Early and late responses to induction of rasT24 expression in Rat-1 cells

We have used a series of Rat-1 cell lines carrying a Zn-inducible human c-Ha-ras oncogene construction (MTrasT24) to evaluate the effect of varied ras oncogene expression on the expression of genes and proteins related to morphologic transformation in vitro. In response to the expression of the ras oncogene, at least two different classes of events occur. These events, referred to as 'early and late' events, are dependent on distinctively different accumulated levels of the ras oncoprotein. Relatively low levels of activated c-Ha-ras p21 protein (1.5-2.5 times the proto-oncogene level) stimulate rapid entry of quiescent (G₀) cells into the cell cycle and result in increased steady state c-myc and glucose transporter mRNA levels which are detectable as early as 3-6 h after zinc addition. In contrast, morphologic transformation develops more slowly and does not appear until 72-96 h after Zn⁺⁺ stimulation in cells with very low basal levels of activated p21 (MR4 cells) and 24-48 h in cells with higher basal levels (MR5 cells). These morphologic changes depend on the accumulation of significant amounts of the ras oncoprotein (> 4 to 5 times the proto-oncogene level) and are accompanied by large increases in the steady state mRNA levels of transin and TGF-α and decreases in PDGF-receptor mRNA and fibronectin protein and mRNA levels. In addition, the level of a novel cytoplasmic protein species (referred to as p29), which is stained by a monoclonal antibody for ras, is dramatically reduced in response to these levels of activated ras protein. Thus changes in morphology and gene expression induced by rasT24 occur sequentially and are quantitatively dependent on activated ras expression.