Nerve growth factor mediates activation of the Smad pathway in PC12 cells.

Ligand-induced oligomerization of receptors is a key step in initiating growth factor signaling. Nevertheless, complex biological responses often require additional trans-signaling mechanisms involving two or more signaling cascades. For cells of neuronal origin, it was shown that neurotrophic effects evoked by nerve growth factor or other neurotrophins depend highly on the cooperativity with cytokines that belong to the transforming growth factor beta (TGF-beta) superfamily. We found that rat pheochromocytoma cells, which represent a model system for neuronal differentiation, are unresponsive to TGF-beta1 due to limiting levels of its receptor, TbetaRII. However, stimulation with nerve growth factor leads to activation of the Smad pathway independent of TGF-beta. In contrast to TGF-beta signaling, activation of Smad3 by nerve growth factor does not occur via phosphorylation of the C-terminal SSXS-motif, but leads to heteromeric complex formation with Smad4, nuclear translocation of Smad3 and transcriptional activation of Smad-dependent reporter genes. This response is direct and does not require de novo protein synthesis, as shown by cycloheximide treatment. This initiation of transcription is dependent on functional tyrosine kinase receptors and can be blocked by Smad7. These data provide further evidence that the Smad proteins are not exclusively activated by the classical TGF-beta triggered mechanism. The potential of NGF to activate the Smad pathway independent of TGF-beta represents an important regulatory mechanism with special relevance for the development and function of neuronal cells or of other NGF-sensitive cells, in particular those that are TGF-beta-resistant.

Integration of the TGF-beta pathway into the cellular signalling network.

Transforming growth factor-betas (TGF-betas) regulate pivotal cellular processes such as proliferation, differentiation and apoptosis. After ligand binding, the signals are transmitted by two types of transmembrane serine/threonine kinase receptors. The type I receptor phosphorylates Smad proteins, intracellular effectors which upon oligomerization enter the nucleus to regulate transcription following assembly with transcriptional co-factors and co-modulators. The cellular distribution of TGF-beta receptors along with their oligomerization mode and their complex formation with different cell surface receptors represent crucial steps in determining the initiation of distinct signalling cascades. In addition, the broad array of intracellular proteins that influence the TGF-beta pathway demonstrates that signal transduction does not proceed in a linear fashion but rather comprises a complex network of cascades that mutually influence each other. The present review describes the intricate control of TGF-beta signal transduction on various levels of the cascade with particular focus (i) on the assembly of different receptor subtypes and (ii) on the multitude of crosstalk with signal transducers from other pathways. Integration of the TGF-beta/Smad pathway into the signalling network has taken on added importance as it substantially contributes to elicit the plethora of cell- and tissue-specific effects of TGF-beta.