Evaluation of growth hormone bioactivity using the Nb2 cell bioassay in children with growth disorders

The Nb-2 cell bioassay could be a useful tool for evaluating human growth hormone (hGH) bioactivity, but is not specific for hGH since it relies on the ability of the hormone to produce effects by cross-reacting with the lactogenic receptor on Nb-2 cells. We studied the biological activity of both endogenous and exogenous hGH in short patients using the Nb-2 bioassay after inhibiting the mitogenic effect of the other lactogenic hormone, that is human prolactin (hPRL), by adding a specific antibody against hPRL to each assay. The in vitro study showed a significant (p<0.0001) increase in Nb-2 cell proliferation when increasing concentrations of highly purified hGH were added to the cell culture. A complete inhibition of Nb-2 cell replication was observed after adding a specific antibody against hGH. The in vivo study showed a significantly (p<0.0001) lower hGH bioactivity (4.90+/-0.28 ng/ml) evaluated during stimulation tests in 9 children with total idiopathic GH deficiency, mean age 9.25+/-1.99 years, in comparison with that found in 11 short children with normal growth velocity, mean! age 8.22+/-1.41 years (12.25+/-1.19 ng/ml). Likewise, serum GH levels measured by immunofluorometric assay IFMA in the same serum samples were significantly (p<0.001) lower in the 9 GH-deficient (1.97+/-0.37 ng/ml) than in the 11 short children (21.85+/-2.71 ng/ml). Moreover, we evaluated GH concentrations using both IFMA and the Nb-2 cell bioassay in serum samples collected from another 11 idiopathic GH-deficient children, mean age 10.71+/-1.18 years, before and then, 6 and 24 hours following the 1st injection of r-hGH (0.1 IU/kg sc), Serum GH values measured by both IFMA and Nb-2 bioassay significantly (p<0.0001) increased 6 hours after r-hGH administration and decreased to reach basal levels after 24 hours. In conclusion, the Nb-2 cell bioassay with minor modifications seems to provide a specific and sensitive assessment of hGH bioactivity. (C),1998, Editrice Kurtis.