UNCOUPLING OF THE PATHWAYS WHICH LINK MAP KINASE TO C-FOS TRANSCRIPTION AND AP-1 IN RESPONSE TO GROWTH STIMULI

The v-Src oncoprotein induces mitogenesis and transformation of cells through multiple effects on diverse signalling pathways that are influ enced by the cellular context in which v-Src is expressed. Here we hav e examined the effects of a temperature-sensitive (ts) v-Src on transc ription of the c-fos proto-oncogene, in serum-deprived and growing Rat -1 fibroblasts. We have also considered the role of mitogen-activated protein (MAP) kinase, a known mediator of ternary complex formation at the c-fos serum response element (SRE), which results in transcriptio nal enhancement in response to growth factors. In cells exponentially growing in the presence of serum, activation of v-Src stimulated MAP k inase and c-fos transcription. In cells made quiescent by serum depriv ation, however, v-Src did not induce a c-fos transcriptional response, nor was there stimulation of ternary complex formation, despite norma l activation of MAP kinase. Thus, activation of MAP kinase and stimula tion of c-fos transcription and ternary complex formation are uncouple d in the absence of serum growth factors. Stimulation of c-fos by v-Sr c in growing cells, however, coincided with formation of a complex wit h an oligonucleotide spanning the c-Sis-inducible element (SIE) upstre am from the SRE, suggesting that the signal transduction and activator of transcription (STAT) family of transcription factors, which bind h ere, may function in response to the v-Src oncoprotein. During these s tudies, we also observed that addition of fresh serum growth factors t o growing Rat-1 fibroblasts expressing ts v-Src at the restrictive tem perature resulted in substantially impaired activation of MAP kinase. This interference with normal growth factor signalling implies that ca talytically inactive Src acts in a dominant negative manner by blockin g normal activation of MAP kinase, although not at the expense of c-fo s transcription. Thus, serum-induced c-fos transcription can also occu r in an MAP kinase-independent manner.