Ligand Characteristics and Receptor Organization
Signal transduction systems exist in every cell to convert external signals into intracellular changes. Cell surface receptors may be located at a distance from one another or clustered in specific areas.
Intracellular receptors are designed to interact with hydrophobic (lipophilic) substances. Classical examples of such ligands include steroid hormones and thyroxine. Due to their chemical nature, these molecules do not require membrane transporters: they can freely diffuse directly through the lipid bilayer of the plasma membrane into the cell.
Structural Features of Intracellular Receptors
Receptors for hydrophobic ligands are located inside the cell—within the cytosol or directly in the nucleus.
In the absence of a signal, cytosolic receptors remain in an inactive state. This inactivity is maintained by specialized chaperone proteins bound to the receptor. The primary function of chaperones is to prevent premature activation of the receptor molecule before the hormone enters the cell.
A key structural feature of both nuclear and cytosolic steroid and thyroid hormone receptors is the presence of a DNA-binding domain. This specific region allows the hormone-receptor complex to interact with regulatory DNA sequences in the nucleus.
Signal Transduction Cascade (Genomic Mechanism)
Signal transmission via intracellular receptors is a nuclear mechanism that alters the rate of gene transcription. It includes the following steps:
- Diffusion: The lipophilic hormone freely crosses the plasma membrane into the target cell.
- Complex Formation: In the cytosol (or nucleus), the hormone meets its receptor. A conformational change occurs, forming a functional hormone-receptor complex.
- Nuclear Translocation: If the complex forms in the cytosol, it translocates into the nucleus.
- DNA Binding: The complex binds to a specific regulatory nucleotide sequence, such as an enhancer or a silencer.
- RNA Polymerase Regulation: The interaction of the complex with DNA alters promoter accessibility for RNA polymerase. Binding to an enhancer increases accessibility, while binding to a silencer decreases it.
- Transcription Modulation: Consequently, transcription (mRNA synthesis) of specific structural genes is activated or suppressed.
- Processing and Export: Mature mRNA molecules are processed and exported from the nucleus into the cytosol.
- Translation: Ribosomes in the cytoplasm synthesize specific proteins based on the mRNA template, increasing or decreasing their translation rate.
- Biological Effect: Changes in protein synthesis directly affect the metabolism and functional state of target cells.
Example of Effect Execution: Action of Calcitriol
The mechanism of action of calcitriol is a classic example of a cytosolic (genomic) signaling pathway.
Calcitriol, being a lipophilic hormone, freely crosses the membrane of the target cell (enterocyte). In the cytoplasm or nucleus, it binds to a specific intracellular receptor. The resulting hormone-receptor complex dimerizes and is transported into the nucleus.
Inside the nucleus, the complex binds to regulatory DNA regions known as hormone response elements (HRE). This activates the transcription of a specific gene encoding a calcium ion transporter protein.
Subsequently, translation occurs on cytoplasmic ribosomes. Specific calcium-binding proteins—calbindins—are synthesized, ultimately altering cellular metabolism to fulfill its biological function.