1. Combating EGFR tyrosine kinase inhibitor resistance in non-small cell lung cancer

Lung cancer is the deadliest and most frequently diagnosed type of tumor. We recently demonstrated that dopamine and cyclic AMP-regulated phosphoprotein, Mr 32000 (DARPP-32), and its truncated splice variant, t-DARPP, promotes non-small cell lung cancer (NSCLC) growth in orthotopic mouse models. Immunostaining of 62 human lung adenocarcinoma tissues showed that t-DARPP expression is elevated with increasing tumor (T) staging score, a metric of tumor progression and growth. Bioinformatics analysis revealed upregulation of t-DARPP correlates with advanced T stage and poor overall survival of NSCLC patients. We showed DARPP-32 and inhibitory kappa B kinase-alpha physically interact to drive NSCLC cell migration through activation of non-canonical NF-kappaB2 signaling.

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) are the recommended first-line therapy for NSCLC patients harboring EGFR mutations. Despite initial responsiveness, EGFR-mutant NSCLC inevitably progresses in most patients within one year. Acquired resistance to EGFR TKIs is a major problem because the mechanisms of resistance are poorly understood. Our in vivo studies show upregulation of DARPP-32 isoforms promotes EGFR-mutant NSCLC growth and drives resistance to EGFR inhibitors. We propose a novel mechanism of acquired resistance to EGFR TKIs in NSCLC, in which DARPP-32 mediates a switch from EGFR TKI-sensitive EGFR homodimers to TKI-resistant EGFR:ERBB3 heterodimers. To date, no proteins have been identified that are capable of mediating such a “dimerization switch” in EGFR-mutant NSCLC. Our data supports a model in which DARPP-32 physically recruits ERBB3 to EGFR to mediate the EGFR:EGFR to EGFR:ERBB3 shift to overcome EGFR TKI-induced apoptosis by potentiating AKT and ERK signaling in NSCLC. Investigation of the role of DARPP-32 isoforms in EGFR-mutant NSCLC remains unexplored, and our work has the potential to lay the groundwork for the development of therapies designed to circumvent TKI refractory NSCLC progression to improve the quality of life and clinical outcome for patients.

2. ASCL1-regulated DARPP-32 and t-DARPP stimulate small cell lung cancer growth and neuroendocrine tumor cell survival

Small cell lung cancer (SCLC) is the most aggressive form of lung cancer, and new molecular insights are necessary for prognostic and therapeutic advances. Here we demonstrate in orthotopic mouse models that dopamine and cAMP-regulated phosphoprotein, Mr 32000 (DARPP-32) and its N-terminally truncated splice variant t-DARPP promote SCLC growth through increased proliferation, Akt/Erk-mediated survival and anti-apoptotic signaling. DARPP-32 and t-DARPP proteins are overexpressed in SCLC patient-derived tumor tissue, but virtually undetectable in physiologically normal lung. RNA sequencing analysis reveals a subset of SCLC patients with high tumoral t-DARPP expression and upregulated Notch signaling genes, including achaete-scute homologue 1 (ASCL1). We show that DARPP-32 isoforms are transcriptionally activated by ASCL1 in human SCLC cells. Taken together, we demonstrate 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.