The leukemia inhibitory factor (LIF) is a fascinating complex cytokine within the human body. It is present in a grand variety of biological processes, and it demonstrates a broad array of roles. These processes include maintaining the pluripotency of stem cells, tissue development, inflammation, and blood vessel formation [1]. Over the last decade, researchers have been drawn to LIF due to its diverse range of applications. The main area of interest is the role it plays in the central nervous system. Many researchers see the LIF as a potential component for therapeutic remedies for neurodegenerative diseases such as HIV-associated neurocognitive disorders (HANDs) and Alzheimer’s disease.

The LIF is classified as a member of the IL-6 family of cytokines. All of the cytokines that make up the IL-6 family use gp130 as a receptor subunit. These cytokines are involved in very different roles and processes at a microscopic level throughout the human body. It has been proven that LIF plays a role in maintaining the pluripotency of embryonic stem cells (ESCs) along with a fellow member of the IL-6 family, Oncostatin M (OSM). It has been shown that ESCs maintain their pluripotency when either the LIF or OSM is bound to the gp130 receptor subunit, essentially demonstrating that the LIF plays a massive role in determining whether ESCs differentiate into more specialized cells or they retain their pluripotency for a longer period of time [2]. This confirms that the LIF is responsible for essential mechanisms during the earliest development of a human embryo. Studies have also shown that the LIF can promote the differentiation of adult exocrine pancreatic cells into insulin-producing beta cells in vitro [3], suggesting that the LIF plays an active role in the development of pancreatic tissue in the human body and in the generation of functional pancreatic cells. It is important to highlight these functions of the LIF cytokine in order to understand its involvement in biological processes that span various systems of the human body.

The LIF has also been demonstrated to be a part of both proinflammatory responses and anti-inflammatory responses in physiological processes. It has been implicated as a proinflammatory agent in conditions such as allergic rhinitis, ulcerative colitis, and pancreatic carcinoma [4,5,6]. On the other hand, the LIF has also been shown to be associated with anti-inflammatory responses in the human body. It has been shown to repress the GnRH gene, which codes for the proinflammatory cytokine GnRH [7]. The duality of LIF in either pro- or anti-inflammatory responses is important to understand because it further highlights that the LIF is just a single part of many more complex mechanisms occurring within the inflammation. The precise role of the LIF in inflammation depends on many external factors and requires more investigation in order to be fully understood.

Another very crucial feature of the LIF is its role in angiogenesis. Angiogenesis is defined as the growth of new blood vessels [8]. One study demonstrates that the LIF promotes the regeneration of the myocardium and ensures the survival of cardiomyocytes after a heart attack. This results in improved angiogenesis and a larger presence of bone-marrow-derived cells in the heart [9]. The LIF has also been associated with the formation of blood vessels in the endometrium. The increase in the LIF in the endometrium is related to improved endometrial receptivity, the early formation of blood vessels, and facilitated embryo adhesion [10]. Understanding the LIF’s role in angiogenesis in structures such as the heart and endometrium is necessary when it comes to comparing this to its role in the CNS. It is significant because it could imply that the LIF plays a role in the regulation of the blood vessels that supply the CNS and in structures such as the blood–brain barrier (BBB).

The LIF has also been shown to be significantly involved in many mechanisms within the peripheral nervous system (PNS). The LIF has been observed to promote cell differentiation, growth, and survival in motor neurons and has been studied in both in vitro and in vivo models [11]. The previous studies further confirm that the LIF is highly involved in neuroprotection in different parts of the nervous system. It has been proven that the LIF takes part in mechanisms that promote the growth and regeneration of sensory neurons around the PNS after nerve damage [12]. A study performed with mice confirmed that the LIF promotes the reparative regeneration of peripheral nerves [13]. The study proved to be impactful due to it demonstrating that the LIF can act as both a polyfunctional cytokine and a neurotrophic factor. All these data can validate that the LIF is an active member of many pathways and mechanisms that form part of the PNS.

The scope of this text is aimed towards elucidating the role the LIF plays in the CNS. It is an integral component of the IL-6 family for processes such as neuronal development, survival, and protection [14,15]. It has been demonstrated that the LIF is present in significant amounts within the cerebrospinal fluid (CSF) and brain parenchyma [16]. This suggests that it is vital for different processes that take place in the CNS. Interestingly, there are large quantities of the LIF stored and released by astrocytes in the CNS. The astrocytes increase the expression of the LIF whenever there is an event that causes neuronal damage. Although the regulatory mechanism of how the astrocytic LIF is expressed is still not clear, it confirms that the LIF is present whenever there is an event of neuronal damage [17]. This would heavily suggest that the LIF has some kind of neuroprotective capabilities. It has also been demonstrated that the LIF plays a critical role in appropriate glial responses, remyelination, and the preservation of neural conductance after mild traumatic injury to the brain [15]. This is another piece of evidence that would lend credence to the idea that the LIF exhibits neuroprotective qualities within the central nervous system. Another important aspect of this study is that it confirms that the LIF is an important element of both neurons and glial cells within the CNS. All these data confirm that the LIF is an important component of both the CNS and PNS. This is very exciting because this opens up even more doors when it comes to assessing the LIF’s candidacy as a therapeutic agent to treat different neurocognitive diseases.

The LIF also plays a critical role in the regulation of neurogenesis. It has been shown that the LIF regulates the formation of neurons in various regions of the brain, including the olfactory bulb and subventricular zone. The LIF has been found to discourage neurogenesis in these studies by acting directly on neural stem cells (NSCs) and causing them to stay in their undifferentiated state for a longer period of time [18,19]. The results suggest that the presence or absence of the LIF is a crucial factor when it comes to determining the levels of neurogenesis in the nervous system. These findings are important because they suggest that the LIF in combination with other factors could be a therapeutic option to promote regeneration in brain tissue. The LIF, along with other factors such as the ciliary neurotrophic factor and other IL-6 cytokines, plays a very important role in the regulation of neurogenesis in the CNS. All of these factors work together to increase or decrease the degree of neurogenesis depending on the needs of the CNS [20,21]. The LIF has been found to be secreted by both astrocytes and pericytes along the CNS. These two types of cells are very involved in both neurogenesis and vascular functions of the CNS [22,23]. This information serves as further confirmation of the notion that the LIF plays a critical role in many processes related to neurogenesis within the brain.

Another very important contribution that the LIF offers to the CNS is its role in cell differentiation. The LIF has been classified as a neuropoietic cytokine that regulates the differentiation of different cell types within the brain [24]. A neuropoietic cytokine is defined as one that plays a role in the control of neuronal, glial, or immune responses to an injury or disease [25]. The LIF has also been shown to promote the differentiation, survival, and proliferation of neural progenitor cells (NPCs) in vitro [26,27]. This evidence would suggest that the LIF is very involved in the early stages of the development of the CNS. The LIF has also been implicated in astrocyte differentiation within the CNS [28]. This is significant because it reveals that the LIF is involved in the differentiation of both neurons and glial cells. It is evident that the LIF is present in many different aspects and processes of all kinds throughout the brain. This is relevant because it contributes to the possibility of it being developed as a therapeutic agent for different types of neurodegenerative diseases such as HIV and Alzheimer’s syndrome.

As mentioned beforehand, the LIF is a cytokine known to have a wide range of functions in different organs such as the bones, liver, kidneys, and CNS, particularly in embryonic cells. The signaling pathways induced by this cytokine are similar among many cells, JAK/STAT 3, PI3K/AKT, and MAPK, which contribute to self-renewal, survival, and differentiation, respectively. The LIF receptor (LIFR) consists of two signaling chains, gp130 and LIFRb [29], which are intracellular domains that are associated with JAK1, a tyrosine kinase known to initiate the signaling cascade. When the extracellular LIF induces the receptor, JAK1 is subsequently transphosphorylated and activated [29]. This results in the phosphorylation of five tyrosine on each chain of the receptor, which recruits more signaling proteins like STAT 3 [30]. STAT 3 is a transcription factor proven to be the mediator of most cellular effects as it translocates to the nucleus and upregulates the transcription of the cytokine important for the self-renewal of cells [30]. One target of STAT 3 is the protein SOCS3, which negatively modulates the JAK/STAT3 and MAPK pathways. On the other hand, the pathway that induces cell survival is PI3/AKT. After the stimulation of the LIF, JAK 1 was associated with subunit p85 of phosphatidylinositol (3,4,5)-triphosphate from phosphatidylinositol (4,5)-diphosphate (PI3 kinase) [29]. This reaction leads to the activation of serine/threonine kinase (AKT), initiating other signaling cascades that ultimately allow cell survival. The MAPK pathway, usually required for differentiation, starts when the SHP2 protein is phosphorylated by JAK1, stimulating Ras/Raf signaling cascade, resulting in the activation of MAPK, a transcription factor known to promote stem cells to become organ-specific. Modulating these three pathways, the LIF stimulates a mixture of effects depending on the cell’s fate.

The effects of the LIF have been widely studied in the central nervous system because it exerts many important functions via the activation of LIFR/gp130, particularly on the development of neural stem cells and neuroglia. The LIF has been proven to stimulate the production, maturation, and survival of oligodendrocytes as well as induce the differentiation of the astrocyte progenitor cell line into GFAP+ astrocytes [29]. LIF receptor signaling provides different options for CNS development, enhancing or inhibiting the neural cells depending on the cell environment and tissue demands. Previous studies on animal models of neurodegeneration have shown that the LIF promotes the regeneration of damaged tissue as neurons increase the expression of this cytokine in response to injury [31]. Although the LIF can have different kinds of expression in our body, most notably the CNS, its role in neurogenesis has been widely studied in structures such as the primary visual cortex, olfactory receptor neurons, and the hippocampus. The LIF has been proven to have a fundamental role in enhancing the proliferation and myelinization of mature and progenitor cells of astroglia in the hippocampus, decreasing the degradation of neurons in models of inflammation and neurodegeneration [32]. This has led to the study of the LIF as a protective factor against neurocognitive disorders.

The JAK-STAT signaling pathway mediates the important role of the innate immune response against a wide range of viral infections, including HIV-1 [30]. The mechanisms of antiviral immune signaling are a complex, orchestrated process that leads to the activation of many proinflammatory cytokines like interferons (INFs), macrophages, and many other immune cells to inhibit viral replication and stimulate the humoral immune response to prevent the propagation of the infection [32]. Viruses have evolved through the years and have many strategies to evade our immune system, but previous studies have shown that the JAK-STAT pathway is one of the most relevant in the regulation of local and systemic inflammation in response to viral infections. Once the viral particles are recognized by the antigen-presenting cells, interferons are released and stimulate the phosphorylation of STAT, which starts the cytokine cascade. Many viruses antagonize the functions of STAT, allowing chronic infections to develop. One of the many viruses that cause persistent infection is HIV-1, which has been characterized to be a modulator of the JAK-STAT pathway via Vpu and Nef proteins [30], blocking STAT phosphorylation following INF-alpha stimulation, to enable its replication without the antiviral action of STAT. Other accessory proteins are Vif, Vpu, and Vpr, which play a role in the recruitment of proteins for ubiquitination and proteasomal degradation [30]. Vif has been proven to interfere with IFN-alpha signaling via the degradation of STAT 1 and STAT 3, inhibiting the inflammatory cascade and facilitating virus survival [32].

The upregulation of JAK-STAT signaling by viral infections shares similar characteristics to that used by the LIF to induce neuroprotection once cells become infected by HIV-1, for example. The LIF counteracts the neurotoxic effect of such inhibiting cytokines like Nef and Vip by modifying the STAT 3 pathway for neuronal renewal and survival instead of the detrimental effects of those proinflammatory cytokines [29]. This explains the particular interest in studying the effects of the LIF as a neuroprotective factor against the long-term sequelae of HIV manifestation in the CNS.

The neuroprotective role of the LIF has been more widely studied in recent years as a possible therapeutic agent against many diseases that have detrimental consequences on our central nervous systems, like HIV-1 infection and its associated neurocognitive disorder. Many studies have recognized the LIF as a key player in modulating the neuroimmune balance in our CNS in two main ways: stimulating neuronal renewal and survival and modulating inflammation [33]. The brain has developed mechanisms for self-protection, and upon infection or brain injury, endothelial cells have been shown to release the LIF to promote the differentiation of neuroglia [31]. This promotes brain repair by stimulating the proliferation of neural stem cells and the regeneration of damaged tissue. The LIF also has been proven to limit the demyelination of oligodendrocytes in mouse models with autoimmune diseases like encephalomyelitis [34]. Apart from the three main activating pathways that the LIF activates to promote neuroprotection (MAPK, PI3K/Akt, and JAK/STAT), studies have shown that cells treated with the LIF increased their activity of antioxidant enzymes in oligodendrocytes and neurons, decreasing the oxidative damage which leads to cell survival [31]. The leukemia inhibitor factor modulates a variety of CNS responses to inflammatory stimuli. The LIF is actively transported across the BBB, and neuroinflammation is a facilitator for the LIF to enter the brain [34]. Animal models with neuroinflammatory diseases have shown that in response to lipopolysaccharide (LPS), a proinflammatory cytokine, microvessels increased their expression of gp130, suggesting that the CNS increases the permeability of the BBB to increase the transport of the LIF to the site of injury when compared with healthy mice [34].

The neuroprotective effects of the LIF have been explored to understand if it may have the same benefits in the PNS. The discovery of modulating factors on peripheral nerve regeneration, such as nerve growth factor (NGF), set a stepping zone in knowledge about the auto capacity of peripheral nerves to heal [35]. However, the recovery of functionality after a nerve injury can be a tedious process, negatively impacting the quality of life of the affected individual. Taking this into consideration, understanding if the LIF modulates axonal regeneration after a nerve injury can open up a new perspective for future therapies. Schwann cells are the main glial cells in charge of migrating to the site of injury to begin the regeneration and repair of affected neurons [36], but the question remains if the LIF can modulate the action of Schwann cells in axonal myelination and thus speed up the recovering of nerve function [37]. A study on sciatic nerve injury in animal models showed that when the LIF knockout genes were inserted into injured peripheral nerves and the proliferation and migration of Schwann cells increased, this led to enhanced boosted myelinization and debris removal compared with elevated LIF expression in control nerves post-injury, suggesting that the LIF has an inhibitory effect on modulating the actions of glial cells in the peripheral nervous system, having an opposite effect on the neuroregeneration seen in the CNS [37]. The duality of the effect of the LIF among the CNS and PNS is a particularity of this cytokine and its capacity to exhibit different effects on different cell types and cellular statuses.

The leukemia inhibitory factor (LIF) has emerged as a multifaceted protective agent, playing pivotal roles in neuroprotection, viral replication, and ocular health. Davis et al. (2019) [38] elucidated that in a rat model of stroke, LIF’s neuroprotective effects hinged on the transcription factor myeloid zinc finger-1 (MZF-1). LIF administration enhanced MZF-1 protein levels, and its neuroprotective attributes were thwarted when MZF-1 was suppressed in vitro. On a different front, Tjernlund et al. (2007) [39] identified LIF’s capacity to inhibit HIV-1 replication by intervening with the Jak/Stat signaling pathway, a conduit HIV-1 harnesses. This interference manifests as diminished HIV-1-mediated Stat 3 phosphorylation, curtailing the virus’s replication. In the realm of ocular health, LIF’s role in neuroprotection becomes evident in its counteraction against acute ocular hypertension (AOH), a precursor to glaucoma. LIF injections in an AOH rat model remarkably ameliorated retinal damage and swelling. This restoration was accompanied by reduced retinal ganglion cell loss and diminished apoptotic markers. The molecular underpinning of this effect was traced back to the augmented activation of STAT3 and mTOR/p70S6K signaling pathways, with the significance of these pathways emphasized when their inhibition reversed LIF’s benefits [38,39].

Adding to LIF’s protective spectrum in the ocular arena, recent studies carried out by Dong et al. (2021) [40] have spotlighted its potential as a neuroprotective agent against oxidative damage in photoreceptor cone cells. An in vivo model with dark-adapted mice exposed to bright light illustrated LIF’s prowess in shielding cone cells from light-induced harm. This protective mantle is tethered to the activation of the Janus tyrosine kinase (JAK)/STAT3 signaling pathway and subsequent modulation of genes related to apoptosis and proliferation. This assertion finds further backing from in vitro experiments using 661 W cells subjected to H₂O₂. Where H₂O₂ heralded oxidative stress and apoptosis, LIF preemptively curtailed these adverse outcomes. A notable observation was the dwindling of LIF’s protective effects upon the introduction of an STAT3 inhibitor, reiterating the instrumental role of the STAT3 pathway in LIF’s neuroprotective schema. Conclusively, the data amplify LIF’s ability to counter oxidative damage in cone cells via the STAT3 pathway, augmenting its therapeutic versatility across neural, viral, and ocular contexts [40].

In the context of glaucoma, a leading cause of irreversible blindness, neuroprotection remains a vital therapeutic approach. Highlighting the potential of leukemia inhibitory factor (LIF)—an IL-6 cytokine family member known for its role in retinal neuroprotection—a recent study explored its effects on acute ocular hypertension (AOH). This model, established by elevating intraocular pressure in rat eyes, demonstrated the damaging consequences of AOH, including tissue swelling and structural retinal damage. However, post-AOH LIF injections remarkably reversed these adverse outcomes. Notably, LIF treatment significantly curtailed the loss of retinal ganglion cells (RGCs), reduced apoptosis markers, and downregulated proteins linked to cell death. This therapeutic efficacy appeared to stem from the activation of the STAT3 and mTOR/p70S6K signaling pathways. Yet, when inhibitors targeting these pathways were applied, LIF’s protective benefits were nullified. These findings underscore LIF’s potential as a promising agent for neuroprotection in glaucoma management, mediated through specific cellular pathways [41].