The gene encoding the small guanosine triphosphatase Ras was one of the first oncogenes to be discovered. Certain tumor cells require continuous presence of activated Ras oncogenes for continued proliferation. Ras has numerous targets, but protein kinase B (PKB, also known as AKT) is a critical one to sustain tumor growth. Lim et al. therefore explored what target or targets of AKT (it also has many) might account for sustained tumor growth. In an assay in which a mixture of cells was transplanted into mice (to provide a characteristic tumor microenvironment), the authors found that in the presence of tumor cells expressing the oncogenic form of Hras, the contribution of cells expressing an activated phosphoinositide 3-kinase (PI3K) subunit (to activate AKT) and LacZ (to allow detection of the cells by their blue staining) to tumors was inhibited if the AKT target eNOS (endothelial nitric oxide synthase) was depleted with short hairpin RNA (shRNA). If eNOS was depleted with shRNA, participation of such cells in tumor formation was rescued by wild-type eNOS but not by a mutant eNOS that was no longer phosphorylated by AKT. Activated eNOS makes nitric oxide, which promotes S-nitrosylation of cysteine residues in proteins, and one such target--bringing us back almost to where we started--is wild-type HRas, which is activated by such nitrosylation. Indeed, other evidence supports facilitation of the effects of oncogenic Ras by wild-type Ras proteins. Activation of AKT promoted nitrosylation of wild-type Ras, and, in assays of tumor initiation or maintenance in mice injected with transformed cells, loss of HRas inhibited tumor formation by cells with activated PI3K. This effect was overcome by expression of wild-type HRas, but an HRas in which the nitrosylated cysteine was mutated provided little recovery of tumor cell growth. In similar studies with human pancreatic tumor cells (a cancer highly dependent on Ras mutations), eNOS and the modified cysteine residue of HRas or NRas were shown to be required for initiation and maintenance of tumor growth in mice. Thus, the authors propose that oncogenic Ras proteins act through AKT and eNOS to activate wild-type Ras proteins and contribute to tumor growth. Inhibition of eNOS is thus a possible therapeutic target for human cancers such as pancreatic cancer that are promoted by Ras oncogenes.
K.-H. Lim, B. B. Ancrile, D. F. Kashatus, C. M. Counter, Tumour maintenance is mediated by eNOS. Nature 452, 646-649 (2008). [PubMed]