7.7: Mechanisms of Hormone Action

Mechanisms Involving Intracellular Hormone Receptors

The realm of intracellular hormone receptors is characterized by their strategic positioning within the cellular confines. In strict accordance with their nomenclature, these receptors reside inside the cell, necessitating a unique set of attributes for hormone interaction.

For hormones to engage with intracellular receptors effectively, they must exhibit the ability to traverse the formidable barrier of the cell membrane, which entails that such hormones must possess properties conducive to membrane penetration. Steroid hormones demonstrate the remarkable capacity to permeate the lipid bilayer of the cell membrane. This inherent quality renders them adept at reaching and binding to intracellular receptors with relative ease, as illustrated in Figure 7.

Another class of lipid-soluble hormones, thyroid hormones, shares similar characteristics in this context. These hormones feature a chemical structure adorned with benzene rings, each bearing iodine atoms. This unique arrangement enables thyroid hormones to meet the lipid-soluble criteria, allowing them to easily cross the cell membrane and enter the intracellular environment, where they can carry out their regulatory functions.

Steroid hormones, known for their versatility, can bind to their receptors either in the cytosol or within the nucleus of the cell. Regardless of where this binding occurs, the resulting hormone-receptor complex ultimately moves toward the cell’s nucleus to attach to a specific DNA segment. In contrast, thyroid hormones take a different approach, bypassing the initial binding steps. They directly interact with receptors that are already bound to DNA, allowing them to streamline the process and dock directly onto these pre-existing DNA-bound receptors.

For both steroid and thyroid hormones, the binding of the hormone-receptor complex to DNA initiates a delicate genetic process. This key interaction activates the transcription of a target gene into mRNA. The mRNA then travels to the cytosol, directing ribosomes in the precise synthesis of proteins. Through this finely tuned sequence, these hormones regulate cellular responses that are essential to the intricate workings of physiological processes.

Mechanisms Involving Cell Membrane Hormone Receptors

Hydrophilic or water-soluble hormones orchestrate a complex series of molecular interactions within cellular communication. Unable to pass through the lipid bilayer of the cell membrane, they initiate their signaling through receptors located on the membrane's outer surface. This hormonal message activates a cascade of cell-signaling events, involving G proteins, adenylyl cyclase, cyclic AMP (cAMP) as a secondary messenger, and protein kinases. G proteins, with their GTP-binding ability, play a central role in this process. Comprising three subunits—α, β, and γ—the G protein activates when the α-subunit binds to GTP, triggering adenylyl cyclase to convert ATP into cAMP. As a second messenger, cAMP activates protein kinase A (PKA), which drives a phosphorylation cascade, modifying numerous cellular proteins, including enzymes. Unlike lipid-soluble hormones that act on gene transcription, hydrophilic hormones exert their influence by orchestrating precise intracellular signaling events, highlighting the complexity and elegance of cellular responses to these water-soluble messengers.

​Figure 8 | Signaling Pathway of Water-soluble Hormones | ⁵  This figure illustrates the signaling pathway of water-soluble hormones, which cannot cross the cell membrane due to their hydrophilic nature. Upon binding to receptors on the cell membrane's outer surface, the hormone initiates a cascade of intracellular events. The hormone-receptor interaction activates a G protein, consisting of α, β, and γ subunits. Once the α-subunit binds GTP, it activates adenylyl cyclase, which catalyzes the conversion of ATP to cyclic AMP (cAMP), a second messenger. cAMP, in turn, activates protein kinase A, triggering a phosphorylation cascade that modifies target proteins, leading to cellular responses. 

In the complex realm of cellular regulation, the phosphorylation of proteins plays a critical role in coordinating a wide range of cellular effects. These effects encompass everything from nutrient metabolism to hormone synthesis and various cellular products. The magnitude and diversity of these effects depend on several factors, including the characteristics of the target cell, the specific G proteins and kinases involved, and the dynamic network of protein phosphorylation.

In this intricate process, cAMP emerges as a key player among second messengers. Numerous hormones, each with its unique signaling message, rely on cAMP to initiate a cascade of intracellular events. For instance, calcitonin promotes bone formation and regulates blood calcium levels, while glucagon manages blood glucose levels.  Thyroid-stimulating hormone precisely triggers the release of thyroid hormones T3 and T4. ADH utilizes cAMP mechanism in inserting aquaporins on the cell membrane of the tubular cell in the renal tubule. 

This phosphorylation cascade enables hormonal responses to be rapid, efficient, and highly specific. Even in response to minimal hormone concentrations in the bloodstream, thousands of signaling events can occur simultaneously. However, this dynamic process is short-lived, as cAMP is quickly deactivated by the enzyme phosphodiesterase (PDE) in the cytosol, ensuring that target cell responses are promptly halted to prevent overstimulation.

In this intricate interplay of cellular responses, there are both accelerators and brakes. Certain G proteins, such as growth hormone–inhibiting hormone (GHIH), known as somatostatin, reduce cAMP levels upon hormone binding. This decrease in cAMP inhibits the secretion of human growth hormone. This duality of action is captured in their nomenclature, with Gs-proteins designated for stimulation and Gi-proteins for inhibition.

Intriguingly, the cAMP second messenger system is but one act in a multifaceted cellular performance. An alternative signaling system employs calcium ions as the second messenger. In this system, G proteins activate phospholipase C (PLC), akin to adenylyl cyclase in function. Once engaged, PLC cleaves a membrane-bound phospholipid, yielding diacylglycerol (DAG) and inositol triphosphate (IP3). DAG activates protein kinases, notably protein kinase C (PKC), thus commencing yet another phosphorylation cascade (Figure 9). Concurrently, IP3 ushers calcium ions from intracellular stores, predominantly from the smooth endoplasmic reticulum. An example of a DAG mechanism is Angiotensin II. While PKC phosphorylates specific target proteins leading to the regulation of vasoconstriction, IP3 mechanism is also activated allowing the release of calcium and muscle contraction.

Besides the cAMP and PLC pathways, activation of stimulatory G (Gs) proteins can indirectly enhance calcium channel opening via the cAMP pathway and PKA, while inhibitory G (Gi) proteins may reduce calcium channel activity by inhibiting adenylyl cyclase and cAMP production, or through direct inhibition of calcium channels. 

Calcium ions, as a key second messenger, exert their influence through two primary pathways. They can directly regulate enzymatic activities and various cellular functions. Alternatively, they can bind to calcium-binding proteins, notably calmodulin, which modulates protein kinases within the cell. This intricate signaling process highlights the remarkable precision and coordination involved in cellular responses to hormones, underscoring the complexity of physiological regulation.