cGMP Intracellular Signal

• cGMP is made from GTP by the enzyme gaunylyl cyclase.

• Atrial natriuretic peptide and nitric oxide function through this Signal.

• These are potent vasodilators.• Inhibitors of cGMP phosphodiestrase is

sildenafil (Viagra).

• The increased cGMP activates cGMP dependent protein kinase (PKG).

• This in turn phosphorylates a number of smooth muscle proteins.

• This leads to relaxation of smooth muscle cell and vasodilation.

Calcium or Phosphatidylinositol

• Ionized calcium is an important regulator of many cellular processes, including:

• muscle contraction, • secretion, • blood clotting, • enzyme activity and • membrane excitability.

• Calcium Metabolism:• Extracellular Ca conc is 5mmol/L.• Ca is restrained from entering the cell and the

intracellular conc of free and ionized Ca is very low 0.05-10μmol/L.

• Some signal must provide communication between the hormone receptor on the plasma membrane and the intracellular Ca reservoirs.

• This is accomplished by products of phosphatidyl inositol metabolism.

• Cell surface receptors such as those for:• acetylcholine, • antidiuretic hormone and • α1 – catecholamines• When occupied by these ligands they are

potent activators of Phospholipase C.

• This involves a specific G protein, which also may activate a calcium channel.

• Phospholipase C cleaves Phosphatidyl inositol 4,5 bisphosphate (PIP2) into :

• 1. 1,2 Diacyl glycerol(DAG) and• 2. inositol 1,4, 5 triphosphate(IP3).

• DAG is a potent activator of protein kinase C.• The activated PKC phosphorylates specific

substrates, which then alter physiologic processes.

• IP3 liberates stored intracellular Ca from the endoplasmic reticulum.

• Ca-Calmodulin complex is formed and this also activate specific kinases which phosphorylates specific substrates, which then alter physiologic processes.

• The same G- protein activation also activates a calcium channel and Ca can enter the cell.

Enzymes and Proteins regulated by Ca and Calmodulin

• Adenylyl Cyclase• Ca –dependent protein kinases.• Ca- Mg ATPase• Nitric oxide synthase• Phosphorylase kinase

Insulin Signaling Pathways

• Insulin is released in response to hyperglycemia.

• Insulin binds to cell surface receptors .• These receptors have intrinsic tyrosine kinase

activity.• The receptors are then auto phosphorylated

on tyrosine residues.• This initiates a complex series of events.

• The phosphorylated receptor then next phosphorylate insulin receptor substrates(IRS 1-4).

• These IRS then bind to Src homology domains on the proteins and are involved in different effects of insulin.

• Effects of this pathway:• 1. Protein translocation (glucose transporters,

insulin receptors)• Enzyme activity (insulin receptor,

Phosphatases, phosphodiestrases)• Gene transcription (PEPCK,

Glucagon,Glucokinase)

JAK STAT Pathway

• Growth hormone• Prolactin• Erythropoietin• Cytokines• Activate a tyrosine kinase, but this activity is

not an integral part of the receptor.

• When the ligand binds to the receptor then the receptor dimerizes and associated cytoplasmic protein kinases such as; Tyk-2, Jak1, Jak2 are phosphorylated.

• Jak-P now becomes an active kinase and it then phosphorylates the receptor on tyrosine residue.

• The kinases then phosphorylate other cytoplasmic proteins.

• One of the cytoplasmic proteins family is called signal transducers and activators of transcription(STAT).

• The phosphorylated STAT protein dimerizes and translocates to the nucleus and bind to specific DNA sequence.

• The phosphotyrosine residues of the receptor also bind to docking proteins through SH2

• domain.• This result in the activation of other pathways.

NF-кB Pathway

• This pathway is regulated by Glucocorticoids.• NF-кB factor is composed of two subunits

termed p50 and p65.• Normally NF-кB factor is sequestered in the

cytoplasm in an inactive form by the inhibitors(IкB).

• Extracellular stimuli such as proinflammatory cytokines, reactive oxygen species, and mitogens lead to the activation of IкB kinase complex, called IKK.

• IKK phosphorylate inhibitor(IкB).• This causes degradation of inhibitor(IкB). • NF-кB factor is free and it translocates to the

nucleus and promote gene transcription.

• NF-кB activators are:• Proinflammatory cytokines• Bacterial and viral infection• Reactive oxygen species• Mitogens

• Inhibitors of NF-кB:• Glucocorticoid hormones are therapeutically

useful agents for the treatment of a variety of inflammatory and immune diseases..

• These actions in part are explained by the inhibition of NF-кB pathway.

• Increase the level of inhibitoe• Compete with the Co activators for the receptor.• Directly bind to p65 subunit of NF-kB.