System-level Studies of Cell Fate Decisions Mediated by Receptor Tyrosine Kinases

Receptor Tyrosine Kinases (RTKs) are ligand-binding cell surface proteins found in Metazoans. They transmit signals from the extracellular environment to the interior of a cell, initiating signaling cascades that lead to diverse phenotypic responses ranging from adhesion to migration, proliferation to differentiation, and survival to apoptosis. The 58 human RTKs activate largely overlapping signaling pathways, yet are able to induce different cell fates in response to diverse stimuli. Here we show that, when expressed in the same cellular background, different RTKs do, indeed, activate many of the same proteins, but they induce patterns of intracellular protein phosphorylation that differ both quantitatively and qualitatively. We further show that intrinsic differences in the recruitment properties of these receptors explain their ability to activate unique patterns of signaling proteins. To further explore the role of RTKs in determining cellular outcome, we investigated how simultaneously activating RTKs and the pro-apoptotic Tumor Necrosis Factor Receptor (TNFR) affects cellular responses. We show that rather than antagonizing TNFR-induced cell death, activation of RTKs enhances apoptosis. By measuring activation of signaling proteins downstream of these two receptors, we find that these networks converge on the stress MAP kinase proteins p38 and JNK, whose activation correlates well with the extent of apoptosis. Finally, we investigate how a single RTK, Epidermal Growth Factor Receptor (EGFR), responds to stimulation with different concentrations of the same ligand. We find that high- and low-affinity ligand-receptor interactions induce activation of different signaling proteins, leading to different cellular outcomes. EGFR thus has a mechanism for inducing qualitatively distinct responses to different ligand concentrations. Together these studies reveal that combinatorial control allows RTKs to utilize a small number of signaling proteins to translate a large variety of inputs into many different cell fates.