RAF-1 KINASE AND ERK2 UNCOUPLED FROM MITOGENIC SIGNALS IN RAT FIBROBLASTS

The MAP kinase pathway impinging on ERK2 has been shown to be integral ly associated with mitogenic signalling in many cell types. Previously , we and others have demonstrated that oncogenic forms of Raf-1 kinase , when expressed in fibroblasts, lead to the constitutive activation o f ERK2, the de-regulation of c-fos expression and increased cell proli feration. Here we describe an exception to this scenario. In Rat6 cell s, although both ERK1 and ERK2 are activated in response to mitogens t hat induce c-fos expression, such as Epidermal Growth Factor (EGF), ly sophosphatidic acid (LPA) or serum, expression of v-Raf fails to induc e c-fos expression and increase proliferation. However, ERK2 is activa ted by v-Raf expression. The co-transfection of an interfering mutant of ERK2 has no effect on the level of c-fos reporter expression in Rat 6 cells whereas the analogous ERK1 mutant reduces its expression. Furt hermore, the spontaneous focus formation observed in Rat6 cells is sus ceptible to the interfering mutant of ERK1 but resistant to that of ER K2. Thus, not only do mitogenic signals appear to by-pass both Raf-1 k inase and ERK2, the Raf-1-ERK2 pathway seems to be functionally compro mised in Rat6 cells as its activation leads neither to c-fos expressio n nor to increased proliferation.