Evidence for circadian rhythms in the serum levels of the vitamin D-dependent calcium-binding protein and in the activity of the 25-hydroxyvitamin D3-1-alpha-hydroxylase in the chick: studies on the mode of action of calciferol

Diet-induced hypothalamic dysfunction and metabolic disease, and the therapeutic potential of polyphenols

BACKGROUND

The prevalence of obesity and metabolic diseases continues to rise globally. The increased consumption of unhealthy energy-rich diets that are high in fat and sugars results in oxidative stress and inflammation leading to hypothalamic dysfunction, which has been linked with these diseases. Conversely, diets rich in polyphenols, which are phytochemicals known for their antioxidant and anti-inflammatory properties, are associated with a reduced risk for developing metabolic diseases.

SCOPE OF REVIEW

This review provides an overview of the effects of polyphenols against diet-induced hypothalamic dysfunction with respect to neural inflammation and mitochondrial dysfunction. Results show that polyphenols ameliorate oxidative stress and inflammation within the hypothalamus, thereby improving leptin signaling and mitochondrial biogenesis. Furthermore, they protect against neurodegeneration by decreasing the production of reactive oxygen species and enhancing natural antioxidant defense systems.

MAJOR CONCLUSIONS

The potential of polyphenols as nutraceuticals against hypothalamic inflammation, mitochondrial dysfunction, and neurodegeneration could hold tremendous value. With hypothalamic inflammation increasing naturally with age, the potential to modulate these processes in order to extend longevity is exciting and warrants exploration. The continued escalation of mental health disorders, which are characterized by heightened neuronal inflammation, necessitates the furthered investigation into polyphenol therapeutic usage in this regard.

Genetic Ancestry, Skin Reflectance and Pigmentation Genotypes in Association with Serum Vitamin D Metabolite Balance

BACKGROUND

Lower serum vitamin D (25(OH)D) among individuals with African ancestry is attributed primarily to skin pigmentation. However, the influence of genetic polymorphisms controlling for skin melanin content has not been investigated. Therefore, we investigated differences in non-summer serum vitamin D metabolites according to self-reported race, genetic ancestry, skin reflectance and key pigmentation genes (SLC45A2 and SLC24A5).

MATERIALS AND METHODS

Healthy individuals reporting at least half African American or half European American heritage were frequency matched to one another on age (+/- 2 years) and sex. 176 autosomal ancestry informative markers were used to estimate genetic ancestry. Melanin index was measured by reflectance spectrometry. Serum vitamin D metabolites (25(OH)D₃, 25(OH)D ₂ and 24,25(OH)₂D₃) were determined by high performance liquid chromatography (HPLC) tandem mass spectrometry. Percent 24,25(OH)₂D₃ was calculated as a percent of the parent metabolite (25(OH)D₃). Stepwise and backward selection regression models were used to identify leading covariates.

RESULTS

Fifty African Americans and 50 European Americans participated in the study. Compared with SLC24A5 111^Thr homozygotes, individuals with the SLC24A5 111^Thr/Ala and 111^Ala/Ala genotypes had respectively lower levels of 25(OH)D₃ (23.0 and 23.8 nmol/L lower, p-dominant=0.007), and percent 24,25(OH)₂D₃ (4.1 and 5.2 percent lower, p-dominant=0.003), controlling for tanning bed use, vitamin D/fish oil supplement intake, race/ethnicity, and genetic ancestry. Results were similar with melanin index adjustment, and were not confounded by glucocorticoid, oral contraceptive, or statin use.

CONCLUSIONS

The SLC24A5 111^Ala allele was associated with lower serum vitamin 25(OH)D₃ and lower percent 24,25(OH)₂D₃, independently from melanin index and West African genetic ancestry.

Calcium in the action of growth factors

The proliferation of cells in vivo and in culture is regulated by polypeptide growth factors, such as epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). Binding of growth factors to their specific cell-surface receptors initiates a cascade of biochemical events in the cell which ultimately leads to DNA synthesis and cell division. Immediate consequences of receptor activation include tyrosine-specific protein phosphorylations, a sustained increase in cytoplasmic pH and a transient rise in cytoplasmic free Ca2+. The PDGF-induced Ca2+ signal is due to Ca2+ release from intracellular stores, whereas EGF seems to activate a voltage-independent Ca2+ channel in the plasma membrane. Monoclonal antibodies to the EGF receptor that stimulate the tyrosine-specific protein kinase fail to raise [Ca2+]i and are not mitogenic for quiescent cells. These results suggest that activation of the EGF receptor tyrosine kinase is not sufficient for the induction of a Ca2+ signal, and that the rise in [Ca2+]i is indispensable for cell proliferation.

Oscillatory behavior of control-systems of calcium homeostasis in chickens

Computer simulation of calcium homeostasis in chicks predicted an oscillatory behavior of bone calcium flow and kidney 25-hydroxyvitamin D3-1-hydroxylase with a periodicity of 56 h and a 9 h phase difference between the two signals. In growing chickens subjected to a light: dark cycle of 22:2 h, and intravenously dosed with 45Ca, the temporal changes in plasma 45Ca could be described by an exponential decline with superimposed diurnal oscillations. The activity of the renal 25-hydroxyvitamin D3-1-hydroxylase in chicks subjected to a 12:12 h light: dark cycle ALSO followed diurnal oscillations, with a nadir at the beginning of the light period and a peak 12 h later. The production of 1,25-dihydroxyvitamin D3 by primary cultures of chicken kidney cells oscillated with a periodicity of 5.6 h or shorter. It is suggested that despite the differences in phase and periodicity between the simulation predictions and actual results, the oscillations in both 1-hydroxylase and bone calcium flow could be coupled through the hormonal systems involved in regulation of plasma calcium.

Light, vitamin D and psychiatry. Role of 1,25 dihydroxyvitamin D3 (soltriol) in etiology and therapy of seasonal affective disorder and other mental processes

This is a review and a prospectus of effects of vitamin D on the brain. Effects of sunlight and equivalent artificial light on physiological and behavioral processes are probably mediated, in large part, through the skin-vitamin D-endocrine system. Experimental evidence from our laboratory reveals sites of action and concomitant direct effects of 1,25(OH)2 vitamin D3 (soltriol) on brain, spinal cord, pituitary and other endocrine tissues. This appears relevant for the activation and modulation of mental and endocrine processes, particularly related to seasonal and daily biorhythms. Effects of sunlight and corresponding artificial light are likely to be mediated through direct actions of soltriol on brain and endocrine tissues that are independent of its effect on calcium levels. Those direct actions are receptor mediated and appear to be dose related as they depend on intensity of light and length of exposure, considering light (photons) as a drug. A role for soltriol, the steroid hormone of sunlight, in the etiology and helio- or phototherapy of affective disorders with cyclic seasonal onset (seasonal affective disorder) is discussed and the significance of research in the new frontier of vitamin D and brain relationships is noted.

Calcium-dependent translocation of calbindin-D28k from intestine to blood

Calbindin-D (vitamin D-induced calcium-binding protein; CaBP) is known to be present in blood at concentrations which vary directly with levels in the intestinal mucosa. Employing a sensitive radioimmunoassay and sampling mesentery venous blood, the present experiments demonstrated a direct relationship between intestinal calcium absorption and serum CaBP. Solutions containing 150 mM NaCl and 45Ca-labeled calcium chloride (5 or 20 mM) were placed in the lumen of ligated duodenal preparations in situ and mesentery venous blood sampled with time. The concentration of absorbed 45Ca in serum was maximal at 5 min, followed by a significant increase in mesentery CaBP maximizing at 15-20 min. Elevation of serum CaBP was not observed when calcium in the dosing solution was omitted or replaced by either glucose or glycine. The possible transfer of absorbed calcium from the enterocyte to the circulation as a CaBP complex was ruled out by calculations revealing that considerably more calcium was transferred than could be accounted for by the low and high affinity binding sites on the protein. It is proposed that vitamin D-dependent enhanced transcellular calcium transport constitutes a stimulus for the increased release of intestinal CaBP into the circulation.