A Clinical Guide to Inositols

A growing body of research demonstrates the potential benefits of the administration of inositol isomers in the treatment of many different disorders, from reproductive to metabolic diseases.

A Clinical Guide to Inositols discusses scientific evidence of inositol-based treatments in different clinical fields to provide clinicians with a practical guide to use inositol supplementation within pathological conditions. Each chapter covers a specific disorder and describes aspects of the application of inositol in clinical practice, discussing the physio-pathologic features of the health condition and scientific evidences of the effects of inositol treatment.This book is a valuable resource to researchers and clinicians looking for a clear understanding of clinical effects of inositol supplementation and a practical guide on inositol-based treatments.

• Covers basic knowledge about the biochemistry and physiology of inositol and their pharmacological targets and metabolites
• Discusses scientific evidence of the benefits of inositol supplementation for the clinical management of different diseases
• Addresses inositol application from the gynecological and obstetrical field to, among others, the metabolic, fetal, andrological, endocrine, and oncological fields

• Language: English
• Publisher: Academic Press
• Release date: Jan 5, 2023
• ISBN: 9780323985550

Book preview

Chapter 1: Introduction to the history of inositols: A tale of scientists

Vittorio Unfera,b; John E. Nestlera,c    a The Experts Group on Inositol in Basic and Clinical Research (EGOI), Rome, Italy

b Systems Biology Group Lab, Rome, Italy

c Division of Endocrinology, Diabetes and Metabolism, Department of Internal Medicine, Virginia Commonwealth University, Richmond, VA, United States

Keywords

Inositol; Metabolism; Insulin; Glucose; Second messenger

Inositol: Early characterization in plants

Inositol is a naturally occurring cyclic polyol. Its discovery dates back to mid-1800s by Scherer, who isolated the compound from muscle tissue. Scherer named the substance myo-inosite, which in chemical language indicated a polyalcohol carbohydrate detectable in muscular fibers [1]. Years later, Maquenne extracted myo-inositol from leaves, then determined its molecular weight and its structure, and observed the absence of any reducing activity. Subsequently, he purified the compound from horse urine, demonstrating that inositol is present both in plants and in animals [2–4]. In 1919, Posternak isolated and characterized phytic acid from leaves. He found out that this compound is indeed the hexa-phosphate of myo-inositol, suggesting that inositol undergoes chemical reactions in the plants [5]. Years later, Needham standardized the purification procedures, allowing quantifications and selective isolations [6]. Posternak later determined the structure of two different inositol isomers, myo- and scyllo-inositol, discovering that natural inositol makes up a family of isomers [7]. On these bases, future investigations shed light on some of the biological features of inositols, including conversion between isomers and ratios in tissues and organs. Many of the roles of inositols in vivo are currently under investigation, and inositol physiology still represents a hot topic of research.

Inositol in animals: From metabolism to its pivotal roles in signaling processes

After optimizing the method for the purification of inositol, Needham focused his research on the physiology of inositol in animals. Through a series of elegant experiments, he demonstrated indeed that animal do synthesize inositol, independently of dietary intake. As a first step, Needham determined the content of inositol in hen's eggs, both unincubated and at the end of development. He found out that inositol concentration increased ten times in the period that goes from preincubation to hatching, concluding that an endogenous source of inositol does exist. Such source may consist either in the release of inositol phosphates from the membranes or in a de novo endogenous biosynthesis or, virtually, in both these processes. To shed light on this mechanistic puzzle, Needham designed an in vivo experiment with rats. The animals were fed with a high-salt, cyclose-free diet for eight months, in order to increase urinary excretion and deprive them of exogenous cycloses. At the end of the experiment, Needham recorded inositol excreted in the urine, concluding that animals were able to synthesize it de novo, also in a regimen of exogenous deprivation [8].

Given these findings, two main questions were still open at that time, engaging scientists in speculations and hypothesis. Which are the physiological roles of inositol? Where is inositol localized in animal organisms?

Fast-forward almost twenty years, Folch was studying cephalin, a membrane phospholipid of neurons, in cow's brain and the role of brain lipids. During his experiments, he serendipitously found out that myo-inositol-based phosphates are constituents of structural lipids, providing the first insight into their crucial role in plasma membranes, especially in the brain [9]. A few years later, the Hokins (husband and wife) discovered that inositol is not only a structural constituent of membranes, but also an active molecule in biological signaling. Indeed, they found that neurotransmitters, such as acetylcholine, induce cytosolic enrichment with inositol phosphates in the post-synaptic neurons. This finding led the Hokins to hypothesize that inositol undergoes release from the membranes and subsequently participates in intracellular signaling processes. They defined this sequence of events as PI effect [10]. The research that Ballou and Dawson carried out in the following decade added new elements to Folch's and Hokins’ findings. Specifically, Dawson first identified and purified the phospholipid phosphatidylinositol 4,5 bisphosphate, which he called tri-phosphoinositide [11], while Ballou discovered that multiple myo-inositol phosphates (including mono-, di-, and triphosphate) exist in the same tissue [12].

Years later, during the period of excitement that followed the discovery of cyclic AMP and the mechanism of second messengers, Berridge was working on calcium mobilization in blowfly salivary glands. Indeed, he was a physiologist who crossed the path of molecular biology and biochemistry, and at that time, he was studying the mechanisms that prompt the secretion of saliva through the activation of membrane ionic channels [13]. Riding the wave of enthusiasm for cyclic AMP, he tested whether this popular molecule could be responsible for the actions he observed during the activation of membrane ionic channels. Indeed, as he reported, he felt excited when he found that cyclic AMP was responsible for the potassium ion uptake, but he also felt puzzled when he observed it had no effects at all to chlorine, which Berridge knew to regulate secretion [14,15]. In those years, findings on cyclic AMP drained all the attentions of scientists, fascinated by the brand-new concept of second messengers, which unfortunately represented an exclusive name for cyclic AMP. Indeed, the greater part of scientists believed that cyclic AMP was the only second messenger molecule, and they almost ignored data indicating the possible existence of other second messengers. Hopefully pursuing his objectives, Berridge found literature evidence suggesting that Ca² + could represent the effector of saliva secretion. Then, he tested if Ca² + was responsible for the opening of chlorine channels. Performing his experiments, in the absence or in the presence of Ca² +, revealed that, as he expected, Ca² + rather than cyclic AMP was the real effector of chlorine uptake [16]. At this point, Berridge research crossed the work of Michell, who strongly believed that the PI effect detected by the Hokins was further propagated by Ca² + [17]. Berridge then started analyzing inositol in membranes, until he demonstrated that an increase in inositol release from membranes leads to higher Ca² + levels in the gland. This was the first demonstration that a compound different from cyclic AMP could regulate extracellular-to-intracellular communications [18]. At this point, Berridge only had to identify the inositol species involved in Ca² + mobilization. He was lucky enough to work a few miles down the road from Dawson's lab, one of the only two laboratories in the world that could provide inositol phosphate standards at that time.

Concurrently with Berridge's research, Irvine—working at Dawson's lab—was trying to identify the real effector of the PI effect. Indeed, he was a biochemist with a great expertise in inositides, as his main research topics included inositol derivative formation, with a special focus on the kinetics of the processes. While Berridge worked on blowfly glands, he carried his experiments on platelets, obtaining poor results as these cells display poor inositol uptake. Like Berridge during his own research, Irvine was inspired by Michell, who persistently believed that the real effector of PI effect was phosphatidylinositol 4,5-bisphosphate, a hydrophilic inositol-based head of membrane lipids. During the collaboration between Berridge and Irvine, they struggled to find the optimal cell model in which they could carry their experiments. In fact, the poor inositol uptake of platelets and the difficulty of analyzing salivary gland cells had left them with few or null ideas on how to proceed. Luckily, Berridge attended a meeting in Amsterdam where he was invited as a speaker. Schülz, the person who spoke right after him, made her report about permeabilized pancreatic cells, which Berridge guessed as the optimal model for his and Irvine's experiments [19,20]. Working on pancreatic cells was troublesome at the beginning, but the intervention of Irvine, who was able to prepare high-quality standards, led to the final demonstration they were looking for. Indeed, they observed that phosphatidylinositol 4,5-bisphosphate is cleaved to form the water-soluble inositol 1,4,5-trisphosphate, and the latter mobilizes Ca² + from the endoplasmic reticulum, activating calcium downstream effectors [21]. They demonstrated this mechanism in pancreatic cells, but the magnitude of their finding prompted the quick research on similar inositol activities in other cellular systems. The collaboration of these keen-minded academics thus first clarified inositol role in signaling. Through their knowledge in biochemistry and molecular biology, they identified inositol trisphosphate, which is today considered as one of the most important second messengers [22]. Given the huge amount of data on inositol available today, one may take this finding for granted, but it is probably the most important discovery ever made on inositol, defining its pivotal role in physiology.

After the discovery of inositol's signaling role, further evidence started emerging on this topic. While Irvine and Berridge were clarifying the role of inositol phosphates, other scientists were working on other signaling mechanisms, especially insulin. The work of Larner already focused on insulin signal mediators, and its group had already isolated the compounds believed to be involved in such signaling [23]. Despite this, it was Saltiel who first highlighted that such insulin mediators were indeed derived from an inositol glycolipid [24,25]. Along with this finding, these scientists started noting differences in insulin mediators. The group of Larner isolated two different mediators of insulin signal, which they found to be characterized by diverse activities. The first mediator activates pyruvate dehydrogenase phosphatase, explaining some of the intracellular actions of insulin, while the second mediator had a pivotal role in the inhibition of protein kinase A. Later, Larner's group characterized these two mediators, revealing that they are indeed inositol phosphoglycans [26]. The first one, inhibiting pyruvate dehydrogenase phosphatase, is composed by galactosamine and d-chiro-inositol [27]. The second one, the inhibitor of protein kinase A, is made out of myo-inositol, glucosamine, galactose, and ethanolamine [28]. Thus, for the first time, inositol stereoisomers were noted to have different activities, an evidence that will further open several debates. Through an elegant experiment with radiolabeled inositol, Larner discovered the tissue-specific conversion rates of stereoisomers [29]. Hence, Larner and colleagues identified d-chiro-inositol deficiency as a common feature of diabetic patients and observed that insulin prompts the biosynthesis of d-chiro-inositol-containing phospholipids [30]. Concomitantly, further studies by Larner in diabetic rhesus monkeys and diabetic rats pointed out the clinical potential of d-chiro-inositol as an insulin-mimetic/insulin sensitizer compound [31].

During those years, Nestler was investigating insulin effects in gynecological and hormonal contexts. He studied lipoproteins and hormones, focusing on their relations with insulin. One of the most important intuition Nestler had concerned the correlation between insulin-like growth factor I (IGF-1) and women hyperandrogenism, which he found studying the regulation of ovarian steroidogenesis [32]. He subsequently observed a similar effect through the action of insulin, finding a direct relation between hyperinsulinemia and hyperandrogenism [33]. Moreover, he found a reduction in ovarian production of estrogens following insulin treatment [34]. Working in collaboration, Larner and Nestler identified inositol glycans as mediators of the signal from insulin and related molecules that impact steroidogenesis [35]. Nestler and colleagues then demonstrated undoubtedly that the d-chiro-inositol-containing phosphoglycans transmitted the signal of insulin, leading to testosterone accumulation in the ovaries [36].

While Nestler was investigating the role of d-chiro-inositol in steroidogenesis, Chiu was approaching studies on myo-inositol. Based on the relation between myo-inositol deficiency and diabetic embryopathies, he decided to investigate whether myo-inositol could affect embryo development or not. Due to its work on in vitro fertilization (IVF), he used the sera of women undergoing IVF as the primary source of myo-inositol. He noted that sera from patients with good IVF outcomes, when added to culture media, supported a better development of mouse embryos in vitro, as they contained higher levels of myo-inositol. Nonetheless, sera of women who had abortions, although supporting embryo development as well, displayed lower quantities of myo-inositol. Moreover, he observed that in some cases, the embryo development improved after the supplementation of myo-inositol [37]. These findings prompted other research to investigate whether the relevant effects of myo-inositol were due to the content in serum or in follicular fluid. Chiu's main findings concerned the follicular fluid, which he found to impact the oocyte quality depending on the myo-inositol content. Indeed, myo-inositol content was also directly related to estradiol content in follicular fluid. Then, he proposed, and later confirmed, that myo-inositol content in follicular fluid should be considered as a marker of good oocyte quality [37].

Inositol in clinical practice: Early findings and stereoisomers

Growing evidence on the roles of inositols in physiology provided a solid rationale for testing the potential of such molecule in clinical studies. Indeed, the first clinical trials involving myo-inositol treatments had been already carried out during early 1950s by Dotti and Felch. They had pointed out the positive effect of inositol on cholesterol levels in hypercholesteremic infarct survivors and in diabetics, but the new evidence opened further scenarios [38,39]. Nestler decided to test d-chiro-inositol as an insulin-sensitizing treatment to induce ovulation in women with PCOS by removing the excess insulin stimulus on the ovary. As expectable, he demonstrated that d-chiro-inositol induced ovulation in obese, insulin-resistant women with PCOS by reducing insulin-overburdening stimulus [40]. Years later, Nestler and colleagues confirmed these promising results on ovulation induction also in lean insulin-resistant patients with PCOS [41]. The clinical research on inositol yielded great results during the following years until 2008, when the group headed by Nestler noted that higher dosages of d-chiro-inositol to women with PCOS without insulin resistance had minor or null efficacy than those previously investigated [42]. Also, in this study, the treatment with d-chiro-inositol obtained null or detrimental effects on hyperandrogenism.

Fascinated by the research of Nestler and colleagues, Unfer was becoming interested in inositol treatments during the early 2000s. Given the huge amount of data available on d-chiro-inositol, he decided to look further on a similar compound, but with different activities. Fueled by Chiu's findings, Unfer focused on the most abundant isomer in nature, myo-inositol. Collaborating with Baillargeon, a colleague of Nestler, Unfer tested for the first time the positive action of myo-inositol during IVF procedures. Interestingly, they noted that myo-inositol oral supplementation to women about to undergo IVF reduced the amount of exogenous FSH to be injected for stimulation [43]. This is a pivotal finding in IVF field, as myo-inositol thus reduces the risk of ovarian overstimulation, a burdensome side effect of FSH treatment. They also observed an increase in the number of retrievable oocytes, defining myo-inositol as a key molecule to achieve better outcomes from IVF procedures.

Unfer and Baillargeon further collaborated to another research project. Indeed, they believed that other than d-chiro-inositol, myo-inositol could induce ovulation in anovulatory patients with PCOS as well. They treated women with PCOS with myo-inositol, expecting to induce the ovulation as in the study from Nestler and colleagues. Indeed, they achieved results comparable to those from Nestler in terms of percentage of ovulating patients, also underlining another positive effect of myo-inositol treatment in women with PCOS. In fact, Unfer pointed out an optimal recovery from hirsutism, a typical feature of hyperandrogenic women with PCOS, following myo-inositol treatment [44].

From that point on, research on inositols began gaining importance, also spanning from the classical areas of interest, namely, metabolism and gynecology. This book gathers all the most important findings obtained in recent years studying inositols and their effectiveness in different clinical settings.

References

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