This model also explains why BRAF inhibitors drive paradoxical activation on the pathway in BCR ABL cells: they do not inhibit BCR ABL, so will not inhibit RAS and, therefore, can drive paradoxical activation of RAF. Additionally, it explains why BCR ABL inhibitors for instance GNF tend not to drive paradoxical activation of your pathway: even though they do not inhibit BCR ABLTI and, for that reason, tend not to inhibit RAS, they aren’t BRAF CRAF inhibitors and so can’t drive their paradoxical activation. It has been reported that imatinib CEP-18770 manufacturer activates MEK and ERK in cells expressing imatinib resistant BCR ABL Yu et al ; Suzuki et al ; Mohi et al ; Chu et al. and our scientific studies now present a mechanistic explanation for all those observations. Far more importantly, we present that whereas the growth of the drug resistant cells was unaffected by nilotinib and PD in vitro and in vivo, these medication synergized to inhibit cell growth and induce apoptosis in vitro, and to suppress tumor growth in mice. Therefore, we display that drug resistant cells develop an unexpected dependency on MEK ERK signaling once the pathway is paradoxically activated. We, thus, posit that in these cells paradoxical activation of this pathway drives both a MEK ERK dependent antiapoptotic signal along with a MEK ERKindependent proapoptotic signal Figure B .
Beneath typical situations the antiapoptotic signal overcomes the proapoptotic signal Figure B , but when MEK is inhibited, the proapoptosis signal predominates Figure C . It’s unclear how MEK inhibition induces apoptosis beneath these circumstances, but one particular likelihood is always that it’s driven because of the formation in the RAF dimers. Prior scientific studies have proven that CRAF opposes cell death inside a MEK ERK independent manner by sequestering the proapoptotic kinases Ask, MST, ROCK, and RIP O?Neill et al ; Navas et al ; Chen et al ; Piazzolla et al. We posit that the recruitment of CRAF into homo and heterodimers releases these binding Rosiglitazone partners, enabling them to induce apoptosis. Our preliminary experiments failed to create a clear part for Ask and MST from the death of BCR ABLTI cells, but the response of these cells to RAF inhibitors supports our model. We demonstrate that SB and L induced robust BRAF binding to CRAF and synergized with PD to induce synthetic lethality Figures G and D . In contrast, PLX, which induced weak BRAF binding to CRAF Figure SC , only weakly synergized using the MEK inhibitor to inhibit cell proliferation Figure I . Moreover, even though sorafenib and RAF induced sturdy BRAF binding to CRAF, they at the same time inhibited MEK signaling and were hence capable to induce cell death without the have to have of a MEK inhibitor.