Effect of Vitamin Intake on Cognitive Decline in Older Adults: Evaluation of the Evidence.

OBJECTIVES: The objective of this review was to evaluate the evidence from human studies on the intake of vitamins, either as monotherapies or in combination with other vitamins, as neuroprotective agents that may delay the onset of cognitive decline in older adults. METHODS: Evidence-based methodologies were used to capture and evaluate the highest levels of evidence. FINDINGS: The current evidence available showed no association for cognitive benefits of vitamins B6 or B12 as a monotherapy, and recent systematic reviews provide no clear evidence that supplementation with vitamin B6, B12 and/or folic acid improves dementia outcomes or slows cognitive decline, even though it may normalise homocysteine levels. Meta-analyses from systematic reviews have shown an association between low vitamin D levels and diminished cognitive function, although causality cannot be confirmed from the available evidence. There is no convincing evidence for an association of vitamin A, vitamin C or vitamin E either as a monotherapy or in combination with other antioxidant vitamins such as ß-carotene and the prevention of cognitive decline. The appraisal of nineteen systematic reviews and meta-analyses has highlighted the heterogeneity between studies, and the need for better consensus on definitions of cognitive decline, duration of testing and agreement on which specific endpoints are clinically relevant. CONCLUSIONS: Evaluation of the totality of the currently available evidence indicates that intake of the above vitamins, either as a monotherapy, or in combination with other vitamins, has no clinically-relevant effect on delaying cognitive decline or delaying the onset of dementia in older adults.

The effect of homogenization and milk fat fractions on the functionality of Mozzarella cheese.

Mozzarella cheese was manufactured from milk containing either a low (olein) or a high (stearin) melting point fraction of milk fat or anhydrous milk fat. The fat was dispersed into skim milk by homogenization at 2.6 MPa before being manufactured into cheese. The melting point of the milk fat did not affect the size or shape of the fat globules, nor was there any effect of homogenization on the polymorphic state of the milk fat. There were no changes in milk fat globule size and shape concomitant with the amount of free oil formed. The polymorphic state of the milk fat did affect the amount of free oil formed and the apparent viscosity of the cheese. The lower melting point fraction yielded a larger amount of free oil. The higher melting point fraction yielded a higher viscosity of melted cheese at 60 degrees C. Mozzarella cheese was also manufactured from homogenized milk, nonhomogenized milk, and a 1:1 ratio of the two, without altering the milk fat composition. Increasing the proportion of homogenized milk yielded a lower free oil content and higher viscosity of the cheese.

The effect of compression, stretching, and cooking temperature on free oil formation in mozzarella curd.

The effect of the extent and rate of compression and stretching on free oil formation in Mozzarella cheese curd was investigated at 55, 65, and 75 degrees C. Confocal laser scanning microscopy was used to determine the maximum cross-sectional diameter, cross-sectional area, elongation factor (maximum divided by minimum cross-sectional diameter), and circularity of fat globules in the cheese curd at the different temperatures, and after stretching or compression. Free oil was not significantly affected by the rate of biaxial compression from 50 to 2000 mm/min at 65 degrees C, the rate of tensile stretching from 1000 to 2500 mm/min at 60 degrees C, or the extent of biaxial compression from 40 to 80% of the original height at 1000 mm/min and 65 degrees C. Increasing the rate of stretching from 1000 to 2500 mm/min increased the elongation factor from 1.91 to 2.61. Cross-sectional area, maximum diameter, and circularity were not affected by the rate of biaxial compression. The extent of curd compression had no effect on the milk fat globule size and shape. Increasing the extent of stretching at 60 degrees C and at 1000 mm/min increased the free oil content (on a fat basis) from 23.8% (curd stretched by 1.4x) to 32.3% (stretched by 4.6x) and the elongation factor of the globules, but did not affect any of the other globule parameters. Increasing the temperature of the cooking-stretching water increased the cross-sectional area, diameter of the globules, and free oil content from 24.1% at 55 degrees C to 34.5% at 75 degrees C for curd compressed to 50% height at 1000 mm/min.

Molecular and cellular actions of a structural domain of human growth hormone (AOD9401) on lipid metabolism in Zucker fatty rats.

A lipolytic domain (AOD9401) of human growth hormone (hGH) which resides in the carboxyl terminus of the molecule and contains the amino acid residues 177-191, has been synthesized using solid-phase peptide synthesis techniques. AOD9401 stimulated hormone-sensitive lipase and inhibited acetyl coenzyme A carboxylase (acetyl CoA carboxylase) in isolated rat adipose tissues, in a similar manner to the actions of the intact hGH molecule. The synthetic lipolytic domain mimicked the effect of the intact growth hormone on diacylglycerol release in adipocytes. Chronic treatment of obese Zucker rats with AOD9401 for 20 days reduced the body weight gain of the animals, and the average cell size of the adipocytes of the treated animals decreased from 110 to 80 microm in diameter. Unlike hGH, synthetic AOD9401 did not induce insulin resistance or glucose intolerance in the laboratory animals after chronic treatment. The results suggest that AOD9401 has the potential to be developed into a therapeutic agent for the control of obesity.

Effect of protein kinase C modulation on gonadotrophin-induced granulosa cell steroidogenesis.

The effect of protein kinase C (PKC) modulation on gonadotrophin-induced ovarian granulosa cell differentiation was investigated by using an activator of PKC, phorbol 12-myristate 13-acetate (PMA) and inhibitors of PKC, sphingosine (SPH) and staurosporine (ST). The effects of PMA (at doses which activate PKC (10 ng mL-1), and down-regulate PKC (1000 ng mL-1)), sphingosine (25 microM) and staurosporine (10(-10)-10(-7) M) on gonadotrophin-induced granulosa cell differentiation were studied by the determination of steroidogenesis and cAMP accumulation in immature rat ovarian granulosa cells treated with or without pregnant mare serum gonadotrophin (100 mU mL-1). PMA (10 ng mL-1) inhibited gonadotrophin-induced granulosa cell steroidogenesis and cAMP accumulation. PMA (1000 ng mL-1)-induced down-regulation of PKC did not affect gonadotrophin-induced steroidogenesis. The inhibitory effect of PMA (10 ng mL-1) on gonadotrophin-induced granulosa cell steroidogenesis was not present in PKC-down-regulated cells. These data indicate that PKC activation by PMA inhibits gonadotrophin-induced steroidogenesis. SPH also inhibited gonadotrophin-induced steroidogenesis and cAMP accumulation. This effect of SPH was not affected by PMA-induced PKC down-regulation, indicating that this action of SPH does not require PKC or is mediated via a phorbol ester-insensitive PKC isoform. ST induced steroidogenesis in the absence of gonadotrophin, but was not synergistic with gonadotrophin. PMA-induced down-regulation of PKC abolished the effect of ST, suggesting that the action of ST requires PKC. The data suggest that ST and PMA, which antagonize each other in gonadotrophin-induced steroidogenesis, act via a PKC-mediated mechanism whereas the cAMP-associated actions of gonadotrophins and SPH are not dependent on PKC.

Effects of phorbol ester and staurosporine on the actions of insulin-like growth factor-I on rat ovarian granulosa cells.

Insulin-like growth factor I (IGF-I) is able to stimulate ovarian granulosa cell steroidogenesis induced by gonadotopins. This gonadotropin-induced potentiation of IGF-I action appears to be due, at least in part, to a gonadotropin-induced increase in membrane-bound IGF-I receptor number and/or decrease in extracellular IGF binding proteins (IGFBPs). Protein kinase C (PKC) has recently been reported to inhibit gonadotropin-induced steroidogenesis in rat ovarian granulosa cells. The role of PKC in the effects of IGF-I on gonadotropin action, however, is unknown. In this study, the effects of phorbol 12-myristate 13-acetate (PMA, a PKC activator) and staurosporine (ST, a PKC inhibitor) on IGF-I action were studied using immature rat ovarian granulosa cells. Activation of PKC by PMA did not affect steroidogenesis or cAMP secretion in cells treated with or without IGF-I. On the other hand, inhibition of PKC by ST alone (10(-9)-10(-7)m) led to an increase in progesterone production in a dose- and time-dependent manner without affecting cAMP secretion. In the presence, but not absence, of ST, IGF-I was able to stimulate progesterone production in the absence of any gonadotropin. PMA decreased ST-induced steroidogenesis and essentially abolished ST-potentiated IGF-I stimulation of steroidogenesis, suggesting the effects of ST on IGF-I action involved a PKC-dependent mechanism. Unlike gonadotropin, ST did not change IGF-I receptor binding. However, ST significantly decreased a major IGF binding protein (IGFBP, ∼30kDa) which is likely to be IGFBP-5, whereas it increased a minor IGFBP (∼24kDa) which is likely to be IGFBP-4. Both effects of ST were dose- and time-dependent. Furthermore, ST inhibited the expression of mRNA for IGFBP-5 suggesting that ST decreased IGFBP-5 levels by inhibiting its transcription and/or decreasing the stability of its mRNA. Interestingly, ST also decreased mRNA levels of IGFBP-4 despite a significant increase in secreted IGFBP-4 levels. The mechanisms involved are not known. Activation of PKC by PMA had no acute effect on these IGFBPs. The regulation by ST of IGFBPs was not antagonized by PMA, and was not affected by PKC-down regulation. Thus, it is likely that ST induces granulosa cell steroidogenesis, potentiates the IGF-I stimulation of steroidogenesis and regulates IGFBP via both PKC-dependent and -independent pathways.

Growth hormone receptor/binding protein: physiology and function.

Soluble truncated forms of the growth hormone receptor (GHR) are present in the circulation of many species and are also produced by many tissues/cell types. The major high-affinity forms of these GH-binding proteins (GHBP) are derived by alternative splicing of GHR mRNA in rodents, but probably by proteolytic cleavage in other species. Questions still remain with respect to the origins, native molecular form(s), physiology, and function of the GHBPs, however. The observation that GH induces dimerization of the soluble GHBP and membrane GHR, and that dimerization of GHR appears to be critical for GH bioactivity suggests that the presentation of GH to target cells, in an unbound form or as a monomeric or dimeric complex with GHBP, may have significant implications for the ability of GH to activate specific postreceptor signaling pathways (tyrosine kinase, protein kinase C, G-protein pathways) known to be utilized by GH for its diverse biological effects. This minireview addresses some of these aspects and highlights several new questions which have arisen as a result of recent advances in our understanding of the structure, function, and signaling mechanisms of the membrane bound GHR.

Postreceptor signaling mechanisms for Growth Hormone.

Recent data have shed significant new light on the mechanisms involved in the transmission of a biologic signal by GH. Following ligand-induced dimerization of the GH receptor, multiple cascades are involved in GH signaling. These include activation of nonreceptor tyrosine kinases, in particular JAK2, which is a mechanism shared by the newly described cytokine receptor superfamily. Furthermore, several classic pathways (for example, guanine-nucleotide-binding proteins and protein kinase C), shared by numerous hormones, growth factors, and neurotransmitters, are also involved in many of the actions of GH. The interrelationships between the various signaling pathways for GH have not yet been fully defined. This review briefly summarizes the current state of knowledge with respect to the processes involved in the effects of GH in target cells.

Involvement of G proteins in the effect of insulin-like growth factor I on gonadotropin-induced rat granulosa cell differentiation.

Insulin-like growth factor I (IGF-I) promotes gonadotropin-induced granulosa cell differentiation and proliferation. In order to investigate whether guanine nucleotide binding proteins (G proteins) may be linked, directly or indirectly, to some of the actions of IGF, the effects of cholera toxin (CT) and pertussis toxin (PT) on the enhancement by IGF-I of PMSG (pregnant mare serum gonadotropin)-induced rat granulosa cell differentiation have been studied. This was done by the determination of progesterone production, aromatase activity and cAMP accumulation after a 48 h incubation with PMSG, IGF-I and PMSG plus IGF-I in cells treated with either CT or PT. Both CT and PT treatment stimulated PMSG-induced progesterone production in granulosa cells after 48 h of culture with PMSG. CT treatment also stimulated aromatase activity in cells treated with PMSG and increased cAMP secretion under basal conditions (untreated cells) and in PMSG treated cells. Both CT and PT increased the stimulation by IGF-I of PMSG-induced progesterone production after 48 h of culture with PMSG plus IGF-I. Furthermore, CT augmented the enhancement by IGF-I of PMSG-induced aromatase activity and cAMP accumulation. In the absence of PMSG, CT did not increase steroidogenesis either alone or in the presence of IGF-I within the time frame studied even though CT was able to stimulate cAMP accumulation in untreated and IGF-I treated cells. These results suggest that G proteins have a role in the signalling cascade involved in gonadotropin-induced granulosa cell differentiation measured as PMSG-mediated steroidogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone inhibits activation of phosphatidylinositol phospholipase C in adipose plasma membranes: evidence for a growth hormone-induced change in G protein function.

Pituitary growth hormone (GH) functions physiologically to oppose the actions of insulin on carbohydrate and lipid metabolism by interfering with metabolic events that occur after insulin binds to its receptor. Which postreceptor effects are involved is presently unknown. Recently, we found that insulin rapidly stimulates a phosphatidylinositol phospholipase C (PI-PLC) in adipose tissue of obese (ob/ob) mice and that this effect of insulin is blocked by treatment of the animals with S-carboxymethylated human GH (RCM-hGH), a derivative having mainly anti-insulin activity. The activation of this PI-PLC by insulin is also inhibited by pertussis toxin. Thus, this study was performed to examine whether the inhibitory effect of GH on the activation of this PI-PLC is exerted at the level of signal transmission by guanine nucleotide binding proteins (G proteins). We found that the nonhydrolyzable GTP analogue, guanosine 5'-[gamma-thio]triphosphate, stimulated basal PI-PLC activity in plasma membranes of adipose tissue of saline-treated ob/ob mice, but it did not stimulate the enzyme in adipose membranes from RCM-hGH-treated mice. Also, RCM-hGH treatment markedly inhibited pertussis toxin-catalyzed ADP ribosylation of G protein alpha subunits in the membranes, suggesting some modification of the G proteins by GH. Immunoblot analysis of adipose membranes from saline- and RCM-hGH-treated mice using antiserum AS/7 (anti-Gi1 alpha and anti-Gi2 alpha) or antiserum EC/2 (anti-Gi3 alpha) showed no difference in the amount of Gi alpha-like protein between the groups. These findings suggest that GH interferes with the ability of a putative Gi-like protein to mediate the activation of PI-PLC in adipose membranes without altering the expression of the G protein.

Isolated adipocytes from growth hormone-treated obese (ob/ob) mice exhibit insulin resistance.

The genetically obese (ob/ob) mouse is a useful model for the study of the diabetogenic action of growth hormone (GH), because treatment of these animals with GH results in decreased responsiveness of their adipose tissue to insulin in vitro. Studies of the mechanisms involved in GH-induced insulin resistance using isolated adipocytes of ob/ob mice have not been possible, however, because of their extreme fragility and the lack of an adequate system for the maintenance of these cells. This study describes a new method for the isolation of ob/ob mouse adipocytes. The isolated cells are stable, viable and metabolically responsive to insulin. In addition, these adipocytes have been maintained in primary culture, in serum-free medium, for up to 3 days. During culture, the cells exhibit large increases in 125I-hGH binding (10-20-fold) and porcine 125I-insulin binding (5-10-fold). The induction of insulin resistance by GH has also been demonstrated in these freshly isolated ob/ob mouse adipocytes. The studies to date indicate that the ob/ob mouse adipocyte system should provide a useful model for detailed studies of the cellular and molecular mechanisms of GH induced insulin resistance.

Intracellular processing of growth hormone receptors by adipocytes in primary culture.

Rat adipocytes in primary culture have been used to study the intracellular processing of growth hormone (GH) receptors. Pretreatment of adipocytes with 20 micrograms/ml cycloheximide resulted in a rapid decline (t1/2 approximately 45 min) of the 125I-human growth hormone (hGH) binding capacity of the cells. This decline occurred at a faster rate in the presence of extracellular unlabeled hGH (400 ng/ml) and was not due to receptor occupancy. These data suggest that GH receptors turn over rapidly and constitutively on the plasma membrane and in the absence of protein synthesis are not replaced. Dissociation of GH-receptor complexes was shown not to occur at pH 5.5, the pH encountered in the acidic pre-lysosomal compartments (endosomes) where intracellular dissociation of many hormone-receptor complexes takes place. These data, together, suggest that the majority of GH receptors are not recycled but instead suffer the same fate as the majority of GH, i.e. degradation. To determine the rate of appearance of GH receptors at the cell surface, adipocytes were first treated with trypsin and then incubated at 37 degrees C to permit incorporation of any available GH receptors into the plasma membrane. Binding of 125I-hGH recovered to pre-trypsin levels by 2 h. This recovery was completely blocked by concomitant treatment with monensin, cytochalasin B, colchicine and 2,4-dinitrophenol. NH4Cl had no effect on receptor recovery. These data suggest that once GH receptors are synthesized in the rough endoplasmic reticulum, they travel via the Golgi apparatus to the plasma membrane (by processes involving both microfilaments and microtubules) and are then inserted into the plasma membrane in an energy-dependent step.

Processing of growth hormone by rat adipocytes in primary culture: differentiation between release of intact hormone and degradative processing.

Rat adipocytes in primary culture have been used to study the intracellular processing of GH. These classic target cells for GH have been shown to process GH through two pathways: a nondegradative pathway which resulted in the rapid release of intact GH, and a slower, degradative pathway which involved the degradation of GH and release of degraded ligand. Differentiation between the two pathways was on the basis of differences in their kinetics and temperature dependence. The present study has investigated the relative characteristics of the two pathways further. Incubation of [125I]human GH ([125I]hGH)-preloaded adipocytes with extracellular unlabeled hGH (400 ng/ml) resulted in an increase in the absolute amount of [125I]hGH released. The increased amount of [125I]hGH released was all intact. Extracellular, unlabeled hGH had no effect on the rate or amount of degraded [125I]hGH released. This suggests that the nondegradative pathway is sensitive to the number of internalized hormone-receptor complexes and that GH which is not immediately degraded or stored in the degradative pathway, is redirected and processed via the faster non-degradative pathway. Ammonium chloride (known to inhibit the lysosomal degradation of many polypeptide hormones) markedly inhibited the absolute amount of [125I]hGH released from preloaded adipocytes. This inhibition was due to an effect on the release of degraded [125I]hGH. NH4Cl had no effect on the rate or amount of intact [125I]hGH released. Finally, it was found that dinitrophenol and sodium fluoride (agents known to deplete cellular energy) inhibited the release of degraded GH but not intact GH suggesting that the degradative pathway involves an energy-dependent step, most likely the fusion of hormone-containing vesicles with the lysosomal membrane. The mechanism of release of intact hormone by energy-independent means is not yet known. These data indicate that the processing of GH by cultured rat adipocytes is complex and involves at least two independently regulated pathways, a predominant degradative route and a nondegradative route. Further studies are required to assess the possible roles of these pathways in the metabolic actions of GH in adipocytes.

Receptor-mediated endocytosis and degradative processing of growth hormone by rat adipocytes in primary culture.

At 37 degrees C, cultured rat adipocytes bound [125I]human GH ([125I]hGH) rapidly, with binding being detectable within 1 min of incubation. The bound [125I]hGH was then internalized (within 10 min) and accumulated in the cell interior until a steady state was reached (by 60 min). At this time, where the rates of GH internalization, processing, and release are equivalent, 55% of total cell-associated [125I]hGH was intracellular. Internalization of [125I]hGH by acutely isolated (noncultured) adipocytes was preceded by a 20-min lag phase indicative of a temporary postbinding defect. The lag phase was not seen with cultured adipocytes. After preloading of [125I]hGH into the cell interior, cultured cells rapidly released [125I]hGH (t1/2 = 20-30 min) into the extracellular medium as both intact (25%) and degraded (75%) GH. The release of intact vs. degraded GH was distinguishable on the basis of kinetics and temperature dependence. In order to determine when internalized [125I]hGH entered a catabolic compartment, cultured adipocytes were incubated with [125I]hGH and the composition of intracellular GH was determined as a function of time. All [125I]hGH internalized during the first 20 min was intact. Between 20 and 30 min some of the internalized [125I]hGH entered a catabolic compartment and degradation products began accumulating within the adipocytes. Release of degraded [125I]hGH from cultured adipocytes began at 60 min. The processing of GH through the complete degradative pathway (binding, internalization, degradation, release) required a period of 1 h at 37 degrees C.

Growth hormone receptors in cultured adipocytes: a model to study receptor regulation.

Acutely isolated rat adipocytes have been maintained in primary culture for several days and the effects of culture on the kinetics of 125I-human growth hormone (hGH) binding to adipocytes have been determined. A marked increase (500-1000%) in specific binding of 125I-hGH was observed over the first 3 days of culture--acutely isolated adipocytes (5.5 +/- 1.4%, mean +/- SE, n = 47) compared to 3-day cultured adipocytes (48 +/- 7%, mean +/- SE, n = 8). Specific binding of 125I-hGH to both acutely isolated and cultured adipocytes was dependent on incubation time and temperature (equilibrium being reached in 1 h at 37 degrees C and 2 h at 22 degrees C). Binding was reversible (t1/2 approximately 1.5 h). Scatchard analysis revealed linear plots and showed that the increase in binding during culture was due to an increase in the number of receptors per cell (approximately 20 000 to approximately 170 000) with little or no change in binding affinity (Ka approximately 1 X 10(9) M-1). Cycloheximide inhibited the increase in binding sites during culture suggesting a requirement for de novo protein synthesis. Addition of unlabelled hGH to the culture medium resulted in a marked down-regulation of the GH receptor by 2 days. The GH-induced decrease in receptor number was to due to receptor occupancy by exogenously added GH. The studies to date indicate that the cultured rat adipocyte should provide a useful model for a comprehensive study of the cellular mechanisms and dynamics of GH receptor regulation.

Growth hormone-binding proteins in high-speed cytosols of multiple tissues of the rabbit.

Soluble, specific binding protein(s) for growth hormone (GH) have been identified and partially characterized in high-speed cytosolic preparations from a number of rabbit tissues. The binding of 125I-labelled human GH to proteins in liver, heart, adipose tissue, skeletal muscle and kidney cytosols was dependent on time and cytosolic protein concentration. By Scatchard analysis, the binding affinities (KA: (2-7) X 10(9) M-1) were somewhat higher than those generally reported for membrane GH receptors. The binding proteins had a greater specificity for somatotrophic hormones than lactogenic hormones, although the kidney appeared to have, in addition, a lactogen-binding protein. By gel filtration, the Mr of the cytosolic GH-binding protein was approximately 100 000 in all tissues. None of the binding proteins was detectable by the poly(ethylene glycol) precipitation method used widely for soluble hormone receptors. The cytosolic GH-binding proteins also cross-reacted with a monoclonal antibody to the rabbit liver membrane GH receptor. These results indicate the ubiquitous presence of apparently naturally soluble GH-binding proteins in the cytosolic fractions of several tissues in the rabbit. Of great interest is their presence in muscle, where GH receptors or binding proteins have not previously been detected, despite muscle being recognized as a classical GH target tissue.

Determination of oxidative deterioration of milk powder and reconstituted milk by measurement of chemiluminescence.

A scintillation counter was used to measure chemiluminescence (CL) from milk powder and reconstituted milk. Only a small amount of CL, just above the background level, was observed from powders, but reconstituted milks emitted easily measurable CL. CL from milk was stimulated by exposure to light and found to decline gradually but non-exponentially after the exposure ceased. CL was at a maximum immediately after the milk was reconstituted from milk powder. Without light exposure, CL then declined gradually but non-exponentially over several hours to reach a constant value related to the level of oxidation of the powder. When determined under standard conditions 60 min after reconstitution, CL of milk reconstituted from stored milk powders correlated well with peroxide values and oxidized flavour as assessed by a taste panel.