Lipid­soluble hormones diffuse across the plasma membrane of cells,
binding to receptors inside the cells where they alter gene expression.
LEARNING OBJECTIVE [ edit ]
Describe how hormones alter cellular activity by binding to intracellular receptors
KEY POINTS [ edit ]
Lipid­soluble hormones are able to diffuse directly across the membranes of both
the endocrine cell where they are produced and that of the target cell, as the cell membranes are
made of a lipid bilayer.
These hormones can bind to receptors that are located either in the cytoplasm of the cell or within
the nucleus of the cell.
When these hormones bind to their receptors, this signals the cell to synthesize more or less
mRNA from a gene or genes, which then results in more or less protein being created from those
mRNA molecules.
The increase or decrease in protein production can alter the cell structurally or alter how and
when it catalyzes chemicalreactions.
TERMS [ edit ]
transcription
the synthesis of RNA under the direction of DNA
steroid
a class of organic compounds having a structure of 17 carbon atoms arranged in four rings; they
are lipids, and occur naturally as sterols, bile acids, adrenal and sex hormones, and some vitamins
gene expression
the transcription and translation of a gene into messenger RNA and, thus, into a protein
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Intracellular Hormone Receptors
Lipid­derived (soluble) hormones such
as steroid hormones diffuse across the
lipid bilayer membranes of
the endocrinecell. Once outside the cell,
they bind to transport proteinsthat keep
them soluble in the bloodstream. At
the target cell, the hormones are released
from the carrier protein and diffuse across
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the lipid bilayer of the plasma
membrane of the target cells. They then adhere to intracellular receptors residing in the
cytoplasm or in the nucleus. The cell signaling pathways induced by the steroid hormones
regulate specificgenes within the cell's DNA. The hormones and receptorcomplex act
as transcription regulators by increasing or decreasing the synthesis of
mRNA molecules from specific genes. This, in turn, determines the amount of corresponding
protein that is synthesized from this RNA; this is known as altering gene expression. This
protein can be used either to change the structure of the cell or to
produce enzymes thatcatalyze chemical reactions. In this way, the steroid hormone regulates
specific cell processes .
Hormone regulation of gene expression
An intracellular nuclear receptor (NR) is located in the cytoplasm bound to a heat shock protein (HSP).
Upon hormone binding, the receptor dissociates from the heat shock protein and translocates to the
nucleus. In the nucleus, the hormone­receptor complex binds to a DNA sequence called a hormone
response element (HRE), which triggers gene transcription and translation. The corresponding protein
product can then mediate changes in cell function.
Other lipid­soluble hormones that are not steroid hormones, such as vitamin D
and thyroxine, have receptors located in the nucleus. The hormones diffuse across both the
plasma membrane and the nuclear envelope, then bind to receptors in the nucleus. The
hormone­receptor complex stimulates transcription of specific genes in the same way that
steroid hormones do. For example, the active vitamin D metabolite,calcitriol, mediates its
biological effects by binding to the vitamin D receptor (VDR), which is principally located in
the nuclei of target cells. The binding of calcitriol to the VDR allows the VDR to act as
a transcription factor that modulates the gene expression of transport proteins that are
involved in calcium absorption in the intestine. VDR activation in the intestine, bone, kidney,
and parathyroid gland cells leads to the maintenance of calcium and phosphorus levels in the
blood and to the maintenance of bone content.