target gene transcription by altering histone ubiquitination levels [7]. Increasing evidence suggests that USP22 has clinicopathological significance in oncology. Identified in many kinds of tumors, USP22 plays a key role in cell cycle regulation, embryo development, and telomere homeostasis [[7], [8], [9]]. For example, knockdown of USP22 in bladder cancer cells led to cell cycle arrest in the G0/G1 phase and resulted in decreased tumor growth in vivo [7,10,11]. Moreover, USP22 was shown to be required for the transcriptional activity of c-MYC, a key player driving tumor growth [7]. Recently, three mammalian DUBs, AMSH, UBPY and USP2a, have been implicated in the endocytic down-regulation of EGFR-RTKs [[12], [13], [14], [15]]. These studies suggested that resistance to EGFR-TKIs is closely related to endocytosis-mediated EGFR activation via ubiquitination. Our previous studies revealed that USP22 promotes tumor progression and induces epithelial-mesenchymal transition (EMT) in lung ADC [16]. Considering the critical roles of USP22 in the pathogenesis of lung ADCs, we sought to understand if USP22 had a non-redundant and independent role in endocytosis-mediated regulation of EGFR-RTKs.