Purpose: In small cell lung cancer cells (SCLC), various autocrine stimuli lead to the parallel activation of Gq/11 and G12/13 proteins. Although the contribution of the Gq/11-phospholipase C-β cascade to mitogenic effects in SCLC cells is well established, the relevance of G12/13 signaling is still elusive. In other tumor entities, G12/13 activation promotes invasiveness without affecting cellular proliferation. Here, we investigate the role of G12/13-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 G12/13 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

ObjectiveChronically HCV-infected orthotopic liver transplantation (OLT) recipients appear to have improved outcomes when their immunosuppressive regimen includes a mammalian target of rapamycin (mTOR) inhibitor. The mechanism underlying this observation is unknown.DesignWe used virological assays to investigate mTOR signalling on the HCV replication cycle. Furthermore, we analysed HCV RNA levels of 42 HCV-positive transplanted patients treated with an mTOR inhibitor as part of their immunosuppressive regimen.ResultsThe mTOR inhibitor rapamycin was found to be a potent inhibitor for HCV RNA replication in Huh-7.5 cells as well as primary human hepatocytes. Half-maximal inhibition was observed at 0.01 µg/mL, a concentration that is in the range of serum levels seen in transplant recipients and does not affect cell proliferation. Early replication cycle steps such as cell entry and RNA translation were not affected. Knockdown of raptor, an essential component of mTORC1, but not rictor, an essential component of mTORC2, inhibited viral RNA replication. In addition, overexpression of raptor led to higher viral RNA replication, demonstrating that mTORC1, but not mTORC2, is required for HCV RNA replication. In 42 HCV-infected liver-transplanted or kidney-transplanted patients who were switched to an mTOR inhibitor, we could verify that mTOR inhibition decreased HCV RNA levels in vivo.ConclusionsOur data identify mTORC1 as a novel HCV replication factor. These findings suggest an underlying mechanism for the observed benefits of mTOR inhibition in HCV-positive OLT recipients and potentiate further investigation of mTOR-containing regimens in HCV-positive recipients of solid organ transplants.

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