6, 7 In addition, studies in PC2-defective cholangiocytes

have shown that the overactivation of this pathway causes Hydroxychloroquine cost the downstream activation of the mammalian target of rapamycin (mTOR) pathway and that both ERK1/2 and mTOR converge in stimulating cyclins and hypoxia inducible factor 1α (HIF1α)-dependent vascular endothelial growth factor (VEGF)-A secretion.8 Mice deficient in PC2 show a severe liver phenotype, high proliferation rate of the cystic epithelium, and high expression of phosphorylated ERK (pERK) 1/2, phosphorylated mTOR, HIF1α, VEGF, and VEGF receptor-2.7-9 The pathophysiological relevance of this model is demonstrated by the reduction of cyst growth in vivo after administration of SU5418 CHIR-99021 order (inhibition of VEGF receptor-2 signaling),7, 9 rapamycin (inhibition of mTOR and of VEGF production),8 or somatostatin, which inhibits cAMP production through its receptor SSTR2.10 Clinical

trials of somatostatin analogues in PLD patients have shown only a modest reduction in cyst growth,11-13 and thus a medical treatment for patients with symptomatic PLD is still not available. Because of its role in the PKA/Ras/Raf/MEK/ERK cascade, the key signaling pathway altered in PLD, and the availability of chemical inhibitors approved for clinical use, we considered Raf as a potential new target molecule for the treatment of PLD and sought to generate experimental proof of this concept. Sorafenib is an oral Raf inhibitor used in the treatment of kidney and liver cancer that has been shown to increase apoptosis and to block cell proliferation and neo-angiogenesis in a wide range of tumor models by targeting Raf/MEK/ERK signaling.14, 15 In this study, we performed in vivo and in vitro experiments to test the hypothesis that sorafenib inhibits liver cyst growth in PC2-defective mice. Contrary to our hypothesis, we found that sorafenib

caused an increase in liver cyst growth in vivo and stimulated pERK, cell proliferation, and Raf-1 kinase activity in Pkd2flox/−:pCxCreERTM (Pkd2cKO) Morin Hydrate cells in vitro. Inhibition of PKA restored the expected inhibitory effect of sorafenib in PC2-defective cells. Consistent with this observation, a significant reduction in liver cyst growth in vivo was achieved when sorafenib was given in combination with octreotide, an analogue of somatostatin known to inhibit cAMP production.10 These data are consistent with a model in which sorafenib inhibits B-Raf, but paradoxically activates Raf-1 in the context of PKA-dependent, Ras-induced B-Raf/Raf-1 heterodimerization. These results also suggest that the potential consequence of paradoxical activation of Raf-1 should be carefully considered when treating conditions characterized by activation of nonmutated Raf.