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Steroid hormones
Introduction:
1. What is steroid?
A steroid is an organic compound with four ringsarranged in a specific molecular
configuration
Examplesinclude the dietary lipid cholesterol, the sex
hormonesestradioland testosterone and the anti-
inflammatory drug dexamethasone.
Steroidshavetwo principal biological functions: certain steroids(such
as cholesterol) are importantcomponentsof cell membraneswhich
alter membrane fluidity, and many steroidsare signaling molecules which
activate steroid hormone receptors.
Fig 1-Steroid ring system: The parent ABCD steroid ring system (hydrocarbon framework) is
shown with IUPAC-approved ring lettering and atom numbering.
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2. What are steroid hormones?
A steroidhormone is a steroid that acts as a hormone.
Classes of steroid hormones
Steroid hormones can be grouped into two classes:
Corticosteroids (typically made in the adrenal cortex, hence cortico-)
Sex steroids(typically made in the gonads or placenta).
Typesof steroid hormones
Within those two classes are five types according to the receptors to which they
bind:
Glucocorticoids
Mineralocorticoids(corticosteroids)
Androgens
Estrogens
Progesterone(sexsteroids).
Function of steroid hormones
Steroid hormones help control
Metabolism
Inflammation
Immune functions
Salt and water balance
Development of sexual characteristics
The ability to withstand illness and injury
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3. Synthesis of steroid hormones
The natural steroid hormones are generally synthesized from cholesterol in
the gonads and adrenal glands. These forms of hormones are lipids. They can pass
through the cell membrane as they are fat-soluble and then bind to steroid hormone
receptors (which may be nuclear or cytosolic depending on the steroid hormone) to
bring about changes within the cell. Steroid hormones are generally carried in the
blood, bound to specific carrier proteins such as sex hormone-binding
globulin or corticosteroid-binding globulin. Further conversions and catabolism
occurs in the liver, in other "peripheral" tissues, and in the target tissues.
Fig-2 Synthesis of steroid hormones
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4. Transport
Steroid hormones are transported through the blood by being bound to carrier
proteins—serum proteins that bind them and increase the hormones' solubility in
water. Some examples are sex hormone-binding globulin (SHBG), corticosteroid-
binding globulin, and albumin.
Fig.4-Steroid hormone action on target
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5. Mechanisms of action and effects
There are many different mechanisms through which steroid hormones
affect their target cells. All of these different pathways can be classified as
having either a genomic effect, or a non-genomic effect. Genomic
pathways are slow and result in altering transcription levels of certain
proteins in the cell; non-genomic pathways are much faster.
Genomic pathways
The first identified mechanisms of steroid hormone action were the
genomic effects. In this pathway, the free hormones first pass through the
cell membrane because they are fat soluble. In the cytoplasm, the steroid
may or may not undergo an enzyme-mediated alteration such as reduction,
hydroxylation, or aromatization. Then the steroid binds to a specific steroid
hormone receptor, also known as a nuclear receptor, which is a large
metalloprotein. Upon steroid binding, many kinds of steroid
receptors dimerize: two receptor subunits join together to form one
functional DNA-binding unit that can enter the cell nucleus. Once in the
nucleus, the steroid-receptor ligand complex binds to
specific DNA sequences and induces transcription of its target gene.
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Non-genomic pathways
Because non-genomic pathways include any mechanism that is not a
genomic effect,there are various non-genomic pathways. However, all of
these pathways are mediated by some type of steroid hormone
receptorfound at the plasma membrane. Ion channels, transporters, G-
protein coupled receptors (GPCR),and membrane fluidity have all been
shown to be affected bysteroid hormones .Of these, GPCR linked proteins
are the most common. For more information on these proteins and
pathways, visit the steroid hormone receptor page.
