TIAM1-RAC1 promote Small Cell Lung Cancer cell survival through antagonizing Nur77-induced BCL2 conformational change

Small cell lung cancer (SCLC), an aggressive neuroendocrine malignancy, has limited
treatment options beyond platinum-based chemotherapy, whereafter acquired
resistance is rapid and common. By analyzing expression data from SCLC tumors,
patient-derived models and established cell lines, we show that the expression of
TIAM1, an activator of the small GTPase RAC1, is associated with a neuroendocrine
gene program. TIAM1 depletion or RAC1 inhibition reduces viability and tumorigenicity
of SCLC cells by increasing apoptosis associated with conversion of BCL2 from its
pro-survival to pro-apoptotic function via BH3 domain exposure. This conversion is
dependent upon cytoplasmic translocation of Nur77, an orphan nuclear receptor.
TIAM1 interacts with and sequesters Nur77 in SCLC cell nuclei and TIAM1 depletion
or RAC1 inhibition promoted Nur77 translocation to the cytoplasm. Mutant TIAM1 with
reduced Nur77 binding fails to suppress apoptosis triggered by TIAM1 depletion. In
conclusion, TIAM1-RAC1 signaling promotes SCLC cell survival via Nur77 nuclear
sequestration.