Janus kinase 2 enhances the stability of the mature growth hormone receptor.

The abundance of surface GH receptor ( GHR) is an important determinant of cellular GH sensitivity and is regulated at both transcriptional and posttranscriptional levels. In previous studies of GHR- expressing Janus kinase 2 ( JAK2)- deficient human fibrosarcoma cells (gamma 2A- GHR), we demonstrated that stable transfection with JAK2 resulted in increased steadystate levels of mature GHR ( endoH- resistant; relative molecular mass, 115 - 140 kDa) relative to precursor GHR ( endoHsensitive; relative molecular mass, 100 kDa). We now examine further the effects of JAK2 on GHR trafficking by comparing gamma 2A- GHR to gamma 2A- GHR cells stably reconstituted with JAK2 ( C14 cells). In the presence of JAK2, GHR surface expression was increased, as assessed by surface biotinylation, I-125- labeled human GH cell surface binding, and immunofluorescence microscopy assays. Although the absence of JAK2 precluded GH- stimulated signaling, GH- induced GHR disulfide linkage ( a proxy for the GH- induced conformational changes in the GHR dimer) proceeded independent of JAK2 expression, indicating that the earliest steps in GH- induced GHR triggering are not prevented by the absence of JAK2. RNA interference- mediated knockdown of JAK2 in C14 cells resulted in a decreased mature to precursor ratio, supporting a primary role for JAK2 either in enhancing GHR biogenesis or dampening mature GHR degradation. To address these potential mechanisms, metabolic pulse- chase labeling experiments and experiments in which the fate of previously synthesized GHR was followed by anti- GHR immunoblotting after cycloheximide treatment ( cycloheximide chase experiments) were performed. These indicated that the presence of JAK2 conferred modest enhancement ( 1.3- to 1.5- fold) in GHR maturation but substantially prolonged the t(1/2) of the mature GHR, suggesting a predominant effect on mature GHR stability. Cycloheximide chase experiments with metalloprotease, proteasome, and lysosome inhibitors indicated that the enhanced stability of mature GHR conferred by JAK2 is not related to effects on constitutive receptor metalloproteolysis but rather is a result of reduced constitutive endosomal/ lysosomal degradation of the mature GHR. These results are discussed in the context of emerging information on how JAKfamily members modulate surface expression of other cytokine receptors.