(101c) Ligand Specificity of C-Fos Expression Emerges from Spatiotemporal Control of Erbb Network Dynamics

Even though ligand-induced activation of different ErbB receptors leads to distinct cellular outcomes, there is as yet no mechanistic understanding of how receptor-specific responses emerge when signals propagate through the same core components. The current work focuses on this topic in the context of how two prototypical ErbB receptor ligands, epidermal growth factor (EGF) and heregulin (HRG), induce differential expression of the transcription factor c-Fos through the core ERK pathway in MCF-7 breast cancer cells. Although EGF and HRG induce transient versus sustained ERK activation, respectively, we found that the duration of c-fos mRNA expression is surprisingly similar for both ligands, whereas c-Fos protein level displayed all (HRG) or none (EGF) behavior. In this presentation, we will discuss our mechanistic modeling which suggests, and our immunofluorescence experiments which confirm, that induction of the nuclear phosphatase mkp1 causes nuclear ERK activation to be transient for both ligands. Since active, nuclear ERK stimulates c-fos transcription, the transience of active, nuclear ERK for both ligands explains the similar c-fos mRNA expression durations. We will also show how combinatorial siRNA knockdown of mkp family proteins verifies the key role these phosphatases play in controlling the c-fos mRNA expression dynamics. Since de novo c-Fos protein is stabilized by active ERK, we deduce that the large, HRG-induced c-Fos response arises from sustained active ERK in the cytoplasm. Thus, we conclude that the all-or-none c-Fos responses arise from ligand-dependent spatiotemporal control of ERK activity. Longer time course gene chip data suggest that these quantitative differences in expression of immediate early transcription factors such as c-fos leads to qualitative differences in subsequent waves of gene expression, and consequently differential cell fate decisions.