Oncogenic Ras‐mediated downregulation of Clast1/LR8 is involved in Ras‐mediated neoplastic transformation and tumorigenesis in NIH3T3 cells

Oncogenic Ras proteins transform cells by way of multiple downstream signaling pathways that promote the genesis of human cancers. However, the exact cellular mechanisms by which downstream targets are regulated are not fully understood. Here, we show that oncogenic Ras reduced Clast1/LR8 transcript levels in mouse NIH3T3 fibroblasts and human WI38 fibroblasts. Clast1/LR8 transcript was undetectable in H460, A549, and H1299 cells showing high Ras activity, but was relatively abundant in DMS53 cells displaying low Ras activity. We also showed that K‐Ras siRNA restored Clast1/LR8 expression in H460 and A549 cells, and that inhibitors of DNA methylation and histone deacetylation reversed oncogenic H‐Ras‐mediated suppression of Clast1/LR8 transcription. Additionally, ectopic expression of Clast1/LR8 inhibited serum‐stimulated phosphorylation of ERK1/2 and Akt in H‐RasV12‐transformed NIH3T3 cells. We further showed that the expression of Clast1/LR8 interfered with oncogenic Ras‐induced NIH3T3 cell transformation and invasion. Finally, our results showed that Clast1/LR8 inhibited Ras‐induced proliferation of, and tumor formation by, oncogenic H‐RasV12‐transformed NIH3T3 cells in vivo. This study identifies the downregulation of Clast1/LR8 as a potentially important mechanism by which oncogenic Ras‐mediated neoplastic transformation occurs. (Cancer Sci 2010)