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The GH binding protein (GHBP), which exists in many vertebrates, is a circulating high affinity binding protein corresponding to the extracellular domain of the GH receptor (GHR). In humans, rabbits, and several other species, the GHBP is generated by proteolysis of the GHR and shedding of its extracellular domain. We previously showed that GHBP shedding is inducible by the phorbol ester phorbol 12-myristate,13-acetate (PMA) and inhibited by the metalloprotease inhibitor, Immunex Corp. Compound 3 (IC3). The metzincin metalloprotease, tumor necrosis factor-alpha (TNF-alpha)-converting enzyme (TACE), catalyzes the shedding of TNF-alpha from its transmembrane precursor, a process that is also inhibitable by IC3. TACE may hence be a candidate for GHBP sheddase. In this study, we reconstitute fibroblasts derived from a TACE knockout mouse (Null cells) with either the rabbit (rb) GHR alone (Null/R) or rbGHR plus murine TACE (Null/R+T). Although GHR in both cells was expressed at similar abundance, dimerized normally and caused JAK2 activation in response to GH independent of TACE expression, PMA was unable to generate GHBP from Null/R cells. In contrast, PMA caused ample GHBP generation from TACE reconstituted (Null/R + T) cells, and this GHBP shedding was substantially inhibited by IC3 pretreatment. Corresponding to the induced shedding of GHBP from Null/R + T cells, PMA treatment caused a significant loss of immunoblottable GHR in Null/R+T, but not in Null/R cells. We conclude that TACE is an enzyme required for PMA-induced GHBP shedding and that PMA-induced down-regulation of GHR abundance may in significant measure be attributable to TACE-mediated GHR proteolysis.

endocrinology

Tumor Necrosis Factor-α Converting Enzyme (TACE) Is a Growth Hormone Binding Protein (GHBP) Sheddase: The Metalloprotease TACE/ADAM-17 Is Critical for (PMA-Induced) GH Receptor Proteolysis and GHBP Generation*