Question

Compare and contrast signal transduction via tyrosine kinase receptors, G-protein receptors, and the IP3/DAG receptor pathways. Explain how it is possible for a primary messenger to illicit more than one response depending on the cell receiving the signal. In other words, how is it that increasing the concentration of one hormone can activate pathways in some tissues and activate or inhibit pathways in other tissues?

Answer 1

The receptor tyrosine kinase(RTK) on getting activated, activates phospholipase C gamma by binding their SH2 domain on the activated tyrosines of the RTK. The activation of phosphor lipase c gamma leads to hydrolysis of PIP2(Phosphatidylinositol 4,5-bisphosphate) to diacylglycerol(DAG) and inositol 1,4,5-triphosphate(IP3). The DAG activates Protein kinase C which phosphorylates other molecules.IP3 activates receptors on Endoplasmic reticulum and mobilizes  Ca2+present there. This leads to the activation of other pathways by  Ca2+.

The phospholipase C is also activated by the Gq subunit of G-protein coupled receptors and then leads to hydrolysis of PIP2 and the events follow.

For the same ligand( first messenger ) there can be different receptors on different cells or even the same receptor ligand interaction can lead to activation of different intracellular signaling molecules in different cells.

for example, in the case of acetylcholine receptors, the release of acetylcholine from a neuron near the striated muscle can lead to increased contraction, while the release of acetylcholine adjacent to a heart muscle leads to lowering of contraction rate. Thus the same ligand has different effects on different cell types.

So same ligand depending on the activation of the particular intracellular signaling molecules or the receptors can lead to differential outcomes.