Hormones and Receptors Flashcards
There are many forms of chemical communication between cells
a. Neurotransmitters–> chemical transmission in the nervous system.
i. neurotransmitters are secreted at specialized junctions called synapses.
b. Paracrine Effectors–> chemical substances secreted by cells may act on neighboring cells without these specialized structures
c. Alternately, if a chemical acts on the same cell that secretes it, it is an autocrine effector.
d. A hormone is a chemical that is produced by certain cells, released into the blood stream in minute amounts and has its physiological effects on target cells at a distance.
i. An endocrine gland consists of a group of cells that produce these hormones.
Hormones
a. A hormone is a chemical that is produced by certain cells, released into the blood stream in minute amounts and has its physiological effects on target cells at a distance.
b. An endocrine gland consists of a group of cells that produce these hormones.
Paracrine and Autocrine effectors
Other chemical substances secreted by cells may act on neighboring cells without these specialized structures and are known as paracrine effectors. Alternately, if a chemical acts on the same cell that secretes it, it is an autocrine effector
Types of Hormones:
The classic hormones fall into three categories
1) derivatives of tyrosine
2) derivatives of cholesterol (steroids)
3) peptides and proteins.
Hormones can also be classified based on their function.
Peptide and Protein Hormones
Summary
a. The secretion of peptide and protein hormones follows the classical pathway for secretion of protein from cells.
b. After synthesis as a pre-prohormone on ribosomes from their respective mRNAs, the hormone is targeted to the rough endoplasmic reticulum.
i. Here the pre-prohormone is cleaved and the prohormone is transported to the Golgi apparatus where it is further processed and packaged into secretory vesicles.
c. The endocrine organ then secretes the hormone in response to specific signals by vesicular exocytosis, in a calcium-dependent manner.
i. A similar mechanism is used for the secretion of catecholamines like dopamine and epinephrine.
d. Once secreted into the bloodstream, the hormone is then carried to its target organ.
e. Most peptide and protein hormones (the exceptions are growth hormone, prolactin and Insulin-like growth factor) are transported in the blood as free hormones.
i. As the blood contains many proteases, the half-life of these hormones is therefore limited.
Peptide and Protein hormone transport in blood
a. Once secreted into the bloodstream, the hormone is then carried to its target organ.
b. Most peptide and protein hormones (the exceptions are growth hormone, prolactin and Insulin-like growth factor) are transported in the blood as free hormones.
c. As the blood contains many proteases, the half-life of these hormones is therefore limited.
How peptide/protein hormones are created
a. The secretion of peptide and protein hormones follows the classical pathway for secretion of protein from cells.
b. After synthesis as a pre-prohormone on ribosomes from their respective mRNAs, the hormone is targeted to the rough endoplasmic reticulum.
i. Here the pre-prohormone is cleaved and the prohormone is transported to the Golgi apparatus where it is further processed and packaged into secretory vesicles.
c. The endocrine organ then secretes the hormone in response to specific signals by vesicular exocytosis, in a calcium-dependent manner.
i. A similar mechanism is used for the secretion of catecholamines like dopamine and epinephrine.
Steroid Hormone
Large Summary
a. The basic precursor for all steroid hormones is cholesterol.
i. Because the names of various steroid hormones derive from the numbering system of cholesterol, it is useful to know how the carbon atoms on a cholesterol molecule are numbered.
b. As steroid hormones are lipophilic and therefore membrane permeant, it stands to reason that their secretion will not be via the vesicular exocytosis as it is for protein and peptide hormones.
c. The steroid hormones are not stored in the cell they are synthesized in and are therefore immediately released into the blood stream.
d. Another consequence of their hydrophobicity is that they need to be carried by carrier proteins in the blood stream.
i. In the blood, steroid hormones exist in equilibrium between free and bound forms and at any given time only a small fraction of the hormone (1-5%) exists in the free form.
e. However, you must realize that it is the free form of the hormone that is biologically active and thus knowing total hormone levels in the blood might not be very informative.
i. The bound form serves as an essential reservoir of the hormone.
f. Unlike the protein and peptide hormones, steroid hormones linger in the bloodstream for a long time and have half lives in the order of many hours to days.
Steroid Hormone- Hydrophobic nature in the blood stream
a. The steroid hormones are not stored in the cell they are synthesized in and are therefore immediately released into the blood stream.
b. Another consequence of their hydrophobicity is that they need to be carried by carrier proteins in the blood stream.
c. In the blood, steroid hormones exist in equilibrium between free and bound forms and at any given time only a small fraction of the hormone (1-5%) exists in the free form.
d. However, you must realize that it is the free form of the hormone that is biologically active and thus knowing total hormone levels in the blood might not be very informative.
e. The bound form serves as an essential reservoir of the hormone.
Half-Life of the steroid hormone
a. Unlike the protein and peptide hormones, steroid hormones linger in the bloodstream for a long time and have half lives in the order of many hours to days.
b. Half-life of peptide/protein hormone is very short
Measurement of Hormone levels:
a. Measurement of plasma levels of hormones is a primary tool of the clinical endocrinologist.
b. The two major methods for measuring hormone levels are
1) bioassays and
2) immunoassays.
c. Bioassays measure hormone activity and in this case hormone function is measured by using an exogenous system e.g. cell lines, to measure hormone activity.
d. Radio-immunoassays (RIA) and enzyme linked immunosorbent assays (ELISA) measure antibody binding to a specific region of the hormone.
i. They might not be useful if an abnormal form of the hormone is being secreted by the patient.
Bioassays and Immunoassays for measuring hormone levels
a. Bioassays measure hormone activity and in this case hormone function is measured by using an exogenous system e.g. cell lines, to measure hormone activity.
b. Immunoassays—> Radio-immunoassays (RIA) and enzyme linked immunosorbent assays (ELISA) measure antibody binding to a specific region of the hormone.
i. They might not be useful if an abnormal form of the hormone is being secreted by the patient.
Hormone Action
Protein and peptide hormones
a. When protein and peptide hormones (as well as some of the hormones derived from tyrosine, like epinephrine and norepinephrine) reach their target, they bind to specific receptors on the plasma membrane of the target cells.
b. Receptors for some hormones, e.g. epinephrine and norepinephrine, belong to the family of G-protein coupled receptors.
i. Binding of the hormone to these receptors results in changes in the levels of intracellular second messengers like cAMP, diacylglycerol and inositol phosphates.
c. Other hormones like growth hormone and prolactin have receptors that belong to the JAK/STAT family of receptors.
i. Activation of these receptors results in coupling and activation of a tyrosine kinase (Janus kinase or JAK), which then causes the phosphorylation of a group of proteins called signal transducers and activators of transcription (STATs).
d. Yet other receptors for hormones like Insulin and IGF-1 belong to a large family of protein tyrosine kinase receptors.
i. In this case the receptors themselves are tyrosine kinases that can be activated upon hormone binding.
e. Activation of catecholamine, protein and peptide hormones can have rapid consequences, like increased cytosolic calcium, exocytosis, phosphorylation of enzymes and ion channels.
i. In addition, they can have effects that are slower and involve changes in gene expression.
Three Major Categories of Protein-Peptide Receptors
G-Protein Coupled Receptors, JAK/STAT receptors, Tyrosin Kinase Receptors
- Receptors for some hormones, e.g. epinephrine and norepinephrine, belong to the family of G-protein coupled receptors.
i. Binding of the hormone to these receptors results in changes in the levels of intracellular second messengers like cAMP, diacylglycerol and inositol phosphates. - Other hormones like growth hormone and prolactin have receptors that belong to the JAK/STAT family of receptors.
i. Activation of these receptors results in coupling and activation of a tyrosine kinase (Janus kinase or JAK), which then causes the phosphorylation of a group of proteins called signal transducers and activators of transcription (STATs). - Yet other receptors for hormones like Insulin and IGF-1 belong to a large family of protein tyrosine kinase receptors.
i. In this case the receptors themselves are tyrosine kinases that can be activated upon hormone binding.
Steroid Hormone Action
a. Unlike the peptide hormones, steroid receptors are nuclear in their location.
b. Once steroid hormones reach their target cells, they enter the cell and bind to their receptors in the cytosol or the nucleus.
c. The receptor-hormone complexes then bind to specific hormone responsive elements (HRE) and activate transcription of specific genes