This biphasic effect was negligible and not significant in WT cholangiocytes. Raf kinases transmit extracellular signals to MEK, a mitogen-activated SP600125 mouse protein kinase that, in turn, phosphorylates ERK. Raf kinases are activated by Ras, a small guanosine triphosphatase that recruits Raf to the plasma membrane promoting the homo- or heterodimerization of B-Raf and Raf-1,29, 30 the two main isoforms of Raf expressed in cholangiocytes.31, 32 B-Raf and Raf-1 have different affinity for MEK and different phosphorylation requirements.33 Furthermore, B-Raf can undergo mutations that are able to generate a constitutively
active kinase, as in the case of B-RafV600E, an oncogene able to promote the formation of benign or malignant tumors.33 Raf inhibitors are very effective in B-Raf mutant cells, but their efficacy is lower in cells Ribociclib order expressing wild type B-Raf, particularly in the presence of an activated Ras. In this
condition, Raf inhibitors can actually paradoxically activate the Raf-MEK-ERK pathway.20, 29, 30 Activated Ras recruits Raf molecules to the cell membrane, inducing the homodimerization B-Raf/B-Raf or the heterodimerization B-Raf/Raf-1.20, 29, 30 As shown in Fig. 5B, at low doses, sorafenib inhibits the B-Raf molecule in the heterodimer while paradoxically activating Raf-1. There is no consensus on the molecular mechanisms leading to the paradoxical activation of Raf-1, but this phenomenon explains why, in cells bearing one mutated B-Raf (BRafV600E), low doses of Raf inhibitors repress cell proliferation and ERK phosphorylation, whereas higher doses are required to shut down Raf-1–mediated ERK phosphorylation in cells with activated Ras, such as liver cyst cells.33 In ADPKD, the growth of cystic cells is not caused by activating mutations of B-Raf, but by the persistent stimulation
of Ras/Raf/ERK signaling caused by the inappropriate production of cAMP (see Fig 8). Our data showing inhibition of B-Raf, and activation of Raf-1 at lower doses of sorafenib in Pkd2cKO cells, provide an experimental confirmation of this hypothesis and explain the cyst click here expansion and cell proliferation induced in vivo by sorafenib in Pkd2cKO mice. Furthermore, we observed that sorafenib-induced Raf-1 stimulation is specific for PC2-defective cells (characterized by higher levels of intracellular cAMP) and is inhibited by PKA inhibitors, suggesting that in PC2-defective cells, PKA-dependent activation of Ras induces the heterodimerization of WT B-Raf with Raf-1.20, 29, 33 Our in vitro findings are in apparent contrast with Yamaguchi et al.,23 who reported that sorafenib inhibits the kinase activity of both B-Raf and Raf-1 in kidney epithelial cells isolated from patients with ADPKD.