Regulation of the HER3/ErbB3 Pseudokinase Domain by an ATP-Competitive Inhibitor

Abstract

HER3 (or ErbB3) is a member of the human epidermal growth factor receptor (HER/EGFR) family of receptor tyrosine kinases. HER3 carries inactivating mutations in its kinase domain and therefore is denoted as a pseudokinase. Despite lacking catalytic activity, the pseudokinase domain of HER3 plays an important role in activating EGFR and HER2 kinases, resulting in potent downstream signaling. Recent advancements in cancer research identify HER3 as an important therapeutic target in breast, lung, gastric, ovarian and colorectal cancers, but due to HER3's pseudokinase status, its inhibition remains a major challenge. Our past work demonstrated that the pseudokinase domain of HER3 tightly binds ATP. This finding suggested an exciting opportunity - it might be possible to regulate HER3 signaling through modulation of its nucleotide-binding pocket. However, due to lack of available compounds, this idea was left untested. Here we are reporting that the allosteric activator function of HER3 can be modulated through this site via the ATP-competitive inhibitor bosutinib. We present a first structure of the HER3 pseudokinase domain with an inhibitor, bosutinib, bound in the nucleotide-binding site. To our knowledge, this is the first structure of any pseudokinase bound to an inhibitor. We then perform in vitro assays to show for the first time that HER3 function can be modulated through small molecule binding to the nucleotide-binding pocket. Although we show that bosutinib has the opposite effect from what is desired in cancer therapies, our study provides the proof of principle for small molecule regulation of the HER3 function. This finding opens an exciting direction for the generation of a new class of HER3-specific therapeutics that directly target the pseudokinase domain.