Stimulatory effect of insulin-like growth factor-1 on renal Pi transport and plasma 1,25-dihydroxyvitamin D3

The impact of dietary calcium and phosphorus on mitochondrial-linked gene expression in five tissues of laying hens

Mitochondria and the energy metabolism are linked to both, the availability of Ca and P to provide the eukaryotic cell with energy. Both minerals are commonly used supplements in the feed of laying hens but little is known about the relationship between the feed content, energy metabolism and genetic background. In this study, we provide a large-scaled gene expression analysis of 31 mitochondrial and nuclear encoded genes in 80 laying hens in the context of dietary P and Ca concentrations. The setup included five tissues and gene expression was analysed under four different diets of recommended and reduced Ca and P concentrations. Our study shows, that mitochondrial gene expression is reacting to a reduction in P and that an imbalance of the nutrients has a higher impact than a combined reduction. The results suggest, that both strains (Lohmann Brown and Lohmann Selected Leghorn) react in a similar way to the changes and that a reduction of both nutrients might be possible without crucial influence on the animals’ health or gene expression.

Description of a novel SLC34A3.c.671delT mutation causing hereditary hypophosphatemic rickets with hypercalciuria in two adolescent boys and response to recombinant human growth hormone

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is an autosomal recessive disorder characterized by hypophosphatemia, rickets, hyperphosphaturia, elevated 1,25(OH)₂D, and hypercalciuria. Mutations in SLC34A3, the gene encoding the sodium-dependent cotransporter NPT2c, have previously been described as a cause of HHRH. Here, we describe two male siblings with rickets and hypercalciuric nephrolithiasis born to unrelated parents, and their response to oral phosphate supplementation and growth hormone therapy. Whole exome sequencing of the oldest brother, and polymerase chain reaction and Sanger sequence analysis of the identified SLC34A3 mutations, was performed for confirmation and to evaluate his siblings and parents. Serum and urine biochemical parameters of mineral homeostasis before and after therapy were evaluated. Whole exome sequencing analysis identified a previously reported heterozygous deletion SLC34A3.g.2259-2359del101bp on the maternal allele, and a novel heterozygous single nucleotide deletion SLC34A3.c.671delT on the paternal allele of the two affected brothers. The parents and the unaffected brother are heterozygous carriers. Recombinant human growth hormone (rHGH) plus oral phosphate in one affected brother improved the renal phosphate leak and resulted in accelerated linear growth superior to that seen with oral phosphate supplementation alone in the other affected brother. Our case study is the first to demonstrate that rHGH can be considered in addition to oral supplementation with phosphorus to improve linear growth in patients with this disorder, and suggests that renal phosphate reabsorption in response to rHGH is NPT2c-independent.

The dietary protein, IGF-I, skeletal health axis

Dietary protein represents an important nutrient for bone health and thereby for the prevention of osteoporosis. Besides its role as a brick provider for building the organic matrix of skeletal tissues, dietary protein stimulates the production of the anabolic bone trophic factor IGF-I (insulin-like growth factor I). The liver is the main source of circulating IGF-I. During growth, protein undernutrition results in reduced bone mass and strength. Genetic defect impairing the production of IGF-I markedly reduces bone development in both length and width. The serum level of IGF-I markedly increases and then decreases during pubertal maturation in parallel with the change in bone growth and standing height velocity. The impact of physical activity on bone structure and strength is enhanced by increased dietary protein consumption. This synergism between these two important environmental factors can be observed in prepubertal boys, thus modifying the genetically determined bone growth trajectory. In anorexia nervosa, IGF-I is low as well as bone mineral mass. In selective protein undernutrition, there is a resistance to the exogenous bone anabolic effect of IGF-I. A series of animal experiments and human clinical trials underscore the positive effect of increased dietary intake of protein on calcium-phosphate economy and bone balance. On the contrary, the dietary protein-induced acidosis hypothesis of osteoporosis is not supported by several experimental and clinical studies. There is a direct effect of amino acids on the local production of IGF-I by osteoblastic cells. IGF-I is likely the main mediator of the positive effect of parathyroid hormone (PTH) on bone formation, thus explaining the reduction in fragility fractures as observed in PTH-treated postmenopausal women. In elderly women and men, relatively high protein intake protects against spinal and femoral bone loss. In hip fracture patients, isocaloric correction of the relatively low protein intake results in: increased IGF-I serum level, significant attenuation of postsurgical bone loss, improved muscle strength, better recovery, and shortened hospital stay. Thus, dietary protein contributes to bone health from early childhood to old age. An adequate intake of protein should be recommended in the prevention and treatment of osteoporosis.

The effect of GH treatment on serum FGF23 and Klotho in GH-deficient children

BACKGROUND

Normal phosphate homeostasis is essential for normal linear growth. The phosphaturic fibroblast growth factor 23 (FGF23)/Klotho axis is a major regulator of phosphate homeostasis; therefore, an intact FGF23/Klotho axis is important for normal linear growth. On the other hand, GH/IGF1 axis has opposing effects on phosphate homeostasis, but the underline mechanisms remain unclear.

AIM

The main objective of this study was to investigate the possible interactions of FGF23 and its co-receptor Klotho, with growth hormone (GH)/IGF1 axis in the regulation of phosphate metabolism in GH-deficient children under GH treatment.

METHODS

We studied 23 GH-deficient children, before and 3 months after the onset of GH treatment. Anthropometry and assessment of biochemical parameters were performed, as well as measurement of FGF23 (intact FGF23/iFGF23 and C-terminal FGF23/cFGF23) and soluble α-Klotho (sKlotho) levels.

RESULTS

After 3 months on GH treatment, the elevation of serum phosphate and TmPO4/GFR (P<0.0001 and P<0.01 respectively) was accompanied by a significant increase in cFGF23 (P<0.01), iFGF23 (P<0.0001), sKlotho (P<0.0001) and IGF1 (P<0.0001). Serum phosphate and TmPO4/GFR were positively associated with iFGF23 (P<0.01 and P<0.05) and IGF1 (P<0.05 and P<0.05). iFGF23 levels were positively correlated with sKlotho (P<0.001), IGF1 (P<0.0001) and height SDS (P<0.0001), whereas sKlotho was positively associated with IGF1 (P<0.0001) and height SDS (P<0.001).

CONCLUSION

The increase in serum phosphate, which we found in GH-deficient children under GH treatment, is not associated with suppression but rather than with upregulation of the phosphaturic FGF23/Klotho axis.

Hyperphosphatemia Management in Patients with Chronic Kidney Disease

Hyperphosphatemia in chronic kidney disease (CKD) patients is a potentially life altering condition that can lead to cardiovascular calcification, metabolic bone disease (renal osteodystrophy) and the development of secondary hyperparathyroidism (SHPT). It is also associated with increased prevalence of cardiovascular diseases and mortality rates. To effectively manage hyperphosphatemia in CKD patients it is important to not only consider pharmacological and nonpharmacological treatment options but also to understand the underlying physiologic pathways involved in phosphorus homoeostasis. This review will therefore provide both a background into phosphorus homoeostasis and the management of hyperphosphatemia in CKD patients. In addition, it will cover some of the most important reasons for failure to control hyperphosphatemia with emphasis on the effect of the gastric pH on phosphate binders efficiency.

Nutritional aspects of bone health

Bone mass, geometry and microstructure, and bony tissue material level properties determine bone strength, hence the resistance to fracture. At a given age, all these variables are the consequence of the amount accumulated and of the structure developed during growth, up to the so-called peak bone mass, and of the bone loss and microstructure degradation occurring later in life. Genetic factors primarily contribute to the variance of the determinants of bone strength. Nutritional intakes are environmental factors that influence both processes, either directly by modifying modelling and remodelling, or indirectly through changes in calcitropic hormone secretion and action. Some effects of nutrition on the offspring bone could take place during foetal life. There are interplays between genetic factors, nutritional intakes and physical exercise. Among the nutrients, sufficient dietary intakes of calcium and protein are necessary for bone health in childhood and adolescence as well as later in life.

Pubertal timing, bone acquisition, and risk of fracture throughout life

Pubertal maturation plays a fundamental role in bone acquisition. In retrospective epidemiological surveys in pre- and postmenopausal women, relatively later menarcheal age was associated with low bone mineral mass and increased risk of osteoporotic fracture. This association was usually ascribed to shorter time exposure to estrogen from the onset of pubertal maturation to peak bone mass attainment. Recent prospective studies in healthy children and adolescents do not corroborate the limited estrogen exposure hypothesis. In prepubertal girls who will experience later menarche, a reduced bone mineral density was observed before the onset of pubertal maturation, with no further accumulated deficit until peak bone mass attainment. In young adulthood, later menarche is associated with impaired microstructural bone components and reduced mechanical resistance. This intrinsic bone deficit can explain the fact that later menarche increases fracture risk during childhood and adolescence. In healthy individuals, both pubertal timing and bone development share several similar characteristics including wide physiological variability and strong effect of heritable factors but moderate influence of environmental determinants such as nutrition and physical activity. Several conditions modify pubertal timing and bone acquisition, a certain number of them acting in concert on both traits. Taken together, these facts should prompt the search for common genetic regulators of pubertal timing and bone acquisition. It should also open epigenetic investigation avenues to pinpoint which environmental exposure in fetal and infancy life, such as vitamin D, calcium, and/or protein supplies, influences both pubertal timing and bone acquisition.

Regulation of serum phosphate

The regulation of serum phosphate, an acknowledged risk factor for chronic kidney disease and cardiovascular mortality, is poorly understood. The discovery of fibroblast growth factor 23 (FGF23) as a key regulator of renal phosphate handling and activation of vitamin D has revolutionized our comprehension of phosphate homeostasis. Through as yet undetermined mechanisms, circulating and dietary phosphate appear to have a direct effect on FGF23 release by bone cells that, in turn, causes renal phosphate excretion and decreases intestinal phosphate absorption through a decrease in vitamin D production. Thus, the two major phosphaturic hormones, PTH and FGF23, have opposing effects on vitamin D production, placing vitamin D at the nexus of phosphate homeostasis. While our understanding of phosphate homeostasis has advanced, the factors determining regulation of serum phosphate level remain enigmatic. Diet, time of day, season, gender, age and genetics have all been identified as significant contributors to serum phosphate level. The effects of these factors on serum phosphate have major implications for what is understood as 'normal' and for studies of phosphate homeostasis and metabolism. Moreover, other hormonal mediators such as dopamine, insulin-like growth factor, and angiotensin II also affect renal handling of phosphate. How the major hormone effects on phosphate handling are regulated and how the effect of these other factors are integrated to yield the measurable serum phosphate are only now beginning to be studied.

Growth hormone, insulin-like growth factor-1, and the kidney: pathophysiological and clinical implications

Besides their growth-promoting properties, GH and IGF-1 regulate a broad spectrum of biological functions in several organs, including the kidney. This review focuses on the renal actions of GH and IGF-1, taking into account major advances in renal physiology and hormone biology made over the last 20 years, allowing us to move our understanding of GH/IGF-1 regulation of renal functions from a cellular to a molecular level. The main purpose of this review was to analyze how GH and IGF-1 regulate renal development, glomerular functions, and tubular handling of sodium, calcium, phosphate, and glucose. Whenever possible, the relative contributions, the nephronic topology, and the underlying molecular mechanisms of GH and IGF-1 actions were addressed. Beyond the physiological aspects of GH/IGF-1 action on the kidney, the review describes the impact of GH excess and deficiency on renal architecture and functions. It reports in particular new insights into the pathophysiological mechanism of body fluid retention and of changes in phospho-calcium metabolism in acromegaly as well as of the reciprocal changes in sodium, calcium, and phosphate homeostasis observed in GH deficiency. The second aim of this review was to analyze how the GH/IGF-1 axis contributes to major renal diseases such as diabetic nephropathy, renal failure, renal carcinoma, and polycystic renal disease. It summarizes the consequences of chronic renal failure and glucocorticoid therapy after renal transplantation on GH secretion and action and questions the interest of GH therapy in these conditions.

The regulation of renal phosphate transport

Renal phosphate transport is mediated by the abundance and activity of the sodium-dependent phosphate transporters, Npt2a, Npt2c, and PiT-2, present within the apical brush border membrane of the proximal tubule. Recent studies have demonstrated differential expression and activity of these sodium-dependent phosphate transporters within the proximal tubule. In general, phosphate transport is regulated by a variety of physiological stimuli, including parathyroid hormone, glucocorticoids, vitamin D3, estrogen, and thyroid hormone. Phosphatonins are now recognized as major regulators of phosphate transport activity. Other factors that affect phosphate transport include dopamine, dietary phosphate, acid-base status, lipid composition, potassium deficiency, circadian rhythm, and hypertension. Studies have shown that the PDZ-containing sodium/hydrogen exchanger regulatory factor (NHERF) proteins, specifically NHERF-1 and NHERF-3, play a critical role in the physiological regulation of phosphate transport, particularly in response to dietary phosphate. In addition, recent studies have found that NHERF-1 is also important in both the parathyroid hormone- and dopamine-mediated inhibition of phosphate transport. This review will detail the various hormones and agents involved in the regulation of phosphate transport as well as provide a brief summary of the signaling pathways and cytoskeletal proteins active in the transport of phosphate in the renal proximal tubule.

Changes in plasma FGF23 in growth hormone deficient children during rhGH therapy

BACKGROUND

Children with growth hormone deficiency (GHD) have increased renal phosphorus reabsorption during rhGH therapy, Fibroblast growth factor 23 (FGF23) is a known regulator of serum phosphorus and may be responsible for this effect.

METHODS

Prospective study in GHD children investigating changes in plasma C-terminal FGF23 (C-FGF23), markers of mineral metabolism, and insulin-like growth factor (IGF-1) in the first year of rhGH therapy. Normal stature children served as baseline controls.

RESULTS

The two groups at baseline were similar, except GHD patients had lower baseline TmP/GFR vs. controls (p < 0.05). C-FGF23 in GHD patients trended upward at follow-up 1 (p = 0.058) and significantly increased at follow-up 2 (p = 0.0005) compared to baseline. TmP/GFR also rose at follow-up 1 (p = 0.002) and follow-up 2 (p = 0.027). The C-FGF23 rise persisted after adjusting for age, gender, sex, total calcium, and phosphorus (p < 0.01) but attenuated after adjusting for TmP/GFR or IGF-1.

CONCLUSIONS

C-FGF23 rises during rhGH therapy in spite of increased Tmp/GFR, an unanticipated observation given the role of FGF23 as a phosphaturic factor. The C-FGF23 rise may be a secondary response during rhGH therapy.

Update on the pharmacogenetics of the vitamin D receptor and osteoporosis

Vitamin D and calcium are essential for normal skeletal growth and for maintaining the mechanical and structural integrity of the skeleton. Reduced intake of calcium and vitamin D may be associated with reduced bone mass and osteoporosis while a chronic and severe vitamin D deficiency may lead to osteomalacia. Given the importance of vitamin D in bone homeostasis, common polymorphisms in the vitamin D receptor gene were the first to be investigated as possible determinants of bone mass and fracture risk. Even though results are still conflicting and the molecular mechanisms by which these polymorphisms influence receptor activity remain in part to be investigated, an additional important issue is represented by their potential pharmacogenomic and pharmacogenetic implications. This review analyzes major pharmacogenetic studies of polymorphisms in the vitamin D receptor gene and osteoporosis.

Abnormalities of IGF-I signaling in the pathogenesis of diseases of the bone, brain, and fetoplacental unit in humans

IGF-I action is essential for the regulation of tissue formation and remodeling, bone growth, prenatal growth, brain development, and muscle metabolism. Cellular effects of IGF-I are mediated through the IGF-I receptor, a transmembrane tyrosine kinase that phosphorylates intracellular substrates, resulting in the activation of multiple intracellular signaling cascades. Dysregulation of IGF-I actions due to impairment in the postreceptor signaling machinery may contribute to multiple diseases in humans. This article will review current information on IGF-I signaling and illustrate recent results demonstrating how impaired IGF-I signaling and action may contribute to the pathogenesis of human diseases, including osteoporosis, neurodegenerative disorders, and reduced fetal growth in utero.

Nutrition: its role in bone health

At a given age, bone mass is determined by the amount of bone accumulated at the end of skeletal growth (the so-called peak bone mass), and by the amount of bone lost subsequently. Nutritional intake is an environmental factor that influences both bone capital accumulation, which is fully achieved by the end of the second decade of life, and bone loss, which occurs during the second half of existence. Nutrients may act directly by modifying bone turnover, or indirectly via changes in calciotropic hormone secretion. The study of the association between nutrition and a bone phenotypic expression may provide inconsistent results, in part because of the low accuracy and reproducibility of the various tools used to assess dietary intakes. Sufficient dietary calcium and protein are necessary for bone health during growth as well as in the elderly.

Acute and chronic effects of growth hormone on renal regulation of electrolyte and water homeostasis

For decades, growth hormone (GH) has been known to influence electrolyte and water handling in humans and animals. However, the molecular mechanisms underlying the GH-induced anti-natriuretic and anti-diuretic effects have remained elusive. This review will examine the existing literature on renal electrolyte and water handling following acute and chronic GH-exposure. Renal responses to GH differ in acute and chronic models. Acute application of GH results in a reduced urinary electrolyte and water excretion, whereas the chronic effects of GH are more diverse, as this state likely represents a complex mixture of primary and secondary actions of GH as well as compensatory mechanisms. During chronic GH-exposure an initial sodium retaining state often occurs, followed by a normalization of the urinary sodium excretion, although extracellular volume expansion still persists. We recently described a possible mechanism by which GH acutely increases renal electrolyte and water reabsorption, by modulation of the kidney specific Na(+), K(+), 2Cl(-) co-transporter (NKCC2). The primary aim of this review is to investigate how GH-induced regulation of NKCC2 may be involved in the complex renal changes previously described during acute and chronic GH. We propose, that the GH-induced increase in NKCC2 activity may explain the initial water and sodium retention seen in a number of studies. Moreover, renal changes seen during prolonged GH-exposure may now be seen on the background of the acute stimulation of NKCC2. Additionally, GH also promotes renal acidification, thus influencing renal acid/base handling. The GH-induced renal acidification is partly compatible with changes in NKCC2 activity. Finally, we review the available data on changes in hormonal systems affecting tubular transport during acute and chronic GH-exposure.

Calcium intake and vitamin D metabolism and action, in healthy conditions and in prostate cancer

An association between Ca intake and the risk of prostate cancer has been reported in some but not all epidemiological studies. Assuming that a pathophysiological relationship would underlie this association, a favoured hypothesis proposes that relatively high Ca consumption could promote prostate cancer by reducing the production of 1,25-dihydroxyvitamin D (1,25(OH)2D; calcitriol), the hormonal form of vitamin D. The present review analyses the plausibility of this hypothesis by considering the quantitative relationships linking Ca intake to 1,25(OH)2D production and action in healthy conditions and in prostate cancer. Changes in the plasma level of 1,25(OH)2D in response to Ca intake are of very small magnitude as compared with the variations required to influence the proliferation and differentiation of prostate cancer cells. In most studies, 1,25(OH)2D plasma level was not found to be reduced in patients with prostate cancer. The possibility that the level of 1,25(OH)2D in prostate cells is decreased with a high-Ca diet has not been documented. Furthermore, a recent randomised placebo-controlled trial did not indicate that Ca supplementation increases the relative risk of prostate cancer in men. In conclusion, the existence of a pathophysiological link between relatively high Ca intake and consequent low production and circulation level of 1,25(OH)2D that might promote the development of prostate cancer in men remains so far an hypothesis, the plausibility of which is not supported by the analysis of available clinical data.

The insulin-like growth factor-I gene and osteoporosis: a critical appraisal

Osteoporosis, a disorder of skeletal fragility, is common in the elderly, and its prevalence is increasing as more individuals with low bone mineral density (BMD), the strongest predictor of fracture risk, are detected. Previous basic and clinical studies imply there is a significant role for insulin-like growth factor-I (IGF-I) in determining BMD. Recently, polymorphisms upstream of the P1 promoter region of the human IGF-I gene have been found to be associated with serum levels of IGF-I, BMD and fracture risk in various ethnic groups. Multiple quantitative trait loci (QTLs) have been identified that underlie serum IGF-I in a mouse intercross between two inbred strains. The most promising QTL on mouse chromosome 6 has provided clues for unraveling the molecular mechanisms that regulate osteoblast differentiation. Genomic engineering resulting in IGF-I deficient mice, and mice with targeted over-expression of IGF-I reinforce the essential role of IGF-I in bone development at both the embryonic and postnatal stages. Thus, it is apparent that significant new insights into the role of the IGF-I gene in bone remodeling occur through several distinct mechanisms: (1) the skeletal IGF regulatory system; (2) the systemic growth hormone/IGF-I axis; (3) parathyroid hormone signaling; (4) sex steroids; and (5) the OPG/RANKL/RANK cytokine system. Molecular dissection of the IGF regulatory system and its signaling pathway in bone may reveal novel therapeutic targets for the treatment of osteoporosis.

Fibroblast growth factor-23 relationship to dietary phosphate and renal phosphate handling in healthy young men

The renal handling of inorganic phosphate (Pi) is controlled not only by PTH, but also by hitherto undetermined mechanisms dependent on phosphate intake. Recently, fibroblast growth factor (FGF)-23 was identified as a novel phosphaturic factor in tumor-induced osteomalacia and autosomal-dominant hypophosphatemic rickets. We hypothesized that phosphate intake could influence FGF-23 concomitantly to the changes in renal Pi handling. Twenty-nine healthy males were subjected to a 5-d low-phosphate diet and a phosphate binder, followed by a high-phosphate diet including supplements. Concomitant modifications in calcium intake allowed minimizing PTH changes in response to dietary phosphate. Serum FGF-23 levels significantly decreased on the low-phosphate diet, then increased with the oral phosphate load. Changes in FGF-23 were positively correlated with changes in 24-h urinary Pi excretion and negatively correlated with changes in the maximal tubular reabsorption of Pi and 1,25(OH)(2)D(3) (calcitriol), whereas PTH was not. In multivariate analysis, changes in FGF-23 remained the most significantly correlated to changes in 1,25(OH)(2)D(3) and maximal tubular reabsorption of Pi. Moreover, FGF-23 was positively correlated to serum osteocalcin, a marker of osteoblastic activity. In summary, FGF-23 was inversely related to renal Pi transport and serum calcitriol levels in healthy young men. These data suggest that FGF-23 may be implicated in the physiological regulation of Pi homeostasis in response to dietary phosphate changes, independent of PTH.

Long-term treatment of acromegaly with lanreotide: evidence of increased serum parathormone concentration

The somatostatin analogue lanreotide is effective in reducing growth hormone levels in patients with acromegaly. Acromegaly is characterized by calcium homeostasis alterations. The aim of our study was to evaluate the effects of lanreotide on bone turnover markers in a group of acromegalic patients and to verify a possible increase of intact parathormone (iPTH) levels in a transient or persistent way. Serum GH, IGF-I and serum and urinary markers of bone metabolism were measured before treatment and on months 3 and 24. In short-term treatment (3 months), lanreotide significantly decreased GH, IGF-I, serum calcium, osteocalcin and alkaline phosphatase levels, but increased iPTH level (49 +/- 16.7 vs pre-treatment 28.3 +/- 7.6 ng/L, p<0.001). During long-term study (24 months) GH and IGF-I were significantly still low; serum calcium and alkaline phosphatase levels returned to pre-treatment levels. iPTH level was significantly still higher compared with pre-treatment (46.4 +/- 9.2 vs 28.3 +/- 7.6 ng/L, p<0.05). No changes were seen in serum albumin, creatinine and vitamin D during short and long term treatment. The changes of most bone markers during lanreotide treatment can be explained by the decrease of GH and IGF-I. The increase of iPTH concentration suggests that lanreotide has ulterior and long-standing actions on calcium homeostasis: intestinal malabsorption of calcium due to the lanreotide could contribute to this "secondary" hyperparathyroidism. The clinical relevance of these long-standing effects needs to be further investigated.

Some Clues and Pitfalls in the Diagnosis of Acromegaly

OBJECTIVE

To describe a case of acromegaly in a young woman using oral contraceptives who had nonspecific symptoms and persistent hyperphosphatemia and to analyze the reasons for delay in diagnosis.

METHODS

A 31-year-old woman underwent evaluation for failure of menstrual flow to resume after discontinued use of oral contraceptives. Clinical and laboratory findings are summarized, and atypical manifestations of acromegaly are discussed.

RESULTS

The patient complained of weight gain, acne, and increased facial hair growth, and laboratory tests showed a low estradiol level and hyperinsulinemia. "Post-pill amenorrhea" and polycystic ovary syndrome were considered possible diagnoses. During subsequent endocrinology consultation, hyperphosphatemia was present on three occasions during a 7-month period. Hormonal studies for evaluation of pituitary function revealed increased insulin-like growth factor-I and growth hormone (GH) levels. Acromegaly was suspected, and the increased GH level failed to suppress after a glucose load. Magnetic resonance imaging revealed a pituitary macro-adenoma with suprasellar extension. Visual field testing showed bitemporal abnormalities. Transsphenoidal resection successfully removed the GH-producing tumor, and hormone replacement therapy was initiated postoperatively.

CONCLUSION

The textbook presentation of GH excess is a late feature of acromegaly. The diagnosis in a young woman can be easily missed when clinicopathologic findings are not classic and the picture is further clouded by factors such as oral contraceptive use. Features may also resemble polycystic ovary syndrome, a more common condition in women of childbearing age. This case also illustrates that hyperphosphatemia, overlooked in this patient, can precede the full-blown clinical manifestations of acromegaly and serve as a subtle clue to the underlying disease. Awareness of these situations and screening for high GH levels in patients with unexplained, persistent hyperphosphatemia are advisable for making an early correct diagnosis and providing appropriate therapy.

Longitudinal changes in maternal serum 1,25-dihydroxyvitamin D and insulin like growth factor I levels in pregnant women who developed preeclampsia: comparison with normotensive pregnant women

This study was undertaken to determine the longitudinal changes of serum 1,25-dihydroxyvitamin D (1,25-(OH)(2)D) and insulin like growth factor I (IGF-I) levels at 20.7, 27.6, and 35.5 week periods of gestation in 40 pregnant women who remained normotensive (NT) and in 10 women who developed preeclampsia (PE). As compared with the first period, significant increases (P < 0.01) in maternal serum 1,25-(OH)(2)D and IGF-I were observed in the NT group. In the PE group, a similar increase in serum 1,25-(OH)(2)D was observed. In contrast, significant (P < 0.05) lower IGF-I levels were observed in the PE group at the moment of diagnosis. In addition a high incidence of subjects with low increase in IGF-I levels (<percentile 10) was found in the PE group (30% versus 5%, P = 0.02). In conclusion, circulating levels of 1,25-(OH)(2)D were not alterated in women before they developed PE. In the opposite, the high percentage of PE women with low increase in circulating IGF-I levels between the 20th and 35th week of pregnancy suggests early alterations of IGF-I synthesis in women developing PE.

Regulation of renal NaPi-2 expression and tubular phosphate reabsorption by growth hormone in the juvenile rat

Growth hormone (GH) is an important factor in the developmental adaptation to enhance P(i) reabsorption; however, the nephron sites and mechanisms by which GH regulates renal P(i) uptake remain unclear and are the focus of the present study. Micropuncture experiments were performed after acute thyroparathyroidectomy in the presence and absence of parathyroid hormone (PTH) in adult (14- to 17-wk old), juvenile (4-wk old), and GH-suppressed juvenile male rats. While the phosphaturic effect of PTH was blunted in the juvenile rat compared with the adult, suppression of GH in the juvenile restored fractional P(i) excretion to adult levels. In the presence or absence of PTH, GH suppression in the juvenile rat caused a significant increase in the fractional P(i) delivery to the late proximal convoluted (PCT) and early distal tubule, so that delivery was not different from that in adults. These data were confirmed by P(i) uptake studies into brush-border membrane (BBM) vesicles. Immunofluorescence studies indicate increased BBM type IIa NaP(i) cotransporter (NaPi-2) expression in the juvenile compared with adult rat, and GH suppression reduced NaPi-2 expression to levels observed in the adult. GH replacement in the [N-acetyl-Tyr(1)-d-Arg(2)]-GRF-(1-29)-NH(2)-treated juveniles restored high NaPi-2 expression and P(i) uptake. Together, these novel results demonstrate that the presence of GH in the juvenile animal is crucial for the early developmental upregulation of BBM NaPi-2 and, most importantly, describe the enhanced P(i) reabsorption along the PCT and proximal straight nephron segments in the juvenile rat.

The phosphatonin pathway: New insights in phosphate homeostasis

Serum phosphate concentrations are maintained within a defined range by processes that regulate the intestinal absorption and renal excretion of inorganic phosphate. The hormones currently believed to influence these processes are parathyroid hormone (PTH) and the active metabolite of vitamin D, 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)2D). A new class of phosphate-regulating factors, collectively known as the phosphatonins, have been shown to be associated with the hypophosphatemic diseases, tumor-induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH), and autosomal-dominant hypophosphatemic rickets (ADHR). These factors, which include fibroblast growth factor 23 (FGF23) and secreted frizzled-related protein 4 (FRP4), decrease extracellular fluid phosphate concentrations by directly reducing renal phosphate reabsorption and by suppressing 1alpha,25(OH)2D formation through the inhibition of 25-hydroxyvitamin D 1alpha-hydroxylase. The role of these substances under normal or pathologic conditions is not yet clear. For example, it is unknown whether any of the phosphatonins are directly responsible for the decreased concentrations of 1alpha,25(OH)2D observed in chronic and end-stage kidney disease or whether they are induced in an attempt to correct the hyperphosphatemia seen in late stages of chronic renal failure. Future experiments should clarify their physiologic and pathologic roles in phosphate metabolism.

Lack of insulin-like growth factor I exaggerates the effect of calcium deficiency on bone accretion in mice

Recent studies provide evidence that the GH/IGF-I axis plays a critical role in the regulation of bone accretion that occurs during puberty and that the peak bone mineral density (BMD) is dependent on the amount of dietary calcium intake during the active growth phases. To evaluate whether IGF-I deficiency exaggerates the effect of calcium deficiency on bone accretion during active growth phases, IGF-I knockout (KO) and wild-type (WT) mice were fed with low calcium (0.01%) or normal calcium (0.6%) for 2 wk during the pubertal growth phase and were labeled with tetracycline. The low calcium diet caused significant decreases in endosteal bone formation parameters and a much greater increase in the resorbing surface of both the endosteum and periosteum of the tibia of IGF-I KO mice compared with WT mice. Accordingly, femur BMD measured by dual energy x-ray absorptiometry or peripheral quantitative computed tomography increased significantly in IGF-I WT mice fed the low calcium diet, but not in IGF-I KO mice. IGF-I-deficient mice fed the normal calcium diet showed elevated PTH levels, decreased serum 1,25-dihydroxyvitamin D and serum calcium levels at baseline. Serum calcium changes due to calcium deficiency were greater in IGF-I KO mice compared with WT mice. PTH levels were 7-fold higher in IGF-I KO mice fed normal calcium compared with WT mice, which was further elevated in mice fed the low calcium diet. Treatment of IGF-I-deficient lit/lit mice with GH decreased the serum PTH level by 70% (P < 0.01). Based on these and past findings, we conclude that: 1) IGF-I deficiency exaggerates the negative effects of calcium deficiency on bone accretion; and 2) IGF-I deficiency may lead to 1,25-dihydroxyvitamin D deficiency and elevated PTH levels even under normal calcium diet.

Growth hormone modulates cholecalciferol metabolism with moderate effects on intestinal mineral absorption and specific effects on bone formation in growing dogs raised on balanced food

The aim of the study was to investigate the influence of growth hormone (GH) on Vitamin D3 metabolism and the subsequent effects on calcium (Ca) homeostasis and skeletal growth in growing dogs. A group of Miniature Poodles received supraphysiological doses of GH (GH group; n = 6; 0.5 IU GH per kg body per day) from 12 to 21 weeks of age and was compared with a control placebo-treated group (n = 8). Biologic activity of GH in the GH compared to the control group was indicated by (a) the 2.5- to 3.5-fold increase in the plasma concentrations of insulin-like growth factor I (IGF-I), (b) the increased production of 1,25-dihydroxycholecalciferol as indicated by the significantly increased plasma 1,25-dihydroxycholecalciferol concentrations and the 12.9-fold increase in renal 1alpha-hydroxylase gene expression, and (c) the inhibited production of 24,25-dihydroxycholecalciferol as indicated by the significantly lower plasma 24,25-dihydroxycholecalciferol concentrations and the similar levels of renal 24-hydroxylase gene expression. Despite the distinct effects on Vitamin D(3) metabolism in the GH group, there were only moderate effects on the intestine, i.e. at 20 weeks of age there was a significant increase of 14.4 and 5.6% in fractional absorption of Ca and phosphate (Pi), respectively, compared to the control group. GH administration resulted in significantly elevated glomerular filtration rate, with no differences in Pi urine excretion as a result of a concomitant increase in the tubular reabsorption of Pi. GH had only limited disturbing effects on endochondral ossification as indicated by the maintenance of the regularity of the growth plates. However, GH had specific anabolic effects on bone formation without concomitant effect on bone resorption that may result in disorders of skeletal remodeling and manifestation of enostosis.

Acute and chronic effect of dietary phosphorus restriction on protein expression in young rat renal proximal tubules

Renal proximal tubules play a vital role in phosphorus (P) homeostasis. It is well known that dietary P restriction up-regulates the activities of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1-OHase), an enzyme that is involved in activation of vitamin D and thereby maintaining P balance. However, the mechanism involved in such regulation is not known. In the present study, we aim to identify proteins that might be involved in the renal adaptation to dietary P restriction using a proteomic approach. Renal proximal tubules were harvested from young rats fed either normal P diet or low P diet (LPD) for 1 to 7 days. Western blotting analysis of 1-OHase and signaling proteins in insulin-like growth factor I axis indicated an increase in expression of these proteins upon dietary P restriction. Using two-dimensional electrophoresis, we found that LPD reduced the total number of protein species expressed in renal proximal tubules. Differentially expressed proteins were analyzed and located using the software Melanie III, and their identities were found using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Our results showed that beta-actin, gamma-actin, major urinary protein, phosphatidylinositol transfer protein beta isoform, and G1/S-specific cyclin D3 are up-regulated and nonspecific lipid transfer protein is down-regulated by LPD.

Protein intake and bone growth

Among osteotrophic nutrients, proteins play an important role in bone development, thereby influencing peak bone mass. Consequently, protein malnutrition during development can increase the risk of osteoporosis and of fragility fracture later in life. Both animal and human studies indicate that low protein intake can be detrimental for both the acquisition of bone mass during growth and its conservation during adulthood. Low protein intake impairs both the production and action of IGF-I (Insulin-like growth factor-I). IGF-I is an essential factor for bone longitudinal growth, as it stimulates proliferation and differentiation of chondrocytes in the epiphyseal plate, and also for bone formation. It can be considered as a key factor in the adjustments of calcium-phosphate metabolism required for normal skeletal development and bone mineralization during growth. In healthy children and adolescents, a positive association between the amount of ingested proteins and bone mass gain was observed in both sexes at the level of the lumbar spine, the proximal femur and the midfemoral shaft. This association appears to be particularly significant in prepubertal children. This suggests that, like for the bone response to either the intake of calcium or weight-bearing exercise, the skeleton would be particularly responsive to the protein intake during the years preceding the onset of pubertal maturation.

Phex cDNA cloning from rat bone and studies on phex mRNA expression: tissue-specificity, age-dependency, and regulation by insulin-like growth factor (IGF) I in vivo

Phosphate regulating gene with homology to endopeptidases on the X chromosome (Phex) inactivating mutations cause X-linked hypophosphatemia (XLH). The disorder is characterized by decreased renal phosphate (Pi) reabsorption in both humans and mice, in the latter shown to be due to a reduction in mRNA and protein of type II sodium-dependent phosphate cotransporter (NadPi-II). To gain insight into the physiological role of Phex, we cloned the rat cDNA and examined tissue-specific and age-dependent mRNA expression. The rat full-length cDNA (2247 nucleotides) shares 96 and 90% identity with the mouse and human cDNA, respectively. We found 6.6 kb Phex transcripts in calvarial bone and lungs, and a weaker signal in liver of newborn rats. In adult animals, Phex mRNA signals were weaker in bone and lungs and absent in liver. Phex mRNA expression in bones and NadPi-I and -II cotransporter mRNA expression in kidney were also determined in hypophysectomized rats. These rats, which lack GH and IGF I, stop growing and exhibit decreased serum Pi levels. Treatment during 6 days with IGF I stimulated growth and increased serum Pi. Phex and NadPi-II cotransporter mRNA levels were higher in IGF I than in vehicle-treated animals, while mRNA expression of NadPi-I, 1alpha-hydroxylase and 24-hydroxylase and serum levels of calcitriol remained unaffected. Age-dependency of Phex expression suggests a role for Phex in Pi retention during growth. Moreover, our findings indicate that an increase in Phex expression in bones under the influence of IGF I may contribute to increased serum Pi by enhancing renal phosphate reabsorption. Because IGF I treatment increased NadPi-II mRNA expression and serum Pi, IGF I appears to act at least partially at pretranslational levels to increase NadPi-II mediated renal Pi retention in growing rats.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Insulin-like growth factor-I stimulates renal 1, 25-dihydroxycholecalciferol synthesis in old rats fed a low calcium diet

The adaptive increase in renal proximal tubule 25-hydroxyvitamin D-alpha-hydroxylase activity (1-OHase) during dietary calcium restriction is mediated by an increase in parathyroid hormone (PTH) and is inhibited by aging. Recent studies in mature (3-4 mo) rats demonstrated that insulin-like growth factor-I (IGF-I) restored stimulation of renal 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)] production by low phosphorus diet (LPD), another major stimulus of 1-OHase. These studies were designed to determine whether IGF-I stimulates 1-OHase during low calcium intake in old rats. Male rats were fed a normal calcium diet (NCD, 6 g Ca/kg diet) or low calcium diet (LCD, 0.2 g Ca/kg diet) for 14 d, and recombinant human IGF-I [rhIGF-I, 1.4 mg/(24h 160 kg body wt)] or vehicle was administrated via miniosmotic pump for 72 h before killing. In 4-mo-old male Sprague-Dawley rats, LCD increased in vitro renal 1-OHase activity in the presence but not in the absence of rhIGF-I. LCD increased in vitro1-OHase activity in young (1-mo-old) but not old (24-mo-old) male Fischer 344 rats. RhIGF-I increased 1-OHase activity in 24 mo-old rats fed LCD to levels that were not different from those in 1-mo-old rats fed LCD. The results indicate that the adaptive increase in 1-OHase activity due to a LCD is lost by 4 mo in rats and can be restored by pharmacologic doses of rhIGF-I.

Preeclampsia is associated with low circulating levels of insulin-like growth factor I and 1,25-dihydroxyvitamin D in maternal and umbilical cord compartments

Insulin-like growth factor I (IGF-I) stimulates renal and placental 1,25-dihydroxyvitamin D [1,25-(OH)2D] and is considered an important regulator of fetal growth. As 1,25-(OH)2D and birth weight are low in preeclampsia, this study was undertaken to determine whether circulating levels of IGF-I were associated with serum 1,25-(OH)2D concentrations in preeclamptic (PE group) and normotensive (NT group) pregnancies. Maternal and umbilical cord serum levels of IGF-I and 1,25-(OH)2D were significantly (P < 0.01) lower in the PE group than in the NT group. The concentrations of these two hormones correlated significantly in the umbilical cord (P < 0.05) and in the maternal (P < 0.001) compartments of the PE and NT groups, respectively. The amount of IGFBP-3 was 64% lower whereas that of IGFBP-1 was 2.9-fold higher in umbilical cord serum of the PE group compared with the NT group. In addition, maternal and umbilical cord serum IGF-I correlated significantly (P < 0.05) with weight and length at birth only in the PE group. In conclusion, the results of this study indicate that circulating IGF-I and 1,25-(OH)2D levels in both maternal and umbilical cord compartments are low in preeclampsia. Furthermore, this study suggests a differential regulatory effect of IGF-I on 1,25-(OH)2D synthesis and fetal growth depending on the presence or absence of preeclampsia.

[Malnutrition and osteoporosis]

Undernutrition, particularly protein undernutrition, contributes to the occurrence of osteoporotic fracture, by lowering bone mass and altering muscle strength. Furthermore, the rate of medical complications after fracture can also be increased by nutritional deficiency. The IGF-I system appears to be directly involved in the pathogenetic mechanisms leading to osteoporotic hip fracture in elderly and to its complications. In the presence of adequate calcium and vitamin D supplies, protein supplements increasing the intakes from low to normal, raises IGF-I levels, improves the clinical outcome after hip fracture, and attenuates the decrease in proximal femur bone mineral density in the year following the fracture. This nutritional approach is associated with a significant reduction of the stay in rehabilitation hospital. This underlines the importance of nutritional supports in preventing and healing osteoporotic fractures.

Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis

Severe chronic metabolic acidosis (CMA) in rats is associated with poor food intake and downregulation of growth hormone (GH), insulin-like growth factors (IGFs), and liver receptors; the administration of recombinant GH (rGH) fails to improve the growth failure. In mice with carbonic anhydrase II deficiency (CAD), a model of moderate CMA with food intake close to normal, we studied serum levels of GH, IGFs, and IGF-binding proteins, and the growth response to rGH. CAD was associated with low serum levels of GH in males. Randomized administration of rGH from approximately 5 to approximately 12 wk to CAD mice improved food efficiency and increased serum IGF-I levels, final length, and weight compared with placebo without affecting blood pH. Although administration of rGH also increased linear growth in healthy animals, the effect was less than that in CAD mice and was only observed when started before 6 wk of life. Thus growth failure in CAD mice is associated with a decrease in GH secretion in males but not in females. Long-term administration of rGH increases linear growth in CAD mice despite persistent CMA.

Serum 25-hydroxyvitamin D levels and bone mineral density in 16-20 years-old girls: lack of association

OBJECTIVE

Hypovitaminosis D has been shown to be associated with low bone mineral density in middle-aged and elderly women. The aim of this study was to evaluate whether such an association might exist in adolescent and young adult girls, approaching peak bone mass.

DESIGN

Cross-sectional study carried out in late winter.

SETTING

Reykjavik area at latitude 64 degrees N.

SUBJECTS

Two-hundred and fifty-nine Icelandic Caucasian girls, aged 16, 18 and 20 years, randomly selected from the registry of Reykjavik.

MAIN OUTCOME MEASURES

Bone mineral density in lumbar spine, hip, distal forearm and total skeleton was measured with dual-energy X-ray absorptiometry (DEXA) and compared with 25-hydroxyvitamin D levels [25 (OH)D] in serum, measured by radioimmunoassay. Calcium and vitamin-D intake were also assessed by a questionnaire.

RESULTS

18.5% of the girls were below 25 nmol L-1 in serum 25 (OH)D which has been recognized as the lower normal limit for adults. No significant association was found between 25 (OH)D levels and bone mineral density.

CONCLUSIONS

Normal calcium and phosphate concentrations in plasma and normal bone mineral density are maintained in adolescent and young adult girls at lower 25 (OH)D levels than published 'normal' levels for middle-aged and elderly.

Effect of growth hormone on renal and systemic acid-base homeostasis in humans

The effects of recombinant human growth hormone (GH, 0.1 U.kg body wt-1.12 h-1) on systemic and renal acid-base homeostasis were investigated in six normal subjects with preexisting sustained chronic metabolic acidosis, induced by NH4Cl administration (4.2 mmol.kg body wt-1.day-1). GH administration increased and maintained plasma bicarbonate concentration from 14.1 +/- 1.4 to 18.6 +/- 1.1 mmol/l (P < 0.001). The GH-induced increase in plasma bicarbonate concentration was the consequence of a significant increase in net acid excretion that was accounted for largely by an increase in renal NH+4 excretion sufficient in magnitude to override a decrease in urinary titratable acid excretion. During GH administration, urinary pH increased and correlated directly and significantly with urinary NH4+ concentration. Urinary net acid excretion rates were not different during the steady-state periods of acidosis and acidosis with GH administration. Glucocorticoid and mineralocorticoid activities increased significantly in response to acidosis and were suppressed (glucocorticoid) or decreased to control levels (mineralocorticoid) by GH. The partial correction of metabolic acidosis occurred despite GH-induced renal sodium retention (180 mmol; gain in weight of 1.8 +/- 0.2 kg, P < 0.005) and decreased glucocorticoid and mineralocorticoid activities. Thus GH (and/or insulin-like growth factor I) increased plasma bicarbonate concentration and partially corrected metabolic acidosis. This effect was generated in large part by and maintained fully by a renal mechanism (i.e., increased renal NH3 production and NH+4/net acid excretion).

Renal reabsorption of inorganic phosphorus in pregnancy in relation to the calciotropic hormones

OBJECTIVE

To measure renal reabsorption of inorganic phosphorus and the calciotropic hormones in early and late pregnancy.

DESIGN

Prospective, cross-sectional study.

SETTING

Endocrine Institute at Assaf Harofeh and E. Wolfson Medical Centers; the Department of Obstetrics and Gynaecology, Sheba Medical Centre and Tel Aviv University.

POPULATION

Three groups of healthy women were studied: pregnant women at the end of the first trimester (n = 20), pregnant women at the end of the third, trimester (n = 22), and nonpregnant controls (n = 27).

METHODS AND MAIN OUTCOME MEASURES

The renal tubular maximal phosphorus reabsorption per decilitre of glomerular filtrate (TmP/GFR) was measured in all women. Circulating levels of intact parathyroid hormone, calcitriol (1,25-dihydroxy vitamin D3) and insulin-like growth factor I were assayed in part of the women (8-11 of each group).

RESULTS

TmP/GFR was elevated in the first trimester group (mean 0.263 mmol/L) compared with controls (95% CI 0.07-0.46, P = 0.003). Third trimester values did not differ from controls. Serum calcitriol in the first trimester group was higher (mean difference 17.68 pg/mL) compared with controls (95% CI 3.89-31.47, P = 0.006) and was higher still (mean difference 20.75 pg/mL) in the third trimester group (95% CI 1.01-40.49, P = 0.042). Serum parathyroid hormone in the first trimester group was lower than in controls or the third trimester group: mean differences were 4.40 pg/mL (95% CI-1.40 to 10.15, P = 0.078) and 8.18 pg/mL (95% CI 0.51-15.85, P = 0.019) respectively. Parathyroid hormone levels correlated negatively to calcitriol levels in the combined control and first trimester groups (r = -0.54, P = 0.022) and negatively to TmP/GFR values in the combined three groups (r = -0.68, P = 0.042). First trimester levels of insulin-like growth factor I were lower than those in controls or in the third trimester: mean differences were 10.24 nmol/L (95% CI 2.05-18.43, P = 0.007) and 13.57 nmol/L (95% CI 4.23-22.91, P = 0.003), respectively.

CONCLUSIONS

The dominant change in mineral metabolism in pregnancy is a rise in calcitriol which most probably is responsible for the relative suppression of parathyroid hormone and thereby for the rise in TmP/GFR in early pregnancy. All the above support the transfer of minerals to the fetus without compromising maternal bone. The significance of circulating insulin-like growth factor I remains unclear.

Gene expression of the insulin-like growth factor system during mouse kidney development

Expression of the insulin-like growth factor (IGF) system was investigated in mouse renal development and physiology, using non radioactive in situ hybridization and quantitative RT-PCR. IGF-I mRNA levels increased after birth and were confined to distal tubules and peritubular capillaries in the outer medulla. IGF-II mRNA levels were high in developing kidneys and peaked after birth. The type I receptor mRNA expression pattern mostly parallelled those of IGF-I and IGF-II. The IGF binding proteins (IGFBP's) showed weak mRNA expression for IGFBP-1 and -6. High fetal mRNA levels were measured for IGFBP-2, showing a similar profile in time as observed for IGF-II. Low fetal IGFBP-3 and -5 mRNA levels increased after birth. IGFBP-2, -4 and -5 mRNA expression was localized to differentiating cells. In the mature kidney predominant expression was confined to proximal tubules (IGFBP-4), thin limbs of Henle's Loop (IGFBP-2), glomerular mesangial cells (IGFBP-5) and peritubular capillaries of the medulla (IGFBP-5). IGFBP-3 mRNA was exclusively expressed in endothelial cells of the renal capillary system. Distinct mRNA expression for each member of the IGF system may point to specific roles in development and physiology of the mouse kidney.

Medical management and complications of X-linked hypophosphatemic vitamin D resistant rickets

To improve the growth failure, bowed legs, and biochemical and radiological abnormalities in patients with X-linked hypophosphatemic vitamin D resistant rickets (XLH), combined therapy of phosphate and calcitriol is the best therapeutic approach at present. However, the complications involving combined therapy, such as hypercalcemia, nephrocalcinosis and hyperparathyroidism, are not fully solved. To achieve better control, new therapeutic approaches have been reported recently, for example, growth hormone (GH) or new vitamin D analogs. GH improved linear growth, decreased phosphate reabsorption and increased 1-alpha-hydroxylase activity. Furthermore, 24R,25-dihydroxyvitamin D3 (24,25) improved the bone lesions in hypophosphatemic (Hyp) mice, and also in XLH, without the adverse effects such as hypercalcemia or hypercalciuria compared with 1,25-dihydroxyvitamin D3. These new approaches should be considered for the treatment of patients with XLH.

Nutrition, genetics and skeletal development

From infancy through young adulthood the activity of bone formation predominates, resulting in a steady accumulation of bone mass. As the rate of growth changes with age, so skeletal modeling progresses through phases of different intensity with time. This is paralleled by concomitant changes in bone and calcium metabolism. Bone modeling and skeletal consolidation probably result from a complex sequence of hormonal changes in interaction with nutritional factors. However, current knowledge of the role, sequence, and genetic regulation of hormonal events during puberty, and of the response of bone tissue in interaction with nutrition is limited. This interaction is now beginning to be elucidated. The importance of this interaction with regard to fracture epidemiology in children and peak bone mass acquisition has been discussed.

Renal calcitriol synthesis and serum phosphorus in response to dietary phosphorus restriction and anabolic agents

Calcitriol [1,25-(OH)2D3] synthesis by the renal 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-hydroxylase) is induced in rats on a low phosphorus diet, but not in the hypophysectomized (HPX) or diabetic rat. However, the normal response is restored by the administration of growth hormone (GH) or insulin-like growth factor-I (IGF-I), or insulin, respectively. To further characterize this in vivo phenomenon, the acute effects of GH, IGF-I, and insulin were studied in the HPX rat. In the HPX rat the low phosphorus diet alone did not significantly alter serum phosphorus or 1alpha-hydroxylase activity, but treatment with GH resulted in a marked decrease in serum phosphorus that was associated with a fivefold induction of enzyme activity. Time course studies showed that by 6 hours after GH administration, hepatic IGF-I mRNA had increased 10-fold while renal IGF-I mRNA had increased by only 52%. Between 6 and 12 hours, serum phosphorus decreased dramatically and 1alpha-hydroxylase activity increased twofold. Treatment of phosphorus-restricted HPX rats with IGF-I resulted in a decrease in serum phosphorus by 2 hours that preceded a fourfold increase in enzyme activity between 6 and 10 hours. Treatment of phosphorus-restricted HPX rats with insulin produced similar results. This is the first demonstration of hypophosphatemia preceding induction of the 1alpha-hydroxylase after administration of IGF-I or insulin to the HPX rat on a low phosphorus diet. Although these growth factors may have a direct effect on the 1alpha-hydroxylase, these data suggest that the influence of GH, IGF-I, and insulin on transcellular phosphorus flux may have an independent effect on enzyme activity. Furthermore, the much greater induction of hepatic compared with renal IGF-I mRNA in response to GH suggests that systemic, rather than the local, IGF-I may be required for induction of the 1alpha-hydroxylase. This effect may be mediated by either the insulin or the IGF-I receptor.