Systems Approach to Dynamic Interplay between cAMP/PKA and ERK Signaling

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Project Summary Extracellular-Signal Regulated Kinase (ERK), the terminal master kinase in the mitogen-activated protein kinase (MAPK) signaling pathway, regulates a variety of critical cell processes, and its aberrant activation contributes to the development of various cancers. In appendiceal, pancreatic, and colorectal cancers, there is significant co-occurrence of activating mutations in the ERK pathway with activating mutations in the cAMP/cAMP-dependent kinase (PKA) pathway, indicating cooperation between these two pathways in cancer development. The exact mechanism of crosstalk between the two canonical signaling pathways has been elusive. This proposal focuses on investigating the mechanisms by which PKA regulates ERK activity in a context dependent manner. Preliminary results in PC12 cells, a model cell line used for spatiotemporal ERK activity studies, indicate both inhibitory and stimulatory regulation by PKA on plasma membrane localized ERK (pmERK) activity depending on the initial activation of the two pathways. When PKA is activated before EGF activation of the ERK pathway, the inhibitory effects of PKA on pmERK seem to dominate. On the other hand, when ERK signaling is already active, PKA activation seems to sustain EGF stimulated pmERK activity. Previous studies have shown PKA phosphorylation of the GTPase Rap1 causes two effects. One effect is that Rap1 leaves the plasma membrane, which decreases pmERK activity. The other effect is that phosphorylated Rap1 interacts with B-Raf, which sustains pmERK activity. Our preliminary results support the hypothesis that the GTPase Rap1 mediates crosstalk between cAMP/PKA and pmERK activity. To test this hypothesis, PKA and Rap1 biosensors will be co-imaged in single cells using an in-house high-throughput, automated liquid handling microscope system. Various perturbations of the cAMP/PKA pathway before and after EGF stimulation will be used to determine the temporal relationship between PKA and Rap1. These experiments will then be repeated with Rap1 biosensors containing a mutated PKA phosphorylation site to test whether removing PKA phosphorylation of Rap1 alone impedes the crosstalk. We will develop a computational model to investigate whether the proposed mechanism is sufficient to replicate the ERK activity observed in preliminary results and to predict how cancer-specific activating mutations in upstream components of both pathways affect ERK activity. These model predictions will then be tested in human colorectal cancer cell lines. The results of this work will uncover a previously unresolved mechanism of PKA regulation of ERK activity, providing more information for the development of therapies for cancers driven by aberrant ERK signaling.