Mathematical modeling is required for understanding the complex behavior of large signal transduction networks. Previous attempts to model signal transduction pathways were often limited to small systems or based on qualitative data only. Here, we developed a mathematical modeling framework for understanding the complex signaling behavior of CD95(APO-1/Fas)-mediated apoptosis. Defects in the regulation of apoptosis result in serious diseases such as cancer, autoimmunity, and neurodegeneration. During the last decade many of the molecular mechanisms of apoptosis signaling have been examined and elucidated. A systemic understanding of apoptosis is, however, still missing. To address the complexity of apoptotic signaling we subdivided this system into subsystems of different information qualities. A new approach for sensitivity analysis within the mathematical model was key for the identification of critical system parameters and two essential system properties: modularity and robustness. Our model describes the regulation of apoptosis on a systems level and resolves the important question of a threshold mechanism for the regulation of apoptosis.

Pain is one of the most severe and debilitating symptoms associated with several forms of cancer. Various types of carcinomas and sarcomas metastasize to skeletal bones and cause spontaneous bone pain and hyperalgesia, which is accompanied by bone degradation and remodeling of peripheral nerves. Despite recent advances, the molecular mechanisms underlying the development and maintenance of cancer-evoked pain are not well understood. Several types of non-hematopoietic tumors secrete hematopoietic colony-stimulating factors that act on myeloid cells and tumor cells. Here we report that receptors and signaling mediators of granulocyte- and granulocyte-macrophage colony-stimulating factors (G-CSF and GM-CSF) are also functionally expressed on sensory nerves. GM-CSF sensitized nerves to mechanical stimuli in vitro and in vivo, potentiated CGRP release and caused sprouting of sensory nerve endings in the skin. Interruption of G-CSF and GM-CSF signaling in vivo led to reduced tumor growth and nerve remodeling, and abrogated bone cancer pain. The key significance of GM-CSF signaling in sensory neurons was revealed by an attenuation of tumor-evoked pain following a sensory nerve-specific knockdown of GM-CSF receptors. These results show that G-CSF and GM-CSF are important in tumor-nerve interactions and suggest that their receptors on primary afferent nerve fibers constitute potential therapeutic targets in cancer pain.

SummaryCancer pain is a debilitating disorder and a primary determinant of the poor quality of life. Here, we report a non-vascular role for ligands of the Vascular Endothelial Growth Factor (VEGF) family in cancer pain. Tumor-derived VEGF-A, PLGF-2, and VEGF-B augment pain sensitivity through selective activation of VEGF receptor 1 (VEGFR1) expressed in sensory neurons in human cancer and mouse models. Sensory-neuron-specific genetic deletion/silencing or local or systemic blockade of VEGFR1 prevented tumor-induced nerve remodeling and attenuated cancer pain in diverse mouse models in vivo. These findings identify a therapeutic potential for VEGFR1-modifying drugs in cancer pain and suggest a palliative effect for VEGF/VEGFR1-targeting anti-angiogenic tumor therapies.

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