Inhibition of MAP kinase kinase causes morphological reversion and dissociation between soft agar growth and in vivo tumorigenesis in angiosarcoma cells

Activated ras causes increased activity of several signal transduction syst ems, including the mitogen-activated protein kinase kinase (MAPKK) pathway and the phosphoinositol-3-kinase (PI-3-K) pathway. We have previously shown that the PI-3-K pathway plays a major role in regulation of ras-mediated t umor angiogenesis in angiosarcoma cells. However, the contribution of the M APKK pathway to tumorigenesis and angiogenesis is not fully understood. Ove rexpression of constitutively active forms of MAPKK has previously been sho wn to transform nonmalignant MH3T3 fibroblasts, but the effect of down-regu lation of MAPKK on tumorigenesis and angiogenesis in a well established tum or has not been fully explored. We introduced a dominant negative MAPKK gen e into SVR murine angiosarcoma cells. Introduction of a dominant negative M APKK causes a significant decrease in proliferation rate in vitro and morph ological reversion, Cells expressing the dominant negative MAPKK have a gre atly decreased ability to form colonies in soft agar compared with wild-typ e cells. Despite the decreased cell growth in vitro and inability to grow i n soft agar, the cells were equally tumorigenic in nude mice. Our results s uggest that the MAPKK pathway is required for soft agar growth of angiosarc oma cells, and separates the phenotypes of soft agar growth versus in vivo tumorigenicity. These findings have implications in the development of sign al transduction modulators as potential antineoplastic agents.