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Do all signalling pathways have something that can inhibit them? If the signal pathway is benefitial and it is inhibited would the inhibitor be caused by a biological problem? Are all inhibitors controlled by some subcellular mechanism?

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Do you think about "artificial" inhibition or an inhibition that occurs naturally in the cell? –  Chris Jul 13 at 9:12

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There are some inhibitors in the cell, which have specific effects on signal transduction. These are mostly second messengers (like cAMP, cGMP, calcium ions, inositol 1,4,5-trisphosphate and others), which are then able to diffuse relative fast to other proteins. There they can either amplify a signal or cause inhibiting processes to start which then terminate the activation of the signal transduction cascade. The problem is that these inhibitors that they work unspecific in their inhibition and termination. Signal transduction is tightly controlled as they directly influence the expression of various genes.

A lot of the proteins in these cascades are kinases which are activated upon phosphorylation and then carry on by phosphorylating the next step of the cascade. The activity is stopped by phosphatases which remove the phosphorylations from the kinases and thus inactivate them until the next signal is passed down the cascade. It works like in this figure (from here):

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Specific inhibitors for single proteins in the signalling cascades are available as drugs against cancer. Since these signalling cascades regulate gene expression including cell cycle control, they are important targets for dysregulation in cancer. A numer of kinases in the MAP kinase pathway for example are mutated there and constantly active. The so called small molecule inhibitors bind to the protein and inhibit the kinase activity, preventing a permanent signal. See this article for more information:

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"There are no inhibitors of signal transduction occuring in cells." Are you talking about small molecules? There are plenty of specific inhibitors of signal transduction, they just happen to be proteins for the most part, and not all of them are phosphatases. Each signaling pathway is very tightly regulated by a variety of mechanisms, including second messangers (which are technically small molecules). –  MattDMo Jul 13 at 18:17
    
Ok, that's right. I was thinking about molecules like the small molecule inhibitors (like all the kinase inhibitors in clinical trials or already licensed like vermurafenib) which specifically inhibit one kinase. And there is no analogon to these in the cell to my knowledge. I will make my answer more precise. –  Chris Jul 13 at 18:32
    
Could a cell be considered like an EXTREMELY complicated subcellular and intracellular signal processing 'machine'? Where any signal pathways are tightly regulated.. –  user128932 Jul 19 at 4:14
    
If you like this, why not? A cell is a extremely complicated mechanism... –  Chris Jul 19 at 19:12

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