We have been studying the transformation mechanisms of oncogenes that act through the Mitogen Activated Protein Kinase (MAPK) signaling pathway. Drm (Down-regulated by mos)/Gremlin is a Bone Morphogenetic Protein (BMP) antagonist, initially identified by our lab, which is expressed in a tissue-specific fashion in vivo. With the exception of primary fibroblasts and a few cell lines, most transformed cells in culture fail to express DRM/Gremlin, and we observed that its expression could be suppressed by oncogene transformation, via the MAPK pathway. We hypothesized this loss of expression may be important for the initiation or progression of specific tumors. Our recent work has focused on analyzing Drm's function and mechanism of action in transformed cells. When retroviral and plasmid clones expressing DRM/Gremlin were introduced into transformed cells that expressed little or no Drm/Gremlin, some showed alterations in growth and transformation-associated phenotypic characteristics. To more carefully examine the effect of Drm/Gremlin expression on tumor cell lines, we constructed an ecdysone-inducible Drm/Gremlin construct and generated cell lines that overexpressed Drm/Gremlin following treatment with the inducer Ponasterone A. In parallel we generated cell lines which constitutively overexpress the protein. The p53-negative, primitive neuroectodermal-derived Daoy and the osteosarcoma-derived Saos2 cell lines exhibited slower growth in culture and reduced tumor-forming ability in vivo when Drm/Gremlin was overexpressed. In contrast several p53-expressing cell lines show either increased growth and tumor forming ability, or no change from control cells. In Daoy and Saos2, overexpression of Drm/Gremlin correlated with overexpression of p21Cip1/Waf1, a known inhibitor of the cyclin-dependent kinases involved in progression through the G1/S cell cycle transition. Levels of other growth associated proteins, including ERK1/ERK2, were not affected. Preliminary evidence suggests that p21 promoter activity is decreased following Drm/Gremlin induction in these cells. In contrast, HT1080 (fibrosarcoma) cells exhibit a reduction in p21Cip1/Waf1 levels and increased rates of proliferation and tumoriginicity. These data suggest that the BMP antagonist Drm/Gremlin can act in novel ways to affect tumor cell growth and function.
