MECHANISM OF HORMONE SIGNALLING

 There are three main ways :

1) ENDOCRINE SIGNALLING :

* When hormone secreted from endocrine gland acts on a target tissue which is distant from the gland.

* Hormone reaches target tissue via blood.

2) PARACRINE SIGNALLING:

* When hormone secreted from endocrine tissue diffuses and acts on neighbouring tissue.

3) AUTOCRINE SIGNALLING :

* When hormone secreted from endocrine gland acts on the same cell.

# MECHANISM OF HORMONE ACTION :

# RECEPTORS :

* Receptors are macromolecules to which ligands ( enzymes ) bind and leads to some biological change.

* Examples of substances binding to receptors which are  ligands  include enzymes, neurotransmitters drugs etc...

# G - PROTEINS :

* They are family of intrinsic membrane proteins which are coupled with receptors and effector molecules.

* G proteins have 3 subunits.

* G protein gets stimulated when the receptor is activated by hormones , neurotransmitters etc... then G protein stimulates effector molecule which generates second messenger.

* G protein leads to generation of many intracellular signals which alter biochemical activity of cell .

* G PROTEIN DISEASES :

1) Diseases due to increased G protein activity :

# McCUNE ALBRIGHT SYNDROME :

* In this syndrome there is mutation in G protein which increases G-protein activity , which leads to to hyper pigmentation on various areas of skin.

2 ) Diseases due to  decreased G - protein activity :

* Pseudohypoparathyroidism.

# TYPES OF RECEPTORS :

1) Receptors present on cell surface :

* Examples are are serpentine receptors , one pass receptors  and ligand operated Ion channels.

* These receptors are generally coupled intracellularly with G -proteins ( examples are adrenaline receptors and cholinergic receptor ).

* These ligands are are not lipids soluble and are either polar or  large in size so they can't pass through the membrane.

2) Receptors present inside the cell :

* Examples are are receptors of steroid hormones like oestrogen , progesterone , glucocorticoids .

* As these hormones are lipids soluble they can easily pass through cell membrane , so receptors are present inside the cell ( cytoplasm or nucleus).

* JAK - STAT PATHWAY Or ACTIVATION OF TYROSINE KINASE ASSOCIATED RECEPTOR :

* There is  tyrosine kinase associated receptor on the cell membrane .

* When ligand bind to the receptor then hormone receptor complex is formed .

* This complex activates Janus kinases also known as Jak .

* As Jack is stimulated it activates another protein called stat protein .

STAT PROTEIN :

*STAT is nothing but signal transducer and activator of transcription.

* Once stat protein is activated it activates nuclear machinery for translation and new proteins will be synthesized and metabolism will be altered.

* Example of hormones which act through jak- stat pathway is growth hormone.