Unidad de control y procesamiento

The anabolic effects of GH on bone are mediated partly through the action of insulin­ like growth factor 1 (IGF-1), which was originally identified as a circulating pituitary- stimulated factor that stimulated [^^S]-sulphate incorporation into cartilage (Salmon and Daughaday, 1957), and also as a factor which had an insulin-like activity that could not be inhibited with insulin anti-sera (Scanes and Daughaday, 1995). The insulin-like

growth factors belong to a family of growth factors which consists of 3 peptides: insulin, IGF-1 and IGF-II, that share peptide sequence similarity and have a role in foetal and postnatal development, as regulators of cellular proliferation and survival. Furthermore the biological activities o f the IGF’s are influenced by IGF binding proteins which modulate the interaction between the IGF’s and their cognate receptors. The IGFBP’s may prolong the interaction with IGF and its receptor, and enhance the biological response to these factors (LeRoith and Butler, 1999).

1.3.5.1 Regulation of IGF-1 by growth hormone

A primary source of IGF-1 is the liver, which is stimulated by GH to produce and

secrete IGF-1 into the circulation (Figure 1.3). For example, serum IGF-1 levels can be

elevated in GH deficient animals by administration of GH, and partial hepatectomy reduces levels of circulating IGF-1. Furthermore, partial regeneration of the liver restores IGF-1 levels (Scanes and Daughaday, 1995). Circulating IGF-1 is bound to a ISOkDa complex consisting of: IGF, acid labile subunit (ALS) and IGF binding protein

3 (IGFBP-3) (Chin et a l, 1994). Both IGF-1 and IGFBP-3 are co-expressed in the liver,

under the control of GH, as treatment of hypophysectomised animals with GH increases levels o f their respective mRNA levels in this tissue (Albiston and Herington, 1992). The dependence of IGFBP-3 levels on GH are also indicative of the efficacy o f GH therapy for short stature, as a correlation has been made between basal and GH generated IGF-I and IGFBP-3 levels in children who present with short stature and

growth failure (Schwarze et a l, 1999).

Although there is a defined relationship between circulating IGF-1 levels produced by the liver and GH output by the pituitary, IGF-I is a relatively poor growth-promoting

agent in comparison to GH when given systemically (Laron, 1999). This reiterates the hypothesis that it is locally produced IGF-1, rather than circulating IGF-1, that mediates

the growth-promoting effects of GH (Skottner et al, 1987). A recent study has shown

that a specific knockout of hver IGF-1 production had no effect on the growth rates of mice. The Cre recombinase/loxP recombination system was used to specifically delete exon 4 of the IGF-1 gene in liver tissue, resulting in serum IGF-1 levels that were about 20% of control mice (89 vs 350ng/ml IGF-1). Bone growth was the same as controls in liver specific IGF-1 knockouts. However, the low circulating IGF-1 levels resulted in a

compensatory increase in GH output from the pituitary (Sjogren et a l, 1999). The roles

of IGF-1 and GH in bone growth are discussed further in the following section. 1.3.6 The effect of growth hormone on bone

A primary role of GH is the stimulation of skeletal and muscle growth during childhood and adolescence, when the major increases in bone mass occur. A maximal bone mass is reached between 20-30 years of age and this is followed by an age dependent decrease in

bone density, which is accelerated in females following the menopause (Ohlsson et al,

1998).

1.3.6.1 Effect of GH deficiency on bone

Growth will occur in the absence of growth hormone, although at a much lower rate, demonstrating that GH has an cumulative effect on skeletal maturation. GH deficiency causes reduced stature (dwarfism) in children, and therefore the primary rationale for GH therapy has been to increase the growth rate of children with a slow growth velocity. For instance, the near-adult height (AH) of 121 children treated with recombinant preparations of human GH, was significantly greater than the pretreatment height SD

(standard deviation) score (-3.1 +/- 1.2) and the predicted AH SD score (-2.2 +/- 1.2) (Blethen e/a/., 1997).

Additionally, growth hormone continues to have a significant effect on bone metabohsm after final height has been reached. For instance, in one group of patients with GHD, photon absorbtiometry was used to demonstrate a decreased mineral content in the

lumbar spine and forearm compared with age and height matched controls (Kaufman et

a l, 1992) It has therefore been suggested that GH treatment should be continued until

peak bone mass is achieved, irrespective of the height attained (Saggese et al, 1996).

For example an increased cortical thickness has been demonstrated by

histomorphometric analysis in GH deficient men treated with recombinant GH

(Bravenboer et al, 1997), and patients with adult onset GH deficiency also respond to

long term GH therapy with a net gain in bone mineral density in several weight bearing

skeletal locations (Johannsson et a l, 1996).

1.3.6.2 Ageing decreases bone mineral density

Bone mineral content decreases with age, and the decline in activity of the GH/IGF-1

axis may be involved in the age-related loss of bone mineral content (Boonen et a l,

1996). For instance, the amplitude of the GH pulse decreases with age (Iranmanesh et

a l, 1991), and basehne IGF-1 levels have also been found to be a predictor of bone

density in elderly women (Boonen et a l, 1996). This has led to a suggestion that GH

therapy could be used to increase bone mineral density in the elderly (Toogood and Shalet, 1998). However, only a slight increase (1.6%) in lumbar bone mineral density was observed after 6 months of treatment with recombinant GH in elderly men (Rudman

et a l, 1990), and in a separate study bone mineral density did not change during a 6

month trial of GH in elderly women (Marcus et a l, 1990).

A more prolonged treatment with recombinant GH may be necessary to effect any

change in bone mineral density in elderly patients (Johannsson et a l, 1996), and the

long-term benefits of increased bone mineral density in patients treated with recombinant GH would include a decreased incidence o f bone fractures. However, the decline in the production of the other pituitary hormones which occurs during ageing further comphcates the interpretation that bone mineral density is exclusively influenced

by reduced activity of the GH/IGF-1 axis (Lamberts et al, 1997).

1.3.6.3 Cellular effects of GH in bone

Although both GH and IGF-1 are required to stimulate bone growth, the relative contributions of each have not clearly been established. The original somatomedin hypothesis proposed that GH stimulates IGF-1 production by the liver, which in turn stimulates bone growth (Daughaday and Rotwein, 1989). Furthermore, a patient with a deletion in the IGF-1 gene demonstrated intrauterine growth retardation and postnatal

growth failure (Woods et al, 1996). However, as described earlier, mice with

conditional knockouts of the IGF-1 gene in the liver continue to grow even in the

absence of circulating hepatic IGF-1 (Sjogren et a l, 1999). This supports the view that

GH stimulates growth through locally produced, and not liver derived IGF-1. However, circulating IGF-1 does have a major influence on the regulation of GH secretion, and this is discussed in the section on the control of GH release.

1.3.6.4 Combined effects of GH and IGF-1 on bone

Injection of GH directly into the rat tibia has also been shown to stimulate longitudinal

bone growth at the site of injection (Isaksson et al, 1982) and during the process of

longitudinal bone growth, chondrocytes in the germinal layer of the growth plate differentiate and undergo clonal expansion in individual chondrocyte columns in the growth plate. Subsequently, cells in the hypertrophic zone mature and are eventually incorporated into bone. GH stimulates the formation of young chondrocytes, whereas

IGF-1 stimulates cells at a later stage of activation (Ohlsson et a l, 1998), as local

injection of GH increases the numbers of thymidine-labelled cells in the prechondrocyte layer of the growth plate, whereas administration of IGF-1 does not stimulate isotope

incorporation into the same cell layer (Ohlsson et al, 1992). These observations support

a functional synergism between GH and IGF-1 (Figure 1.4).