Search

While molecular targeted therapies have improved prognoses of advanced stage lung adenocarcinoma expressing oncogenic driver mutations, acquired therapeutic resistance continues to be a major problem. Epidermal growth factor receptor (EGFR) activating mutations are among the most common targetable genetic alterations in lung adenocarcinoma, and EGFR tyrosine kinase inhibitors (TKIs) are recommended first-line therapy for EGFR mutation positive cancer patients. Unfortunately, most patients develop resistance to EGFR TKIs and rapid disease progression occurs. A better mechanistic understanding of therapy refractory cancer progression is necessary to develop new therapeutic approaches to predict and prevent acquired resistance to EGFR TKIs. Here, we identify a new mechanism of ERBB3-mediated resistance to EGFR TKIs in human lung adenocarcinoma. Specifically, we show that dopamine and cyclic AMP-regulated phosphoprotein, Mr 32000 (DARPP-32) physically recruits ERBB3 to EGFR to mediate a switch from EGFR homodimers to EGFR:ERBB3 heterodimers to bypass EGFR TKI-mediated inhibition to potentiate ERBB3-dependent activation of oncogenic AKT and ERK signaling that drives therapy refractory tumor cell survival. In a cohort of paired tumor specimens derived from 30 lung adenocarcinoma patients before and after the development of EGFR TKI refractory disease progression, we reveal that DARPP-32 as well as kinase-activated EGFR and ERBB3 proteins are overexpressed upon acquired EGFR TKI resistance. In vivo studies suggest that ablation of DARPP-32 protein activity sensitizes gefitinib-resistant lung tumor xenografts to EGFR TKI treatment, while DARPP-32 overexpression increases gefitinib-refractory lung cancer progression in gefitinib-sensitive lung tumors orthotopically xenografted into mice. Taken together, our findings introduce a DARPP-32-mediated, ERBB3-dependent mechanism used by lung tumor cells to evade EGFR TKI-induced cell death, potentially paving the way for the development of new therapies to prevent or overcome therapy-refractory lung adenocarcinoma progression.

Background Small cell lung cancer (SCLC) is the most aggressive form of lung cancer, and new molecular insights are necessary for prognostic and therapeutic advances. Methods Dopamine and cAMP-regulated phosphoprotein, Mr 32000 (DARPP-32) and its N-terminally truncated splice variant, t-DARPP, were stably overexpressed or ablated in human DMS-53 and H1048 SCLC cells. Functional assays and immunoblotting were used to assess how DARPP-32 isoforms regulate SCLC cell growth, proliferation, and apoptosis. DARPP-32-modulated SCLC cells were orthotopically injected into the lungs of SCID mice to evaluate how DARPP-32 and t-DARPP regulate neuroendocrine tumour growth. Immunostaining for DARPP-32 proteins was performed in SCLC patient-derived specimens. Bioinformatics analysis and subsequent transcription assays were used to determine the mechanistic basis of DARPP-32-regulated SCLC growth. Results We demonstrate in mice that DARPP-32 and t-DARPP promote SCLC growth through increased Akt/Erk-mediated proliferation and anti-apoptotic signalling. DARPP-32 isoforms are overexpressed in SCLC patient-derived tumour tissue, but undetectable in physiologically normal lung. Achaete-scute homologue 1 (ASCL1) transcriptionally activates DARPP-32 isoforms in human SCLC cells. Conclusions We reveal new regulatory mechanisms of SCLC oncogenesis that suggest DARPP-32 isoforms may represent a negative prognostic indicator for SCLC and serve as a potential target for the development of new therapies.