Proteasome inhibitor enhances growth hormone-binding protein release

We used murine Ba/F3 cells transfected with human growth hormone receptor ( hGHR) cDNA to investigate the regulatory mechanisms of human growth hormone -binding protein (hGH-BP) release. The extracellular domain of hGHRs were c leaved and released as hGH-BPs (a soluble form of hGHR). The hGH-BP release was enhanced by phorbol 12,13-dibutyrate (PDBu), and suggested to be media ted by activation of PKC, the same as in human IM-9 cells. Thus, Ba/F3 cell s have hGH-BP-releasing pathways similar to those of human cells. The prote asome inhibitors MG-132 and clasto-lactacystin beta -lactone also increased hGH-BP release from Ba/F3-hGHR cells, and MG-132 and PDBu synergistically increased hGH-BP release. The results obtained by using three PKC inhibitor s Go 6976, GF 109203X and Go 6983 suggest that the enhancement of hGH-BP re lease by MG-132 and PDBu is mediated by different mechanisms probably invol ving different PKC isozymes. (C) 2001 Elsevier Science Ireland Ltd. All rig hts reserved.