Purpose: In small cell lung cancer cells (SCLC), various autocrine stimuli lead to the parallel activation of G_q/11 and G_12/13 proteins. Although the contribution of the G_q/11-phospholipase C-β cascade to mitogenic effects in SCLC cells is well established, the relevance of G_12/13 signaling is still elusive. In other tumor entities, G_12/13 activation promotes invasiveness without affecting cellular proliferation. Here, we investigate the role of G_12/13-dependent signaling in SCLC.

Experimental Design: We used small hairpin RNA–mediated targeting of Gα₁₂, Gα₁₃, or both in H69 and H209 cells and analyzed the effects of Gα₁₂ and/or Gα₁₃ knockdown on tumor cells in vitro, tumor growth in vivo, and mitogen-activated protein kinase (MAPK) activation.

Results: Lentiviral expression of small hairpin RNAs resulted in robust and specific Gα₁₂ and Gα₁₃ knockdown as well as markedly inhibited proliferation, colony formation, and bradykinin-promoted stimulation of cell growth. Analyzing the activation status of all three major MAPK families revealed nonredundant functions of Gα₁₂ and Gα₁₃ in SCLC and a marked p42/p44 activation upon Gα₁₂/Gα₁₃ knockdown. In a s.c. tumor xenograft mouse model, Gα₁₂ or Gα₁₃ downregulation led to decreased tumor growth due to reduced tumor cell proliferation. More importantly, Gα₁₂/Gα₁₃ double knockdown completely abolished H69 tumorigenicity in mice.

Conclusions: Gα₁₂ and Gα₁₃ exert a complex pattern of nonredundant effects in SCLC, and in contrast to other tumor types, SCLC cell proliferation in vitro and tumorigenicity in vivo critically depend on G_12/13 signaling. Due to the complete abolishment of tumorgenicity in our study, RNAi-mediated double knockdown may provide a promising new avenue in SCLC treatment. Clin Cancer Res; 16(5); 1402–15