Evidence suggesting that the direct growth-promoting effect of growth hormone on cartilage in vivo is mediated by local production of somatomedin

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous mediators of growth

In the present review it is not possible to discuss the effects of the numerous endogenous mediators of growth. What we have attempted to do is to indicate the areas of controversy and the need for further research. In our view, four main questions arise. First, what are the relative contributions of the direct and indirect effects of GH? Indeed, if GH can produce all its effects by local production of IGF, is the original somatomedin hypothesis still tenable? Second, how is the biological activity of the IGF modified by the presence of binding proteins? Because of the role of binding proteins in modulating IGF bioactivity, care must be taken when interpreting results from immunoassays for IGF because this will only represent the concentration of IGF not the level of biological activity, a situation which is analogous to that which pertains with certain polypeptide hormones (for review, see Robertson et al. 1987). Third, how are the activities of the osteoblast and osteoclast coupled so that in the mature adult, bone formation and bone resorption are roughly equivalent? Understanding of this process will undoubtedly involve the elucidation of the roles and interactions between a number of locally acting growth factors and systemic hormones and will lead to the understanding of certain metabolic bone diseases such as osteoporosis. Last, how is the response to local stimuli such as mechanical stress transduced? This is again probably dependent on the activity of local growth factors but may also involve changes in the interactions between bone cells and their underlying matrix components (Skerry et al. 1988).

Bovine growth hormone: human food safety evaluation

Scientists in the Food and Drug Administration (FDA), after reviewing the scientific literature and evaluating studies conducted by pharmaceutical companies, have concluded that the use of recombinant bovine growth hormone (rbGH) in dairy cattle presents no increased health risk to consumers. Bovine GH is not biologically active in humans, and oral toxicity studies have demonstrated that rbGH is not orally active in rats, a species responsive to parenterally administered bGH. Recombinant bGH treatment produces an increase in the concentration of insulin-like growth factor-I (IGF-I) in cow's milk. However, oral toxicity studies have shown that bovine IGF-I lacks oral activity in rats. Additionally, the concentration of IGF-I in milk of rbGH-treated cows is within the normal physiological range found in human breast milk, and IGF-I is denatured under conditions used to process cow's milk for infant formula. On the basis of estimates of the amount of protein absorbed intact in humans and the concentration of IGF-I in cow's milk during rbGH treatment, biologically significant levels of intact IGF-I would not be absorbed.

A short course of recombinant human growth hormone treatment stimulates osteoblasts and activates bone remodeling in normal human volunteers

The effects of recombinant human growth hormone (rhGH) on biochemical markers of bone turnover and bone mineral content (BMC) were investigated in 20 normal male volunteers (aged 22-31 years) randomized to treatment for 7 days with either rhGH (0.1 IU/kg subcutaneously twice a day) or placebo. Serum somatomedin C rose during treatment (p less than 0.001) but was not significantly different from baseline at day 14. The fasting urinary hydroxyproline/creatinine (p less than 0.001) and calcium/creatinine ratios (p less than 0.01) increased during treatment and remained elevated for 4 and 2 weeks, respectively. Serum bone gamma-carboxyglutamic acid-containing protein (BGP) increased during treatment (p less than 0.001) and remained elevated for 6 months (p less than 0.02). Serum bone alkaline phosphatase (B-AP), after an initial fall in the treatment period (p less than 0.001), increased slightly in the following months (p less than 0.01). In the rhGH group BMC was significantly higher than the prestudy value at day 14 (p less than 0.05) but was unaltered at the end of study. The simultaneous increase in markers of bone resorption and formation during rhGH treatment followed by a decline in resorption parameters within a few weeks and the prolonged effect on BGP and B-AP demonstrate that rhGH treatment stimulates osteoblasts and activates bone remodeling.

Condensation of hypertrophic chondrocytes at the chondro-osseous junction of growth plate cartilage in Yucatan swine: relationship to long bone growth

Chondrocytes of the cartilaginous growth plate are found in a spatial gradient of cellular differentiation beginning with cellular proliferation and ending with cellular hypertrophy. Although it is recognized that both proliferation and hypertrophy contribute significantly to overall bone growth, mechanisms acting on the chondrocyte to control the timing, the rate, and the extent of hypertrophy are poorly understood. Similarly, mechanisms acting on the terminal chondrocyte to cause its death at the chondro-osseous junction have not been investigated. In this study we examine the condensation of terminal hypertrophic chondrocytes in proximal and distal radial growth plates of Yucatan swine at 4 weeks of age. The animals were raised in a controlled environment where activity and feeding patterns were synchronized to a given time in the light/dark cycle. We analyzed cellular condensation both as a function of circadian rhythms in a 24-hr time period, and as a function of overall rate of growth. The data suggest that the magnitude of circadian influences on long bone growth is significantly damped at the level of the hypertrophic chondrocyte compared to that seen by previous investigators studying circadian influences on chondrocytic proliferation. Secondly, the condensation of hypertrophic chondrocytes at the chondro-osseous junction varies inversely with rate of growth in length of the bone. At any time period, a higher percentage of terminal chondrocytes in the condensed form was found in the slower-growing of the two growth plates. We relate these findings to current hypotheses concerning controls of chondrocytic hypertrophy and possible controls over the timing of hypertrophic cell death.

Regulation by fasting of rat insulin-like growth factor I and its receptor. Effects on gene expression and binding

We have examined, in liver and extrahepatic tissues, the effects of fasting on total insulin-like growth factor I (IGF-I) mRNA levels, on levels of different IGF-I mRNAs generated by alternative splicing of the primary IGF-I transcript, and on IGF-I receptor binding and mRNA levels. A 48-h fast decreased total IGF-I mRNA levels by approximately 80% in lung and liver, approximately 60% in kidney and muscle, and only approximately 30-40% in stomach, brain, and testes. In heart, IGF-I mRNA levels did not change. The levels of the different splicing variants, however, were essentially coordinately regulated within a given tissue. Specific 125I-IGF-I binding in lung, testes, stomach, kidney, and heart was increased by fasting by approximately 30-100%, whereas in brain 125I-IGF-I binding did not change in response to fasting. In tissues in which fasting increased IGF-I receptor number, receptor mRNA levels increased approximately 1.6- to 2.5-fold, whereas when IGF-I receptor number was unchanged in response to fasting, receptor mRNA levels did not change. These data demonstrate that the change in IGF-I and IGF-I receptor mRNA levels during fasting is quantitatively different in different tissues and suggest that regulation of IGF-I and IGF-I receptor gene expression by fasting is discoordinate.

Growth hormone stimulates c-fos gene expression by means of protein kinase C without increasing inositol lipid turnover

Growth hormone (GH) is required for the terminal differentiation of preadipose Ob1771 cells that have entered the differentiation program as evidenced by the expression of early marker genes (pOb24 and lipoprotein lipase). Induction of c-fos mRNA within 15 min and induction of insulin-like growth factor I mRNA within a few hours take place in response to GH. The role of GH is mediated, at least in part, by means of the activation of protein kinase C, as shown by the inhibition of epidermal growth factor binding and by the expression of the c-fos gene, and is thus analogous to the action of prostaglandin F2 alpha and 4 beta-phorbol-12,13-didecanoate in this respect. However, in contrast to that of the c-fos gene, the regulation of insulin-like growth factor I gene expression by GH is not mediated by means of the activation of protein kinase C, and, in line with this, prostaglandin F2 alpha and 4 beta-phorbol-12,13-didecanoate were ineffective. GH and prostaglandin F2 alpha were able to stimulate the formation of diacyglycerol within a few seconds, but GH did not elicit an accumulation of inositol phosphates, in contrast to that generated by prostaglandin F2 alpha. We conclude that the transduction signal of GH action in c-fos mRNA induction is the formation of diacylglycerol and that the mechanism whereby GH can activate protein kinase C is associated with a phospholipase C-mediated hydrolysis of glycerophospholipids other than inositol phospholipids.

Effect of bovine somatotropin on the distribution of immunoreactive insulin-like growth factor-I in lactating bovine mammary tissue

The distribution pattern of immunoreactive insulin-like growth factor-I in normal lactating bovine mammary tissue and in tissue obtained after bovine somatotropin treatment was determined by indirect immunofluorescence. In normal tissue, insulin-like growth factor-I immunoreactivity was observed almost exclusively associated with stromal elements. Intralobular stromal cells, small blood vessels, and capillaries all expressed moderate to high immunoreactivity. In contrast, mammary epithelial cells displayed only sparse cytoplasmic immunoreactivity. Immunoreactive material was also present in the periductular connective tissue area, possibly associated with the basal plasma membrane of epithelial cells. Somatotropin treatment of animals resulted in elevated serum insulin-like growth factor-I concentrations and altered the distribution of insulin-like growth factor-I-stainable material in mammary tissue. After somatotropin treatment, immunoreactivity was still detected in mammary stroma; however, prominent staining was also observed in the cytoplasm of mammary epithelial cells. Given the possible role of insulin-like growth factor-I in the regulation of bovine mammary epithelial cell growth and function, our findings raise the possibility that somatotropin may induce insulin-like growth factor-I production in mammary tissue, or other tissues, to influence indirectly the growth or function of the epithelial cells. This offers a possible mechanism for bovine somatotropin stimulation of lactation.

Progressive glomerulosclerosis develops in transgenic mice chronically expressing growth hormone and growth hormone releasing factor but not in those expressing insulinlike growth factor-1

An increase in glomerular size occurs in normal maturation after subtotal renal ablation and disease states such as diabetes mellitus. The role that growth hormone (GH), growth hormone releasing factor (GHRF), and insulinlike growth factor-1 (IGF-1) play in these processes has been investigated using transgenic mice chronically expressing these hormones. The glomeruli were enlarged in all 3 strains of mice. Mesangial proliferation followed by progressive glomerulosclerosis was observed in the GH and GHRF animals only. In the IGF-1 mice the large glomeruli remained morphologically normal except for the enlargement. These data suggest that the glomerulosclerosis was due, in part, to disordered mesangial cell growth in response to circulating GH. The mesangial lesions in mice with chronically high plasma GH levels mimicked those in human diabetes mellitus. These models provide a means to study the hormonal regulation of glomerular growth and the role that specific hormones might play in the pathogenesis of glomerulosclerosis.

Receptors for and effects of insulin and IGF-I in rat glomerular mesangial cells

Receptors for and biological effects of insulin and insulin-like growth factor I (IGF-I) were studied in cultured rat renal mesangial cells. Specific binding of 125I-IGF was over 200-fold greater (5.8%/0.2 mg cell protein) than the specific binding of 125I-insulin (0.2%/2 mg cell protein). Fifty percent inhibition of 125I-insulin binding was obtained with 8 x 10(-9) M unlabeled insulin. For 125I-IGF-I, 50% inhibition required 1.8 x 10(-9) M unlabeled IGF-I. 125I-IGF-I was also displaced by IGF-II and insulin but at 10-and 100-fold lower potencies, respectively, than IGF-I. Cross-linking of 125I-insulin and 125I-IGF-I to their receptors, using disuccinimidyl suberate (DSS), and identification of the receptor with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography showed a band with a molecular mass of 135 kDa, probably corresponding to the alpha-subunit of the insulin receptor and a major band with a molecular mass of 145 kDa for the alpha-subunit of the IGF-I receptor. Both insulin and IGF-I stimulated the incorporation of [3H]thymidine into DNA. A half-maximal effect was obtained at 1.6 x 10(-8) M for insulin and 1.2 x 10(-9) M for IGF-I. No additive effect on DNA synthesis was observed. Insulin at 8 x 10(-10) M increased the accumulation of [14C]glucose in mesangial cells, whereas IGF-I was 10-fold less potent.

The relationship between height velocity and growth hormone secretion in short prepubertal children

We have performed 24 h growth hormone (GH) profiles in 50 short prepubertal children aged between 5.2 and 12.9 years, growing with height velocity standard deviation scores (SDS) between 0.4 and -3.9. There was an asymptotic relationship between height velocity and spontaneous GH secretion described by the equation: height velocity SDS = A-B(e-cx), where A, B and C are constants and x is a measure of spontaneous GH secretion. We considered GH pulse amplitude to be the better description of spontaneous GH secretion as duration of the GH pulse (the time component of area under the curve) contributed little to the relationship between height velocity and area under the pulse. The distribution of GH secretion was continuous and there was no dividing point between GH insufficiency and sufficiency. Similar overlap was observed when the results of GH responses to insulin induced hypoglycaemia were considered; 14% of slowly growing children (height velocity SDS less than -0.8), had a response greater than 15 mU/l. Likewise serum IGF-I concentrations could not clearly separate slowly growing children from normal individuals. We conclude that height velocity, which ultimately determines height achieved, is controlled predominantly by GH pulse amplitude. The findings suggest that short normal children growing along or parallel to the third height centile could be made to grow faster by the administration of exogenous GH.

Effect of growth hormone on short normal children

The growth of 26 short normal prepubertal children (mean age 8.4, height velocity standard deviation score for chronological age between +0.4 and -0.8) was studied for two years. Sixteen children were treated with somatrem (methionyl growth hormone) during the second year, and the remaining 10 children served as controls. During one year of treatment the height velocity standard deviation score for chronological age increased from the pretreatment mean of -0.44 (SD 0.33) to +2.20 (1.03). These values represented a change in height velocity from a pretreatment mean of 5.3 cm/year (range 4.6-6.9) to 7.4 cm/year (range 5.7-9.9). In the control group the height velocity standard deviation score was unchanged. Bone age advanced by 0.75 (0.33) years in the treated group compared with 0.70 (0.18) years in the control group. There was a significant increase in the height standard deviation score for bone age (0.63 (0.55] in the treated group. Multiple regression analysis of predictive factors contributing to the change in height velocity standard deviation score over the first year of treatment showed that the dose of growth hormone and pretreatment height velocity standard deviation score were important, together yielding a regression correlation coefficient of 0.80. The only metabolic side effect of treatment was an increase in fasting insulin concentration, which may be an important mediator of the anabolic effects of growth hormone. Treatment had no effect on thyroid function, blood pressure, or glucose tolerance. At the end of the treatment year seven of the 16 treated children had developed antibodies to growth hormone, but they were present in low titre with low binding capacity and in no child was growth attenuated. Biosynthetic growth hormone improved the height velocity of children growing along or parallel to the third height centile, but the effects on height prognosis need to be assessed over a longer period.