Editors' ChoiceCancer

Of Oncogenes and Apoptosis

Sci. Signal.  03 Jan 2012:
Vol. 5, Issue 205, pp. ec7
DOI: 10.1126/scisignal.2002817

Ras proteins, K-Ras, N-Ras, and H-Ras, are guanosine trisphosphatases that control various cellular processes, and mutations in K-Ras are commonly associated with many forms of cancer. K-Ras can initiate an apoptotic cascade through an interaction with proteins of the RASSF family and activation of the kinases MST1 and MST2. Matallanas et al. sought to understand how this death pathway was compatible with oncogenesis associated with K-Ras mutations using an isogenic pair of cell lines derived from colorectal cancer cells. HCT116 cells have a mutant KRASD13 allele and a wild-type KRAS allele; Hke3 cells arose through the deletion of the mutant KRAS allele, which reverted to the oncogenic phenotype. In HCT116 cells, MST2, K-Ras, and the MST2 substrate LATS1 coimmunoprecipitated; MST2 activity was high and knockdown of RASSF1A reduced this activity. In HCT116 cells, a greater proportion of MST2 coimmunoprecipitated with RASSF1A than coimmunoprecipitated with Raf-1, which is another MST2 binding partner. Raf-1 depletion enhanced the activity of MST2; RASSF1A depletion reduced its activity. In the Hke3 cells, MST2 activity was low, and overexpression of RASSF1A stimulated MST2 activity. Serum deprivation of HCT116 cells triggered apoptosis, which was prevented by knockdown of RASSF1A, MST2, or LATS1; Hke3 cells exhibited limited apoptosis, and this was not affected by knocking down these proteins. Coimmunoprecipitation experiments in the presence of various small interfering RNAs indicated that in HC116 cells, but not in Hke3 cells, apoptosis was mediated by sequestration of Mdm2 (an E3 ubiquitin ligase) through an interaction with LATS1 and increased abundance of p53. Mutant K-Ras can stimulate the production of epidermal growth factor (EGF), and HCT116-conditioned medium stimulated epidermal growth factor receptor (EGFR) signaling in Hke3 cells, which indicated that this pathway was active in the HCT116 cells. Knockdown of wild-type K-Ras in the HC116 cells or pharmacological inhibition of EGFR enhanced apoptosis of these cells, which suggested that autocrine EGF signaling counteracts the apoptotic effects of mutant K-Ras. Analysis of colorectal cancer patient samples showed that KRAS mutations were increased in MST2-negative samples, whereas there was no correlation between BRAF mutations and MST2 abundance. Furthermore, sequence analysis showed that MST2-positive tumors retained one wild-type KRAS allele, and MST2-negative samples could lose the wild-type allele. Thus, wild-type K-Ras is necessary to counteract the apoptotic pathway initiated by mutant K-Ras.

D. Matallanas, D. Romano, F. Al-Mulla, E. O’Neill, W. Al-Ali, P. Crespo, B. Doyle, C. Nixon, O. Sansom. M. Drosten. M. Barbacid, W. Kolch, Mutant K-Ras activation of the proapoptotic MST2 pathway is antagonized by wild-type K-Ras. Mol. Cell 44, 893–906 (2011). [PubMed]