Differential contribution of the ERK and JNK mitogen-activated protein kinase cascades to Ras transformation of HT1080 fibrosarcoma and DLD-1 colon carcinoma cells

Although an important contribution of ERK and JNK mitogen-activated protein kinase (MAPK) activation in Ras transformation of rodent fibroblasts has b een determined, their role in mediating oncogenic Ras transformation of hum an tumor cells remains to be established. We have utilized the human HT1080 fibrosarcoma and DLD-1 colon carcinoma cell lines, which contain endogenou s mutated and oncogenic N- and K-ras alleles, respectively, to address this role. Study of these cells is advantageous over Ras-transformed rodent mod el cell systems for two key reasons. First, the ras mutations occurred natu rally in the progression of the tumors from which the cell lines were deriv ed, rather than due to overexpression of an exogenously introduced gene, Se cond, although these tumor cells possess defects in multiple genetic loci, it has been established that mutated Ras contributes significantly to the t ransformed phenotype of these cells. Clonal variant lines of HT1080 and DLD -1 have been isolated which have lost the oncogenic was allele and exhibit a corresponding impairment in growth transformation in vitro and in vivo. W e found that upregulation of Raf/MEK/ERK and JNK correlated with expression of oncogenic Ras in HT1080, but not DLD-1 cells. Furthermore, inhibition o f ERK activation in parental HT1080 cells caused the same changes in cell m orphology and actin stress fiber organization seen with loss of expression of activated N-Ras(61K). Thus, we suggest that constitutive activation of t he Raf/MEK/ERK and JNK pathways is necessary for Ras-induced transformation of HT1080 but not DLD-1 cells. These results emphasize that cell type diff erences exist in the signaling pathways by which oncogenic pas causes trans formation.