Hyperactivating mutations in Ras are found in a significant percentage of cancers, with a particularly high frequency in pancreatic and colon carcinomas. The hyperactivation of Ras drives a vast number of distinct downstream signaling pathways that play an important role in disease progression. One outcome of overactive oncogenic Ras signaling is the inhibition of anoikis, a form of apoptotic cell death caused by detachment from the extracellular matrix (ECM). It is well-established that one mechanism of anoikis inhibition by Ras involves impaired mitochondrial cytochrome c release. In addition, here, we have found that Ras-mediated anoikis inhibition can also occur downstream of mitochondrial cytochrome c release. Our data suggest that anoikis inhibition following cytochrome c release is not dependent on changes in the abundance of the Apaf-1, pro-caspase-9, or pro-caspase-3. Instead, our data suggest that Ras signaling leads to an inhibition in the ability of caspase-9 to bind Apaf-1 which thereby inhibits proper formation of the apoptosome and caspase activation. Interestingly, in stark contrast to the inhibition of cytochrome c-induced apoptosis in ECM-detached cells, the overexpression of oncogenic Ras in ECM-attached cells results in enhanced sensitivity to exogenous cytochrome c. This sensitization was found to be due to upregulation of apoptosomal proteins (e.g. Apaf-1, pro-caspase-9) in an ERK/MAPK dependent manner. In aggregate, our data suggest that in addition to inhibiting the release of cytochrome c in both attachment and detachment, oncogenic Ras drives additional mechanisms that prevent apoptosome formation and caspase activation in detachment. Furthermore, our data support a model whereby ECM-attached cells containing oncogenic Ras mutations could be selectively eliminated by cytochrome c or agents that mimic its action.

Citation Format: Chelsea M. McCallister, Raju Rayavarapu, Nicholas Pagani, Brendan Heiden, Melissa Shaw, Sydeny Shuff, Zachary T. Schafer. Ras-mediated regulation of cytochrome c-induced caspase activation is dependent on the status of extracellular matrix attachment. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3563.