Growth hormone binding protein 2001.

Abstract

The present state of knowledge about growth hormone binding proteins (GHBP) is reviewed, with particular emphasis on the high affinity GHBP, which represents the circulating ectodomain of the growth hormone receptor (GHR). GHBP is conserved through vertebrate evolution, is produced in many tissues (especially liver) by either alternative GHR mRNA splicing (rodents) or by proteolytic cleavage from the GHR (humans, rabbits and several other species). The metalloprotease TACE (tumor necrosis factor-alpha converting enzyme) is the likely enzyme responsible for cleavage, but the structural requirements for TACE recognition or catalysis, and hence the precise cleavage point in the GHR, are unknown. GHBP is widely distributed in biological fluids, with marked concentration differences amongst them. GHBP binds about half of the circulating GH under basal conditions but is easily saturated at high GH levels; it subserves complex functions, including a circulating buffer/reservoir function for GH, prolongation of plasma GH half-life, competition with GHRs for GH, and probably unproductive heterodimer formation with the GHR. The net effect of these partly enhancing and partly inhibitory functions on GH action in vivo is complex and difficult to ascertain. Serum GHBP levels roughly parallel GHR expression (particularly in liver) through the life span, with very low levels in fetal life, upregulation to adult levels during childhood, and decline in senescence. Rodent pregnancy is associated with a massive increase in GHBP expression. Although the regulation of GHBP expression/production is not necessarily tightly linked to GHR expression, in general, low GHBP levels occur in conditions associated with GH resistance (e.g., malnutrition, uncontrolled diabetes, catabolic states, renal failure, hypothyroidism). Conversely, obesity, a condition with enhanced GH responsivity, is associated with elevated GHBP levels. This suggests that in many (but not all) instances of abnormal GH action, the GHBP level reflects GHR status. Laron syndrome (genetic GHR deficiency/dysfunction due to mutations in the GHR gene) is associated with low or undetectable GHBP in 75-80% of patients; GHBP measurement can therefore be used diagnostically. Depending on the design of assays for serum GH, endogenous GHBP may interfere to varying degrees, and GH assays should be individually validated and optimized in this regard. The ultimate biological role and physiological significance of the GHBP remain to be established.