Introduction
Retinoids are the class of chemicals related to vitamin A. The breadth of retinoids’ effects in the body is perhaps greater than that of any other vitamin, including vision, cell proliferation/differentiation, bone tissue, and the immune system. “Retinoid” is a general term that encompasses a range of molecules, including retinol, retinoic acid, and retinyl esters, as well as synthetic retinoids. Vitamin A stands out as one of the most intriguing vitamins due to its diverse roles and the serious risks associated with overexposure. In this article, I aim to demystify Vitamin A by exploring how retinoids regulate gene expression, thereby influencing cellular turnover, stem cell proliferation, and differentiation.
What is Accutane?
Despite retinoids existing in many forms, most retinoid signalling comes from the primary metabolite all-trans retinoic acid (ATRA). ATRA binds to several types of nuclear receptors, including Retinoic Acid Receptor (RAR), Retinoid X Receptor (RXR), and Peroxisome proliferator-activated receptors (PPARs). [1] The Retinoic Acid Receptor (RAR) is the primary target of Retinoid Acid, while PPARs are a set of nuclear receptors that are particularly relevant sebum production (read more).
Vitamin A is classified as a dietary vitamin because the body cannot synthesize it on its own. The precursor to vitamin A, beta-carotene, can be obtained from plant sources that possess orange and red colours, such as carrots.
Additionally, retinyl esters can be obtained from animal sources, such as beef liver, which is the storage form of vitamin A that accumulates in the liver and adipose fat. [2] These retinyl esters do not have a significant role aside from serving as substrates for conversion into other retinoid products in the body, such as 11-cis-retinal for vision (read more). [3]
Retinol itself does not primarily contribute to the biological roles of vitamin A, as it must first be converted into retinoic acid. [4] It is believed that Accutane also serves as a substrate for conversion into retinoic acid within the cell.
The advantage of applying isotretinoin (Accutane) rather than retinoic acid is that it bypasses the body’s metabolizing enzymes (P450), which would otherwise break down excessive retinoic acid. This allows for greater accumulation of retinoic acid in the cell nucleus. [5]
Isotretinoin (Accutane) is an isomer of retinoic acid; it occurs naturally in very small doses but primarily exerts its effect through the metabolite all-trans-retinoic acid (ATRA). The advantage of administering isotretinoin rather than simply ATRA is that it has a longer half-life, allowing for less frequent administration. Isotretinoin is distinct from ATRA because it has a cis bond on the 13th carbon.
[Fig 1] All trans retinoic acid (ATRA) vs. cis-13 retinoic acid. (Vaccinationist, Public domain, via Wikimedia Commons)
Differentiation vs Proliferation
Almost every cell in the body undergoes a life cycle that can be broken into four phases. The first stage being G1 (gap 1) phase, which is where the cell synthesises proteins and mRNA to prepare for cell division. Next the S (synthesis phase) takes place, where DNA is replicated to ensure the genome can be perfectly copied across to the new cell.
Next follows the G2 (gap 2) phase, where chromatin condenses into chromosomes. The chromatin is the long string of DNA, that first must be packaged into tight structures called chromosomes. Finally, once all this preparation has taken place the cell can divide though mitosis, and a new cell is formed.
[Fig 2] Depiction of cell cycle. (Servier Medical Art is licensed under CC BY 4.0 https://smart.servier.com/smart_image/mitosis/)
The new copy of chromosomes are separated into a new nucleus and new cell is a copy of the first. Cells can be taken outside of this cycle of replication into G0, or quiescent phase. This might occur because there simply isn’t sufficient nutrients to support the growth of new cells, or because some cells don’t require regeneration unless there is injury.
Most cells in adults exist in the G0 phase, and don’t need proliferation. For example, cells in the liver very rarely divide, but when a section of liver is surgically removed or damaged, the cells rapidly proliferate to repair the damage.
During cell proliferation, tissues growth individual cells grow whilst dividing, and therefor maintain cells of a roughly constant size. If the cells did not also grow as they divide, then the tissue mass would remain constant as it divides into smaller and smaller cells. Cells can also change purpose or function through a process of differentiation.
[Fig 4] Cell Proliferation vs. Differentiation
A progenitor or stem cell can become specialised to perform specific tissues or functions. Some cells have a short life span and must be replaced by continual cell proliferation such as blood cells and epithelial cells of the skin or digestive tract. Stem cells are the unique cells in the body that can self-replenish, and can convert into specialised tissue cells through a process called differentiation. However high levels of retinoic acid can directly inhibit stem cell proliferation, interfering with growth and repair.
Stem cells also divide to produce new stem cells and act as reserve for throughout an entire lifetime. However, some cells can proliferate in an uncontrolled manner and avoid the typical cycle of cell death. These cells are called cancer cells and form tumours that disrupt normal tissue function and can ultimately lead to death.
[Fig. 5] The variety of cells that a stem cell can differentiate into. Haileyfournier, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
What does Accutane Do?
Retinoic acid is needed to help signal for cells to become differentiated and specialised from progenitor or stem cells. However high levels of retinoic acid can directly inhibit cell growth. This is most relevant to foetal development, where cells are rapidly proliferating and differentiating.
The mother needs healthy levels of vitamin A ensure that stem cells differentiate appropriately to form new limbs in a process called morphogenesis. The absence of vitamin A leads to uncontrolled proliferation of epithelial stem cells that fail to differentiate. For this reason, there has been a strong interest in retinoids reducing cancer risk.
The skin is one organ that relies on pools of progenitor stem cells to maintain tissue health and regeneration throughout adulthood, and for this reason it’s particularly reliant on Vitamin A to regulate the process of differentiation. Epidermal stem cells go through a process of differentiation to become specialised into skin cells, known as epithelial cells.
In this process the cells change shape and begin producing a protein called Keratin, increasing the strength and resilience of the cell. The cells also change shape to become flattened till they eventually form the outermost layer of dead skin cells called the epidermis, which acts as a protective barrier.
Accutane accelerates the process of skin cell turnover by promoting skin cell differentiation. This can lead to improvements in skin texture, particularly in older individuals, as stem cell proliferation naturally slows down with age, reducing the rate of tissue regeneration. However, increasing differentiation may deplete the pool of progenitor stem cells.
While retinoids can enhance skin appearance, they may do so at the expense of the long-term ability of cells to proliferate. This could potentially have unintended consequences for other tissues in the body that rely on stem cell pools for growth and maintenance, such as epithelial cells in the gut or progenitor cells in the brain. One of the other tissues Accutane affects in this way is hair, which is why Accutane treatment is some commonly associated with hair loss (read more).
How Retinoids Regulate Differentiation
One of the key signalling pathways by which Retinoids influence differentiation is the Wnt/β-catenin pathway. The scope of this growth signalling pathway is broad and is key to understanding the effects of Retinoids more generally through the body.
β-catenin is a growth-signalling protein central to the Wnt pathway, which is essential for cell adhesion, tissue growth, development, and homeostasis. It is essential for maintaining pluripotent stem cell proliferation, and in its absence, these cells undergo differentiation, leading to the loss of their stemness.
Wnt proteins (named ‘wingless’ due to their shape) activate the ‘canonical’ Wnt/β-catenin pathway, leading to the transcription of β-catenin target genes. In the absence of Wnt ligands (binding molecules), β-catenin is continuously marked for degradation within a ‘destruction complex.’
[Fig 6] The Destruction Complex (Axin, APC, GSK-3β) continuously breaks down β-catenin.
This destruction complex, which traps β-catenin, consists of Axin, APC, GSK-3β (glycogen synthase kinase 3 beta), and CK1. When Wnt proteins bind to receptors (Frizzled and LRP5/6) on the cell surface, the destruction complex is inhibited, allowing β-catenin to stabilize and accumulate in the cytoplasm. β-catenin then translocates into the nucleus, where it interacts with TCF/LEF transcription factors to regulate the expression of target genes related to cell proliferation and differentiation. [9]
ATRA (the primary active metabolite of Accutane) can block the action of β-catenin by enhancing the destruction complex’s activity. ATRA achieves this by inhibiting PI3K-AKT, which upregulates GSK-3β’s degradation of β-catenin. [10]
Retinoic acid also appears to directly impact the transaction of LEF/TCF by β-catenin, which are the primary transcription factors that mediate the effects of β-catenin. One of β-catenin’s key roles is maintaining stem cell populations. When β-catenin activity is blocked, stem cells undergo differentiation, losing their pluripotent self-renewing properties. [11]
One of Accutane’s medical applications is in treating cancers, where tumours maintain their self-renewing stem cell properties to rapidly proliferate. ATRA can disrupt tumorigenesis by blocking β-catenin and triggering differentiation. [12] While Accutane exerts this differentiating effect on cancer stem cells, it can also induce differentiation in healthy tissues throughout the body that rely on stem cell populations for maintenance, such as bones, skin, the gut, and the brain.
Unintended Consequences Of Retinoids
By speeding up the process of differentiation, Accutane in essence exerts an anti-proliferative effect. This is the case not only for skin, but for a variety of other organs and tissues too. This is most strikingly observed in embryos overexposed to vitamin A. If these embryos reach full term, they often suffer from underdeveloped limbs and cleft palates. [13]
This explains why Accutane is classified as a teratogen (a substance that disrupts normal foetal development and causes congenital disabilities). It is also the reason for the strict guidelines on birth control for women undergoing Accutane treatment.
However,the anti-proliferative effects of Accutane can also be observed in many adult tissues that rely on pools of stem cells for continual renewal and growth, including the skin, intestines, bone marrow, cornea, hair follicles, and brain (particularly the hippocampus). Retinoids such as Accutane trigger the conversion of these stem cells into specialized cells through differentiation.
In doing so, retinoids maintain a delicate balance between proliferation and differentiation, which is why certain tissues are particularly affected by Accutane treatment. The hippocampus, a region of the brain that relies on stem cells to continue developing new neurons during adulthood, is essential for forming new memories. Accutane significantly inhibits hippocampal neurogenesis, disrupting hippocampal-dependent learning (read more here). [14]
Although there is evidence that Accutane may be detrimental to cognitive function, the results are sometimes mixed. For instance, when rats were treated with Accutane prior to a two-stage maze task in which both stages were identical, it was found that Accutane impaired explicit memory during the second stage. [15] However, one month after Accutane exposure ended, explicit memory was recovered. This finding was supported by a study in mice that similarly showed a disruption in learning a radial maze task. [16]
Crandall et al. (2004) demonstrated that after 42 days of treatment with retinoic acid, hippocampal cell proliferation had almost halved. From this, they concluded that the decline in memory was directly related to Accutane’s impact on neurogenesis. Nonetheless, when rats were maintained on a long-term vitamin A-deficient diet, they also suffered from deficits in memory and hippocampal neurogenesis. This suggests that retinoic acid signalling must be delicately balanced, as both excessive and insufficient levels can damage memory formation. [17]
Article Summary
- Retinoids and Vitamin A: Retinoids are chemicals related to vitamin A, impacting vision, cell growth, bone tissue, and the immune system. They include molecules like retinol, retinyl estsers, and synthetic variants like Accutane. Most retinoid signalling in the body is from all-trans retinoic acid (ATRA).
- Accutane (Isotretinoin): Accutane is a retinoic acid isomer used medically, particularly for acne treatment. It bypasses metabolic enzymes that degrade retinoic acid, allowing higher accumulation in cells and a longer-lasting effect. Accutane accelerates skin cell turnover by promoting differentiation, which can enhance skin texture but may reduce stem cell reserves, impacting long-term tissue maintenance.
- Cell Cycle and Proliferation: Cells undergo a life cycle with stages of growth, DNA replication, and division. Most adult cells are in a quiescent (G0) phase and do not proliferate unless needed for repair. Retinoic acid helps signal differentiation from stem cells, but excessive levels can inhibit cell growth, which is particularly important in fetal development.
- Retinoids and Differentiation: Retinoic acid is critical for differentiating stem cells into specialized cells. The Wnt/β-catenin pathway is central to cell growth and differentiation, and ATRA disrupts β-catenin, leading to reduced stem cell proliferation and increased differentiation. This process is important for cancer treatment but also affects healthy tissues relying on stem cells for renewal, such as skin, gut, and brain tissues.
- Accutane and Cognitive Effects: The brain relies on delicate balance between cell proliferation and differentiation, and so Accutane treatment can have some neurological effects. Accutane impacts the hippocampus, a brain region crucial for memory, by reducing neurogenesis (growth of new neurons). Studies show mixed effects, with some evidence of memory impairment during treatment, but potential recovery afterward. Both excessive and insufficient retinoic acid levels can impair memory and neurogenesis.
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References
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