Objective: Insulin-like growth factor binding protein-6 (IGFBP-6) is a relatively unknown member of a family of six specific structurally related IGF binding proteins which are involved in the modulation of the biological effects of the IGFs. A distinctive property of IGFBP-6 is its preferential affinity for IGF-II relative to IGF-I. In order to obtain more insight into the clinical significance and regulation of circulating levels of IGFBP-6 we developed a specific radioimmunoassay (RIA) for this protein.
Design and patients: Selected human biological fluids and plasma from 847 normal subjects were analysed. In addition, plasma samples from patients with different disorders (i.e. GH-deficiency, acromegaly, cancer, corticosteroid-treated children suffering from different kinds of severe illness and chronic renal failure) were investigated.
Measurements: The IGFBP-6 assay is competitive, utilizing a rabbit polyclonal antibody raised against a synthetic peptide comprising amino acids 90-118 of the hIGFBP-6 sequence and an additional tyrosine residue. It is calibrated against recombinant human (rh)IGFBP-6. The 125I tracer is prepared by iodination of the synthetic peptide. There is no significant cross-reactivity with other IGFBPs and no interference with the IGFs.
Results: Extensive normative range values for IGFBP-6 were determined using 847 plasma samples from normal males and females, ranging from 0 to 75 years of age. IGFBP-6 levels increased gradually (about two-fold) with age. In childhood the plasma levels of IGFBP-6 in females tended to be slightly higher than those for males. For the adult population the reverse was observed. Overall, the mean +/- SD value for males was higher than that for females (149 +/- 57 vs. 139 +/- 45 micrograms/l, P < 0.004). GH status did not appear to influence IGFBP-6 level since normal levels were found for both untreated acromegalic patients and GH-deficient subjects. GH treatment of the latter group of patients did not alter IGFBP-6 in plasma. Pharmacological doses of glucocorticosteroids affected circulating IGFBP-6 levels only slightly. IGFBP-6 levels in plasma samples derived both from children with acute lymphoblastic leukaemia and from patients with various types of solid neoplasms were generally within the normal range. In contrast, plasma samples from four of six patients with non-islet cell tumour induced hypoglycaemia (NICTH) showed elevated concentrations of IGFBP-6 (SDS > 2.9). An excess of IGFBP-6 was also found in plasma of both dialysed and non-dialysed prepubertal growth retarded children with chronic renal failure (CRF) (mean SDS: 23.0 and 9.3, respectively). IGFBP-6 levels were inversely correlated with glomerular filtration rate. In a group of CRF patients who underwent renal transplantation circulating IGFBP-6 levels were markedly lower (mean SDS: 4.6). The presence of IGFBP-6 could also be demonstrated in several other human biological fluids. Low amounts were detected in saliva (3-12 micrograms/l) and breast milk (6-45 micrograms/l) while the levels in amniotic fluid and follicular fluid were comparable with those determined in normal plasma. The IGFBP-6 content of cerebrospinal fluid (CSF) ranged between 25 and 87 micrograms/l, which is rather high in relation to the relatively low concentration of total protein in this body fluid.
Conclusions: Measurements of IGFBP-6 have been shown so far to be of relatively minor clinical relevance. The exceptions are chronic renal failure patients and subjects with large tumours and non-islet cell tumour induced hypoglycaemia who may exhibit elevated circulating levels of this IGFBP. The physiological significance of this observation remains to be elucidated. The possibility of quantifying IGFBP-6 by specific RIA will facilitate further in vitro and in vivo studies of its regulation and function in man.