Cell Signaling Mechanisms

Steroid Hormone Signaling: Cortisol and Intracellular Receptors

Steroid hormones such as cortisol are lipid-soluble molecules that can cross the plasma membrane and interact with intracellular receptors. This process leads to changes in gene expression within the target cell.

• Cortisol passes through the plasma membrane into the cytoplasm.

  • Steroid hormones are hydrophobic and can diffuse through the lipid bilayer of the cell membrane.

• Cortisol binds to an intracellular receptor in the cytoplasm.

  • This forms a cortisol-receptor complex.

• Cortisol-receptor complex enters the nucleus where it binds to specific genes.

  • The complex acts as a transcription factor, regulating gene expression.

• Cortisol-receptor complex acts as a transcription factor.

  • It binds to DNA at specific response elements, promoting or inhibiting transcription of target genes.

• The transcribed mRNA is translated into a specific protein.

  • This leads to the physiological effects associated with cortisol.

Example: Cortisol regulates metabolism by increasing the transcription of genes involved in gluconeogenesis in liver cells.

Additional info: Steps such as "Cortisol binds to a receptor on the outside of the cell" and "Cortisol-receptor complex forms in the cytoplasm" are not applicable to steroid hormone signaling, as these hormones use intracellular, not cell-surface, receptors.

G Protein-Coupled Receptor (GPCR) Signaling: Histamine and the H1 Receptor

Histamine is a chemical messenger involved in inflammatory and allergic responses. It exerts its effects by binding to specific cell-surface receptors, such as the H1 receptor, which is a type of G protein-coupled receptor (GPCR).

• Histamine binds extracellularly to the H1 receptor.

  • The H1 receptor is located on the plasma membrane of target cells.

• Upon binding, the H1 receptor undergoes a conformational change and binds intracellularly to an inactive G protein, activating it.

  • This activation involves the exchange of GDP for GTP on the G protein.

• The active G protein binds to the enzyme phospholipase C, activating it.

  • Phospholipase C then catalyzes the production of second messengers (e.g., IP3 and DAG), leading to cellular responses such as increased calcium release.

• Histamine is highly hydrophilic.

  • As a result, it cannot cross the plasma membrane and must bind to cell-surface receptors.

Example: In allergic reactions, histamine released from mast cells binds to H1 receptors on smooth muscle cells, causing contraction and symptoms such as bronchoconstriction.

Additional info: GPCR signaling is a common mechanism for transmitting signals from hydrophilic ligands that cannot cross the plasma membrane.