Signal Transduction Pathways Lecture Notes PDF

SIGNAL TRANSDUCTION PATHWAYS Contents of this lecture: 1- Definition of signal transduction 2- Types of signaling 3- Types of hormones and mechanism of action 4- Types of cellular receptors and mechanism of action 1-Definition: It is the process of converting the extracellular signals to cellular r...

SIGNAL TRANSDUCTION PATHWAYS Contents of this lecture: 1- Definition of signal transduction 2- Types of signaling 3- Types of hormones and mechanism of action 4- Types of cellular receptors and mechanism of action 1-Definition: It is the process of converting the extracellular signals to cellular responses. It occurs through cellular receptors. Figure 1 : steps of transduction 2- Types of signaling: 1-Endocrine signaling: 2 Signaling molecules (hormones) are released from distant cells ( endocrine organ ) to act on target cells. 2- Paracrine signaling: Signaling molecules (neurotransmitters) are released from organs or cells in close proximity to target cells. 3-Autocrine signaling: Signaling molecules (growth factors) are released from the same target cells. Steps of Hormonal Signaling 1. Biosynthesis of a particular hormone in a particular tissue. 2. Storage and secretion of the hormone. 3. Transport of the hormone to the target cells, tissues, or organs. 4. Recognition of the hormone by an associated cell membrane or an intracellular receptor protein. 5. Relay and amplification of the received hormonal signal via a signal transduction process. 6. Potential feedback to a hormone-producing cell. 3 3-Types of hormones: Lipophilic hormones Hydrophilic hormones (GROUP 1) (GROUP 2) 1. Lipophilic refer to they 1. hydrophilic refer to they ability to dissolve in fat ability to dissolve in water solvents and water solvents 2. They are not soluble in 2. They are not soluble in fat water. solvents. 3. Contain polar groups as 3. Mostly organic non polar protein or carbohydrates in compounds nature. 4. Examples as thyroid 4. Examples as steroid and sex hormones. offshoot hormones. 5. They work through 5. They work through intra extracellular receptor cellular receptor 6. Not Need transport proteins. 6. Need transport proteins. 7. Short half life. 7. Long half life 4- TYPES OF CELLULAR RECEPTORS: Cellular receptors are specific proteins bounded to cell membrane where signal molecules attach to the target cells to give cell response. Cell receptors are classified to two main groups : A- intra-cellular receptors: Which are found inside the cell, in cytoplasm or in the nucleus. They are attached by lipophilic hormones. hydrophobia B- Cell surface receptors (extra-cellular receptors): 4  Lipophobia Which are found in the cell membrane. They are attached by hydrophilic hormones. Figure 2: types of cellular receptors Lipophilic 5- Mechanism of action of Group 1 hormones: Group 1 hormones bind to intracellular receptors. The hormones diffuse through the cell membrane of the target cell to bind specific protein receptor in the cytosol ( as steroid, calcitrols and retonic acid) or in nucleus ( thyroid hormone ) to form a complex. e The hormone-receptor complex undergoes activation ( change in size, confirmation and surface charges to be able to go to nucleus and bind to DNA at a specific sequence that is called ( Hormone Response Element = HER),that work as sliencer or enhancer. The hormone receptor complex bound to HER works as a transcription regulating factor ( it induce or suppress the rate of transcription of the hormone-dependent gene. jo o This action affect rate of production of mRNA and released protein and the overall process. 5  Figure 3: mechanism of action of group 1 hormones hydropillic 6- Mechanism of action Group 2 hormones: Group 2 hormones bind to cell surface receptors (extracellular receptors). When Group 2 hormones attach to the receptor, second messenger which is located in the cytoplasm undergo reactions which affect the cell function. Group 2 hormones are classified to 4 groups according to the second messenger: 1- Group 2a using cAMP as second messenger such as glucagon, catecholamines (adrenaline and noradrenaline), TSH,FSH and LH. 2- Group 2b using cGMP as second messenger. Hormone that use the guanylate cyclase c-GMP system is atrial natriuretic factor (ANF). 3- Group 2c using Ca and phospholipids as second messenger. This is specially found in case of vasopressin, TRH, GnRH. 4- Group 2d using intrinsic protein kinase/phosphatase cascade as second messenger. Hormones of this group as: Insulin and growth factors. 6