The thyroid hormones, thyroxine and triiodothyronine, are tyrosine-based hormones produced by the thyroid gland, primarily responsible for regulation of metabolism.
An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half life than T3. T3 is the more biologically active form, and each tissue catalyzes the conversion from one form to the other: in this way, it can have his own pool of active hormone, according to its own exigencies.
Thyroid hormone is essential for normal development, growth and metabolism. In particular, it increases cardiac output, heart rate and ventilation rate; increase metabolism of proteins and carbohydrates and, more generally, it increases basal metabolic rate. It has some important effects on growth development, since it potentiates the effects of catecholamines, it potentiates brain development and it thickens endometrium in females.
Until the last few years, TH-mediated changes in gene expression were thought to be primarily, if not solely, initiated by direct nuclear TR binding to a TRE in the promoter of a target gene. The thyroid hormone, actually, mediates its effects through two different kind of mechanism: the best known nuclear action, traditionally, and the ‘non-conventional’ extranuclear action. The last includes a series of important effects, dependent on thyroid and its hormone, that don’t involve the nuclear import of the hormone and consequent binding to its receptor, but rather the cytosolic or extracellular interaction of the hormone with its own receptors or heterologous receptor proteins.
Thyroid hormone and phosphatidylinositol-3-kinase
Rapid nongenomic actions of thyroid hormone. 2006
Thyroid hormone action during brain development: more questions than answers. 2010
Thyroid hormone mediated changes in gene expression can be initiated by cytosolic action of the thyroid hormone receptor beta through the phosphatidylinositol 3-kinase pathway. 2006
Phosphatidylinositols are cell membrane phospholipids, important for cell’s structure as well as signaling and communications: enzymes called ‘phosphatidylinositol kinases’ are capable of phosphorilate this molecules at different positions, turning them into active signalers.
Phosphatidylinositol 3-kinases (PI 3-kinases or PI3Ks) are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which in turn are involved in cancer.
There are two major isoforms of TR, TRα1 and TRβ1. TRα1 is expressed in heart, brain, skeletal muscle, and adipose tissue, whereas TRβ1 is expressed at higher levels in liver and kidney, as well as fibroblasts.
The PI3K pathway, as mentioned above, exerts some of its main functions in the cardiovascular system. In vascular endothelial cells, the predominant TR isoform is TRα1, which is also expressed in brain, skeletal muscle, and adipose tissue. An important downstream target of PI3K is Akt, endothelial nitric oxide synthase (eNOS) is, as it's well known, a really important mediator of vascular regulation, and it's phosphorylated and activated by Akt.
Treatment of endothelial cells with triiodothyronine increases the association of TRα1 with the p85α subunit of PI3-kinase, leading to the phosphorylation and activation of Akt and endothelial nitric oxide synthase (eNOS). The activation of Akt and eNOS by T3 is, actually, abolished by the PI3-kinase inhibitors, but not by the transcriptional inhibitor.
Acting through the PI3K, T3 is capable of reducing the infarct size and improving the cerebral blood flood, leading to decreased stroke size and improved neurological deficit score after focal cerebral ischemia.
In human fibroblasts TH acts through the TRβ, also expressed at higher levels in liver and kidney: the liganded thyroid receptor β (TRβ) interacts with the regulatory subunit of PI3K (p85α) in the cytosol. This leads to activation of PI3K and its downstream signaling cascade: sequential phosphorylation and activation of the serine/threonine kinase Akt, mammalian target of rapamycin (mTOR) and its substrate p70S6K. mTOR activation is rapid, with detectable phosphorylation as early after T3 treatment.
In fibroblasts expressing the WT TRβ, introduction of a dominant negative mutant TRβ abrogated the effect of TH. Furthermore, co-immunoprecipitation experiments showed that TRβ1 interacts with the p85α subunit of PI3K. The interaction between TRβ and PI3K most likely takes place in the cytosol. Within minutes after activation by T3, phosphorylated Akt, as part of the PI3K pathway, is translocated from the cytosol into the nucleus. This TH action is very rapid and independent of protein synthesis, which is typical of non-genomic action.
Two aspects distinguish this mechanism of TH action from most other non-genomic effects of the hormone: it requires TR binding and its ultimate effect is ‘genomic’. This means that, even if it doesn’t acts as a proper transcription factor, specific genes are induced by this mechanism anyway. The calcineurin inhibitor ZAKI-4α, for example, is a TH responsive gene, and the promoting effect of thyroid hormone on this gene is blocked by inhibitors of PI3K and by a dominant negative p85α subunit of PI3K. Rapamycin, an inhibitor of mTOR, also abrogates the TH-dependent induction of ZAKI-4α, suggesting the requirement of sequential activation of kinases initiated by the activation of PI3K.
Between the genes up-regolated through this indirect effect of thyroid hormone, there are thypoxia-inducible factor's sub unit 1α (HIF-1α), its target genes, glucose transporter 1 (GLUT1) and platelet-type phosphofructokinase (PFKP), and the monocarboxylate transporter 4 (MCT4), all functionally related as they have important roles in cellular glucose metabolism.
Another important downstream target of Akt is endothelial nitric oxide synthase (eNOS), which is phosphorylated and activated by Akt.
Thyroid hormone, integrin receptors and MAP-kinases
Thyroid hormone action during brain development: more questions than answers. 2010
Membrane receptors mediating thyroid hormone action. 2005
Thyroid hormone, however, is able to act also outside the cell, at the plasma membrane level; more interestingly, it’s able to bind to non-TH receptors. For example, there is a thyroid hormone cell surface receptor on the extracellular domain of integrin αVβ3, a protein best known for its interactions with the extracellular matrix (ECM). Binding of thyroid hormone to this receptor leads to activation of the mitogen-activated protein kinase (MAPK) signal transduction cascade.
MAP kinases are serine/threonine-specific protein kinases that respond to extracellular stimuli (mitogens, osmotic stress, heat shock and proinflammatory cytokines) and regulate various cellular activities, such as gene expression, mitosis, differentiation, proliferation, and cell survival and apoptosis.
MAPK-dependent thyroid hormone actions include plasma membrane ion pump stimulation and specific nuclear events. Among the nuclear events there are serine phosphorylation of the nuclear thyroid hormone receptor, leading to co-activator protein recruitment and complex tissue responses, such as thyroid hormone-induced angiogenesis or tumor cell growth. The existence of this cell surface receptor means that the activity of administered hormone could be limited through structural modification of the molecule to reproduce or inhibit only those hormone actions initiated at the cell surface.
After the treatment with T3 or T4, MAPK is activated and moves into the nucleus, causing the displacement of thyroid hormone receptor β too. Inside the nucleus, MAPK phosphorilates and activates TRβ1.
One of the most interesting aspects of this hormonal action, is that, in this circumstance and differently from the traditional nuclear pathway, the most active form of the hormone is T4.
Given the activities that thyroid hormone and MAPK are known to have, it isn’t surprising that other nuclear proteins are influenced positively from this signaling pathway.
Among these proteins are the estrogen receptor (ER) and the oncogene suppressor protein, p53, whose transcriptional activity is changed by serine-phosphorylation in thyroid hormone-treated cells. Also serine-phosphorylated signal transducer and activator of transcription-1α (STAT1α) is found in the nucleus of cells exposed to thyroid hormone. Considering the situation under a more general point of view, the consequence of these effects is amplification of signals that originate at the plasma membrane from cytokines, such as interferon-γ, or growth factors, like epidermal growth factor (EGF) and transforming growth factor-α (TGF-α).
It is well known that a lot of cellular signaling pathways have, as an ultimate or intermediate effect, a change in intracellular potential due to the concentrations of some ions. Several laboratories have described actions of thyroid hormone on membrane ion pumps or channels; some of these effects have been shown to be independent of TR, and can be classified as membrane-initiated actions of the hormone. The effects are non-genomic, since they are not primarily dependent upon intranuclear complexing of thyroid hormone and its receptors. the activity of the Na+/H+ antiporter is regulated by thyroid hormone. This hormonal effect is MAPK-dependent, indicating that transduction of an integrin-originated hormone signal by MAPK doesn't need to proceed downstream to the nucleus, but can remain at the cell surface. Hormonal activation of the antiporter also depends upon generation of an intracellular Ca2+ signal, consistent with the rise in [Ca2+]i.
Other channels regulated in a similar fashion are the inward-rectifying K+ channel and of the Na+ current (INa): they are modulated by thyroid hormone – specifically by T3 – through mechanisms that appear to exclude participation of the nuclear receptor. The hormonal effect on INa might increase inotropy via consequent increase in [Ca2+]i.
The fact that thyroid hormone interacts with integrin receptors makes sense not only experimentally, but also conceptually: these receptors aren't just the link between the cell and the extracellular matrix, but they're involved in regulating processes of cell growth, division, survival and differentiation. The fact that MAP kinases have the role of transducer in their signaling, after all, provides a clear proof of this.
Also, thyroid hormone has ecently been found to promote angiogenesis, a process where integrins take actively part.
The fact that the involved form of the hormone is T4, and not the more traditionally biologically active T3, is because T4's structure better fits the RGD's binding site on αVβ3's receptor, as it can be seen in the picture below.
It's still unknown, and providing an interesting field of research, the reason of this selectivity, different from nearly all the processes where thyroid hormone exerts its action. The fact that there is at least one context where the more functional form is T4, and not T3, maybe helps in maximizing and optimizing the action of the hormone, but this is just a speculation.
Non-genomic actions of thyroid hormone in brain development
Thyroid hormone action during brain development: more questions than answers. 2010
Non-genomic actions of thyroid hormone in brain development. 2008
It is well known that one of the main actions of thyroid hormone is the regulation of the growth and development of central nervous system. T3-induced transcriptional regulation is not the sole contribution to the brain developmental program.
One well-recognized non-genomic action of TH is the T4-dependent regulation of actin polymerization. Chemical disruption of neuronal actin cytoskeleton markedly impairs neuronal growth cone motility and pathfinding ability.
In astrocytes and neuronal processes of granular neurons grown in the absence of TH, only 40-60% of the cellular actin is polymerized, and the actin cytoskeleton is disorganized. Both T4 and rT3, but not T3, rapidly increase the F-actin content to 90-95% of the cellular actin and restore the microfilaments without altering total actin content or gene expression.
These cell culture findings mimic those observed during cerebellar development in vivo, as it can be observed in rats.
These data indicate that TH-initiated changes in actin polymerization during brain development is one action of this hormone that does not require the conventional thyroid hormone's receptors; this is an especially attractive mechanism of action, because the two effector hormones, T4 and rT3, are the two predominant thyroid hormones produced during fetal life.
Genomic and non-genomic actions of thyroid hormone: not a rivalry, but a sinergy
Membrane receptors mediating thyroid hormone action. 2005
It is noteworthy, now, to observe that the genomic and non-genomic actions of thyroid hormone aren't two totally distinguished processes. They may have different kinetics and timing, and interact with some pathways that aren't involved in the more traditional nuclear signaling.
Nevertheless, as it has already been mentioned, it must not be forgotten that the primary biological roles of thyroid hormone are growth, development and homeostasis, and the pathways where TH exerts its non-genomic actions, like PI3K's and MAPK's pathways, take part in this process.
So, a virtual model where the thyroid hormone is the mediator of multiple and independent cellular actions should be replaced by a more harmonic one, where the changes induced by the non-genomic role of this hormone render the cell more active and responsive, and, as a matter of fact, more able to respond to the slower and long-term nuclear action.
The first thing someone could think about are, for example, ion membrane channels; it has been previously discussed, in the section about MAP kinases, that T3, actually, rapidly modulates membrane potential, cellular depolarization, and contractile activity by regulating ion flux across plasma membrane ion channels. In a range of time to onset that can vary from 30 seconds up to 20 minutes, the thyroid hormones positively modulates the following activities:
- Na+ channel bursting or prolonged opening
- Inward Na+ current
- Inward rectifying K+ current
- Na+/H+ antiporter
These findings have been collected studying animal models, but these events are very likely to happen in human beings too; anyway, further research and experiments will no doubt provide more information.
The Na+ entry, in particular, is supposed to increase the intracellular calcium concentration ([Ca2+]i), a plenty of other effects exerted by thyroid hormone.
Not only the membrane-initiated actions of thyroid hormone can be considered factors that contribute to the setpoints of membrane ion pumps; rather, they can also ensure the basal levels of transcription of genes that contain TREs, providing another link between the genomic and non-genomic actions of TH.
TH non-genomic receptors
THE GENES
TRα
TRβ
αVβ3 integrin receptor
Other sources
- GravesDisease.com
- Vivo.Colostate.edu
- Wikipedia
