Raf-1 causes growth suppression and alteration of neuroendocrine markers in DMS53 human small-cell lung cancer cells

Ras mutations are common in lung adenocarcinomas and squamous-cell cancers, which are non-small-cell lung cancers (NSCLCs). However, small-cell lung c ancers (SCLCs) rarely have ras mutations, suggesting that ras activation ma y not confer a growth advantage in these cells. In one SCLC cell line DMS53 , activated ras expression induced increased neuroendocrine differentiation and decreased cell proliferation. We show here that DMS53 cells undergo di fferentiation and G(1)-specific growth an est in response to ras/raf/mitoge n-activated protein kinase kinase (MEK)/mitogen-activated protein kinase (M APK) pathway activation. To assess the consequences of activating the raf/M EW/MAPK pathway downstream of ras, we transfected a DMS53 cell line with De lta Raf-1:ER, an activatable form of c-raf-1. Delta Raf-1:ER activation sup pressed cell proliferation and cloning on soft agar by 90% without evidence of apoptosis. Cell cycle analysis showed a reduced proportion of cells in S phase, and was associated with induction of the cyclin-dependent kinase ( cdk) inhibitor p16(INK4). Expression of the cell cycle-specific proteins pR b, Rb2/p130, p107, cyclin A, cdc-2, and E2F-1 was decreased after Delta Raf -1:ER activation in DMS53 cells. The activity cdk4 and cdk2 was also reduce d, as consistent with cell cycle arrest in cells with activated Delta Raf-1 :ER cells. In addition, Delta Raf-1:ER reduced the expression of neuroendoc rine markers, gastrin releasing peptide, and ret gene in DMS53:Delta Raf-1: ER cells. These results provide further evidence that activation of the raf /MEK/MAPK signaling pathway, which is associated with transformation in man y circumstances, can reduce the growth of SCLC cells, and suggest that acti vation of this pathway might be clinically efficacious in some settings.