Comparative Study of in Vitro and in Vivo Modulation of Lactogenic and Somatotropic Receptors by Native Human Growth Hormone and Its Modified Analog Prepared by Recombinant Deoxyribonucleic Acid Technology

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Abstract
A modified analog of human GH (hGH), prepared by recombinant DNA technology, that lacks 13 amino acids at the amino acids at the amino terminus (Met14 hGH), was able to compete with [125I]hGH for binding to lactogenic receptors in Nb2-11C rat lymphoma cells, to somatotropic receptors in IM-9 human lymphocytes, and to both lactogenic and somatotropic receptors in the microsomal fraction of virgin female rat liver. Exposure of intact Nb2 or IM-9 cells to Met14hGH did not reduce the number of surface or intracellular receptors, as compared to the control without hormone. A parallel exposure to 500-fold lower concentrations of hGH resulted in 77-93% reduction in both surface and intracellular receptors. In contrast to [125I]hGH, [125I]Met 14hGH was not taken up by the intact Nb2 lymphoma cells. Infusion of anesthetized female virgin rats for 3 h with hGH down-regulated both lactogenic and somatotropic receptors in the liver. A similar infusion with up to 200-fold higher amounts of Met14hGH did not lower the number of total receptors, indicating lack of down-regulation. Some decrease in the binding to free receptors was observed, suggesting that Met14hGH is capable of binding to liver receptors in vivo.

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