Inositol & its Phosphates: Basic Science to Practical Applications

Considerable evidence has accumulated indicating a significant role of dietary inositol and its phosphates, and phospholipid derivatives in human health. In a typical American diet, the amount of myo-inositol per 2500 kcal diet is approximately 900 mg, and most of the inositol compounds are phospholipid derivatives. Virtually all of the ingested myo-inositol (99.8%) is absorbed from the human gastrointestinal tract. In general a normal circulating fasting plasma myo-inositol concentration is approximately 0.03 mM and this material turns over with a half-life of 22 minutes. Contrary to the misconception held by some, inositol hexaphosphate (InsP₆) likewise is also rapidly absorbed (79 ± 10%) and distributed widely throughout the body. While there are interspecies variations, in general they all point to rather rapid absorption. Following an InsP₆-poor diet, the basal plasma value in humans is 0.07 ± 0.01 mg.L^-1 that rises to 0.26 ± 0.03 mg.L^-1 after ingestion of InsP₆-normal diet.

Water-soluble and non-soluble inositol phosphates and phosphatidylinositol are key constituents of all plants and animals, and participate in their essential metabolic processes. In animal tissues, although the dietary route is one of the three pathways to maintain inositol homeostasis, the receptor mediated salvage pathway involving IMPase 1 and a de novo biosynthetic pathway involving inositol synthase play crucial role in cell signaling and maintaining inositol homeostasis, particularly the polyphosphorylated inositol. Water-soluble inositol polyphosphates including inositol pyrophosphates (up to 60 different possible compounds and at least 37 of these have been found in nature) are extremely important biologically in cell signaling, DNA repair etc. Phosphatidylinositol is the primary source of the arachidonic acid required for biosynthesis of eicosanoids, including prostaglandins via the action of the enzyme phospholipase A2, as well as the main source of diacylglycerols that serve as signaling molecules in animal and plant cells. Phosphatidylinositol is converted to polyphosphoinositides with important signaling and other functional activities in animal cells, and involve a number of different kinases, particularly 3-phosphorylated forms.

Inositol and its phosphates are integral parts of cellular signaling. InsP₆ is plentiful in organisms as diverse as yeast, actinobacteria, mammals and plants. In the plant kingdom, InsP₆ accumulates during seed development; at the time of germination it is broken down into lower inositol phosphates and micronutrients to maintain seedling. InsP₆ is the most abundant of intracellular inositol phosphates in eukaryotes. In mammals InsP₆ maintains homeostasis, stores phosphate and acts as strong antioxidant and neurotransmitter. InsP₆ and other inositol phosphates including the pyrophosphates, play critical roles in cellular pathways involved in signal transduction, control of cell proliferation and differentiation, RNA export, DNA repair, energy transduction, ATP regeneration etc. By enabling communication between nucleus, cytoplasm, and the outside environment, InsP₆ and the other inositol phosphates play a crucial role in many aspects of cell biology.

While the quantitation of inositol in biological specimens and that of InsP₆ in food is rather simple and the methodologies are well-established, accurate quantitation of inositol phosphates, especially InsP₆ in biological specimens in vivo has not been easy, many being dogmatic about even its presence in human tissues. In spite of that, it appears that finally accurate and specific methods to quantitate and follow InsP₆ in cells, tissues, plasma, urine etc. may have been developed that might be useful in both research and clinical laboratories.

Studies have shown that InsP₆ is rapidly absorbed from the gastrointestinal tract, distributed through the plasma to various organs including the brain, and excreted from the lungs via exhaled air and through the kidneys via urine as inositol, InsP_1-5 and InsP₆. That it is distributed in brain indicates that it crosses the blood-brain barrier. Also challenging the dogma is that at least in mice, InsP₆ is distributed in the red blood cells as well. InsP₆ is rapidly taken up by malignant cells wherein it is precipitously dephosphorylated to various lower phosphates; depending on the cell types, the proportion of these lower inositol phosphates varies. Inositol is also rapidly absorbed by humans.

Myo-inositol and InsP₆ have independent anticancer action against divergent types of malignancies in a consistent and reproducible manner. The anti-cancer action is dose-dependent and inositol acts synergistically with InsP₆ to enhance the anticancer action of the latter. Since inositol and InsP₆ are common constituents of cereals and legumes, comparative study of high bran diet with equivalent amount of InsP₆ showed that while the ‘high fiber’ diet failed to reduce the incidence of experimental cancer, InsP₆ alone at equivalent dose significantly inhibited mammary cancer formation. Combined with the other studies, these data strongly suggest that supplementation with pure inositol + InsP₆ may be better than gobbling a large amount of fiber.

Towards the eventual goal of translating the cancer inhibitory effect of inositol & InsP₆ seen in experimental animals in vivo to cancer prevention and therapy in humans, studies of human cancer cells lines were performed in vitro; and human cancer cells xenotransplanted in nude mice were used to test the therapeutic potential. A consistent and reproducible broad-spectrum anticancer action of InsP₆ ± inositol was observed in various models; in human breast cancer model InsP₆ showed synergism with standard chemotherapeutic agents Adriamycin or Tamoxifen. Suppression of cancer growth was seen in xenotransplanted rhabdomyosarcoma model and regression of preexisting cancer was seen in hepatoma model. InsP₆ ± inositol also inhibited various steps in cancer metastasis, inositol potentiated the action. These findings are compelling reasons for application of InsP₆+inositol for human cancer control.

Despite compelling experimental evidence of the efficacy of InsP₆ + inositol as an anticancer cocktail, there has been very limited enthusiasm for clinical studies of cancer prevention and/or therapy. Notwithstanding this lack of interest, results from the handful of clinical studies are extremely encouraging. Myo-inositol with or without InsP₆ has shown to be effective in preventing precancerous lesions of lung cancer. Experimentally, InsP₆ acts synergistically with standard chemotherapeutic drugs such as doxorubicin and tamoxifen; emerging clinical data support that. InsP₆ + inositol available as dietary supplement has been reported to cause abrogation of breast, lung and colorectal cancers, with or without standard therapies; reduce chemotherapyinduced negative side-effects and provide a better quality of life.

Radiation is a common environmental carcinogen; humans are most often exposed to it from ultraviolet (UV) rays from the sun during sunbathing. There are other forms of radiation that we are exposed to owing to occupational or medical necessity. Radiation damage to health can be acute or chronic, the latter culminating in cancer. Uranium used in nuclear technology is also an important source of radiation. InsP₆ has shown to be a much more efficient chelator of uranium than the currently available therapy for uranium poisoning as in nuclear disasters. Inositol and InsP₆ have been proven to be protective against ultraviolet (UV) light-induced acute and chronic effects, including skin cancer.

InsP₆ and inositol independently may help in diabetes; InsP₆ by stimulating insulin secretion by the pancreatic β cells. Both inositol and InsP₆ can prevent oxidative damages to the cells and tissues inflicted through advanced glycosylated end products by acting as antioxidants. InsP₆ and inositol could replenish the low intracellular inositol since they do not need insulin to gain entry into the cell. They also have positive effect on the abnormalities of fat metabolism such as hypercholesterolemia and fatty liver.

Atherosclerotic cardiovascular disease results in serious life threatening conditions such as heart attack, stroke etc. Hypercholesterolemia is a risk factor; studies show that InsP₆ can lower serum cholesterol level. Oxidative damage seems to play an important role in the pathogenesis of atherosclerosis as well as in heart attack. Studies show that InsP₆ by virtue of its antioxidant function can prevent or ameliorate some of the deleterious sequel.

Owing to the ability to chelate calcium, InsP₆ has been studied as a preventive and therapeutic agent for abnormal calcification in various tissues such as blood vessels in atherosclerosis and skin in calcinosis cutis; kidney and salivary gland stones; prevention of dental caries and bone loss as in osteoporosis, etc. The results are very promising for wide-range practical application of InsP₆ especially against the serious debilitating problem of osteoporosis amongst the seniors, globally.

There are experimental data showing beneficial actions of inositol and inositol hexaphosphate (InsP₆) in Alzheimer’s disease and Parkinson’s disease. The different stereoisomers of inositol, myo-, epi-, scyllo- and chiro-inositol have shown efficacy in Alzheimer’s disease. The antioxidant and cation chelating properties of InsP₆ are considered to be some of the mechanisms; those of myo-inositol are not clear. Insofar as psychiatric diseases, lithium salts have been in use since the 1950s for bipolar disorder. While on one hand there is reduction of myo-inositol by lithium, myo-inositol on the other hand has been found useful in psychiatric disorders, and at least in one study of bipolar disorder. Thus, further studies of myo-inositol, and InsP₆ in the treatment of bipolar disorder are warranted. A clinical trial of InsP₆ in bipolar disorder is underway.

Sickle cell disease (SCD) is a hereditary blood disorder due to a mutation in the hemoglobin gene, and characterized by abnormal hemoglobin S (HbS). Under hypoxic conditions, HbS polymerizes, inducing distortion of RBCs into rigid sickleshaped cells; in particular, homozygous SCD patients (SS) may experience painful episodes and complications due to vaso-occlusion (VOC) during their lifetime. There is no cure, but symptomatic management to improve anemia and lower the complications through blood transfusions is currently the most accepted therapy for those patients. Chronic red blood cell (RBC) exchanges or transfusion are effective in preventing the recurrence of VOC. While simple blood transfusions are important therapy for SCD, the goal of efficient transfusion is to increase the oxygen delivery and to prevent the sickling. It has been demonstrated that InsP₆-loaded red blood cells (InsP₆-RBCs) reduce the risks of sickling of sickle RBCs (SS RBCs) exposed to hypoxia.

Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders among females. Although there is a diverse range of causes that are not entirely understood, recent studies indicate the potential role of myo-inositol in the pathophysiology of PCOS: i) insulin resistance in PCOS women can be attributed to a deficiency of myo-inositol’s intracellular metabolites, D-chiro-inositol (DCI) and inositol-phosphoglycan (IPG), mediators of insulin action; ii) PCOS patients have a higher urinary clearance of DCI; and iii) DCI functions as an intracellular messenger in mammalian oocytes, playing a role in the follicular milieu, meiotic resumption, and oocyte maturation. Recently, myo-inositol has been used in PCOS patients more and more as a natural insulin sensitizer and until today, there are more than 70 clinical trials on determining the effect of myo-inositol and/or DCI as insulin-sensitizing compound in PCOS women.

Inositol and its phosphates, specifically InsP6 have antioxidant properties, the latter is a very strong antioxidant indeed. This is brought about by chelating iron through the Fenton Reaction. The exact mechanism how inositol acts as an antioxidant is not clear; however it could be by phosphorylation to inositol phosphates. Inositol hexaphosphate-citrate, a new molecule has double the valence than InsP6; thus it offers to be an even more powerful antioxidant. Owing to the antioxidant properties, these molecules have important biological functions beneficial to human and animal health, as well as industrial and environmental application.

InsP₆ with or without inositol enhances NK cell activity and stimulates polymorphonuclear cell priming function and the macrophages to produce cytokines. InsP₆ can inhibit human immunodeficiency virus (HIV) replication; both InsP5 and InsP₆ are critical in HIV particle assembly. InsP₆ plays significant roles in mucosal protection in the gastrointestinal tract, and inflammation.

A fundamental defect in cancer cells is the abnormal and uncontrolled cell proliferation. Along with that, there is de-differentiation to an immature or primitive phenotype. InsP₆ and inositol normalize the abnormal cell proliferation rate and induce increased differentiation so that the cancer cells begin to look and behave akin to normal cells. Examples of these are shown in colon cancer, erythroleukemia and mammary cancer, and rhabdomyosarcoma cells. In colon cancer cells that produce the cancer marker ß-D-Galactose-[1→3]-N-acetyl-D-galactosamine (Gal-GalNAc), InsP₆ inhibits the expression of the marker in the intracellular mucus without suppressing the production of mucus, a normal function of colon epithelial cells.

DNA damage is one of the earliest changes in cancer formation. Within the cell and the nucleus, inositol and its phosphates are found wherein they are involved in DNA damage repair machinery, chromatin remodeling, RNA editing and mRNA transport.

Epigenetic changes such as DNA methylation, histone modifications etc., are also important in cancer formation. InsP₆ reverses the carcinogen-induced epigenetic changes in at least two different carcinogen-induced lung cancer models. InsP₆ has also been found to repress telomerase activity in prostate and brain cancers. For cancer to metastasize, the cells need access to blood vessels to disseminate. Formation of new blood vessels - angiogenesis facilitates the process. InsP₆ has been demonstrated to inhibit angiogenesis both in vitro and in vivo. This is mediated by suppression of proangiogenic factors such as vascular endothelial growth factor (VEGF).

There are emerging new uses of InsP6 in both the existing as well as in developing technologies, and in protective roles in our environment; these are in addition to the vast and wide applications in healthcare. Discussed here is its use in preservation of food and wine, improvement of the taste of wine, application in dentistry, lithium ion battery, diagnostic imaging, mass spectrometry, fuel cell and nanotechnology, etc.

Owing to the various health benefits of inositol and inositol phosphates, especially InsP₆, and their availability as nutritional supplement for human consumption, it is imperative that their safety be addressed. The safety of myo-inositol has not been an issue, however, there have been lingering doubts in some quarters about the safety of InsP₆. There had been some unsubstantiated concerns in the past regarding the intake of foods high in InsP₆ that might reduce mineral bioavailability; recent studies using extracted salts of InsP₆ have refuted those. Both inositol and InsP₆ have been listed as “generally recognized as safe” by the US FDA. There should therefore be no concern for their consumption as appropriate.