Aberrant HER3 ligand heregulin-expressing head and neck squamous cell carcinoma is resistant to anti-EGFR antibody cetuximab, but not second-generation EGFR-TKI.

The anti-epidermal growth factor receptor (EGFR) antibody cetuximab is standard therapy for head and neck squamous cell carcinoma (HNSCC). However, most HNSCC tumors are resistant to it and require alternative treatments. Here, we explored the mechanism of cetuximab resistance and evaluated its clinical relevance in HNSCC. An unbiased comprehensive transcriptome analysis was performed on cetuximab-resistant HNSCC FaDuCR cells. The causative resistance genome was knocked down with siRNA, cell signaling was immunologically analyzed, and drug efficacy was evaluated in vitro and in vivo. The mRNA in situ hybridization (ISH) of the causative genome was performed using 28 excised HNSCC tumors and its relationship with cetuximab efficacy was analyzed. FaDuCR cells were resistant to cetuximab, whereas parental FaDu cells were susceptible to it. FaDuCR cells expressed consistently higher levels of phosphorylated Akt than FaDu cells despite cetuximab exposure. A comprehensive transcriptome analysis revealed that the HER3-ligand heregulin was upregulated in FaDuCR cells compared to FaDu cells. Heregulin knockdown in FaDuCR cells repressed HER3 and Akt phosphorylation and recovered cetuximab anticancer efficacy. In contrast, pan-HER family tyrosine kinase inhibitors such as afatinib decreased HER3 and Akt phosphorylation in FaDuCR cells and inhibited FaDuCR tumor growth. Two of the 28 HNSCC tumor samples presented aberrant heregulin expression comparable to that of FaDuCR cells and were resistant to cetuximab therapy. In HNSCC, heregulin-mediated HER3-Akt activation causes resistance to cetuximab but not to second-generation EGFR-tyrosine kinase inhibitors. Subpopulations with aberrant heregulin-expressing HNSCC might be resistant to cetuximab.