What are the different ways in which GPCR regulates cellular functions upon hormone binding?

G-protein-coupled receptors (GPCRs) are a large family of membrane receptors that sense extracellular signals such as hormones, neurotransmitters and sensory stimuli. Upon ligand (hormone) binding, GPCR undergoes a conformational change and activates heterotrimeric G-proteins by promoting the exchange of GDP for GTP on the Gα subunit. The GTP-bound Gα subunit and Gβγ dimer then dissociate and modulate various downstream effectors inside the cell.

GPCRs regulate cellular functions through the following four main mechanisms:

i) By Activating Protein Kinase A (PKA)

Many hormones like epinephrine act via Gs-protein-coupled receptors. The activated Gαs subunit stimulates the enzyme adenylyl cyclase, which converts ATP to cyclic AMP (cAMP). The increased levels of cAMP then activate protein kinase A (PKA). Activated PKA phosphorylates various target proteins, enzymes and transcription factors, leading to diverse cellular responses like increased heart rate, glycogen breakdown and lipolysis.

ii) By Modulating Ion Channels

GPCRs also regulate ion channel activity, especially through the Gβγ subunits. For example, muscarinic acetylcholine receptors in cardiac muscle activate G-proteins that directly open K⁺ channels, slowing down the heart rate. In some neurons, GPCRs regulate Ca²⁺ channels, thereby controlling neurotransmitter release. These rapid effects are important in neuronal signaling and muscle function.

iii) By Activating Phospholipase C (PLC)

Some GPCRs couple with Gq proteins, which activate the enzyme phospholipase C-β (PLC-β). PLC hydrolyzes the membrane phospholipid PIP₂ into two second messengers: inositol trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ increases intracellular Ca²⁺ by releasing it from the endoplasmic reticulum, while DAG activates protein kinase C (PKC). Together, these signals modulate functions such as secretion, smooth muscle contraction and gene expression.

iv) By Regulating Gene Transcription

GPCR signaling pathways can ultimately lead to changes in gene expression. For example, cAMP can activate PKA, which then translocates into the nucleus and phosphorylates the transcription factor CREB (cAMP response element-binding protein). CREB binds to DNA and regulates transcription of target genes. Similarly, signals from DAG and Ca²⁺ via PKC and other kinases can also affect transcription. This regulation is crucial for long-term changes like cell growth, differentiation and memory formation.









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