pSTAT3 transactivates EGFR in maintaining EGFR protein homeostasis and EGFR-TKI resistance

Abstract

EGFR protein trafficking is critical for regulating multiple biological processes, including cell growth and survival. However, how EGFR protein homeostasis is maintained remains unclear. In this study, we show that a reduction in plasma membrane-associated EGFR triggers EGFR transcription by promoting pSTAT3 nuclear localization. Nucleus-localized pSTAT3 binds to the EGFR gene promoter to transactivate EGFR. Moreover, erlotinib, an EGFR tyrosine kinase inhibitor (TKI), can also increase pSTAT3 nuclear accumulation, resulting in increased EGFR transcription and erlotinib resistance. Importantly, pharmacological inhibition of pSTAT3 can significantly overcome the resistance of cancer cells to erlotinib. Together, these findings demonstrate that pSTAT3 is pivotal for maintaining EGFR protein homeostasis and suggest that activation of the pSTAT3-EGFR axis contributes to EGFR-TKI resistance.

1. Introduction

Epidermal growth factor receptor (EGFR) plays pivotal roles in multiple biological processes, including cell proliferation, migration, and survival [1–4]. Abnormal activation of the EGFR signaling pathway serves as a driving force in lung cancer development [4,5]. EGFR gene amplification, deletion, and point mutation can lead to abnormal EGFR activation [6,7]. L858R mutation and exon 19 deletion are the most common EGFR-activating mutations in non-small cell lung cancer (NSCLC) patients [4,8,9]. The specific targeting of EGFR-activating mutations by 1st-generation EGFR tyrosine kinase inhibitors (TKIs), such as erlotinib and gefitinib, is beneficial for NSCLC patients [8,10]. However, most NSCLC patients develop acquired resistance to 1st-generation TKIs within 10-16 months, in which the EGFR T790M mutation accounts for approximately 50% of all 1st-generation EGFR TKI resistance [9,11]. Moreover, HER2 amplification, MET amplification, BRAF mutation, PIK3CA mutation, Ras mutation, or activation of IGF1R also contribute to EGFR TKI resistance [12–15].

In addition to gene mutation(s), increased EGFR protein expression plays a critical role in regulating cancer development [6]. The upregulation of EGFR protein recycling, stability, or gene transcription can facilitate EGFR protein expression [6]. A series of stress signals, including serum starvation, UV irradiation, hypoxia, oxidative stress, and erlotinib/gefitinib treatment, can inhibit EGFR recycling to the plasma membrane and consequently lead to EGFR protein degradation [16]. It has been reported that the ubiquitin E3 ligase c-Cbl can facilitate EGFR mono-ubiquitination, which leads to EGFR lysosome-dependent degradation [17]. Moreover, several E3 ubiquitin ligases, including FBXL2, CHIP, HUWE1, Siah1, and CGRRF1, can promote EGFR poly-ubiquitination and result in the proteasomal degradation of EGFR [18–22]. With respect to EGFR gene transcription, several transcription factors, such as Sp1, c-Jun, and Stat5b, can directly transactivate EGFR [23–25]. However, the role of the balance between EGFR protein degradation and gene transcription in maintaining EGFR protein homeostasis to adapt to stress signaling remains unknown.

In this study, we show that a reduction in plasma membrane-associated EGFR expression promotes EGFR transcription in a pSTAT3-dependent manner to maintain EGFR protein homeostasis. Moreover, erlotinib promoted pSTAT3-mediated EGFR transcription, which resulted in resistance to erlotinib in NSCLC cells. Together, the results of this study highlight that pSTAT3 directly transactivates EGFR, which plays a critical role in maintaining EGFR protein homeostasis, and that the activation of pSTAT3-EGFR signaling is a novel molecular mechanism by which NSCLC cells develop acquired resistance to EGFR TKIs.