An important aspect of multi-step
tumorigenesis is the mutational activation of genes of the RAS family, particularly in sporadic
cancers of the pancreas, colon, lung and myeloid system. RAS genes encode small
GTP-binding proteins that affect gene expression in a global way by acting as major switches in signal transduction processes, coupling extracellular signals with
transcription factors. Oncogenic forms of RAS are locked in their active state and transduce signals essential for transformation, angiogenesis, invasion and
metastasis via downstream pathways involving the RAF/
MEK/ERK cascade of cytoplasmic
kinases, the small
GTP-binding proteins RAC and RHO,
phosphatidylinositol 3-kinase and others. We have used subtractive suppression hybridization (SSH), a PCR-based
cDNA subtraction technique, to contrast differential gene expression profiles in immortalized, non-tumorigenic rat embryo fibroblasts and in HRAS- transformed cells. Sequence and expression analysis of more than 1,200 subtracted
cDNA fragments revealed transcriptional stimulation or repression of 104 ESTs, 45 novel sequences and 244 known genes in HRAS- transformed cells compared with normal cells. Furthermore, we identified common and distinct targets in cells transformed by mutant HRAS, KRAS and NRAS, as well as 61 putative target genes controlled by the RAF/MEK/ERK pathway in reverted cells treated with the
MEK-specific inhibitor
PD 98059.