Immunological relationship and binding capacity of prolactin receptors in cytosolic and membrane fractions of rabbit mammary gland

Dominant negative and cooperative effects of mutant forms of prolactin receptor

In addition to a long form of 591 amino acids (aa), two other forms of PRL receptor (PRLR), differing in the length of their cytoplasmic domains, have been identified in the rat. The Nb2 form, lacking 198 aa in the cytoplasmic domain, is able to transmit a lactogenic signal similar to the long form, whereas the short form of 291 aa is inactive. The ability of PRL to activate the promoter of the beta-casein gene or the lactogenic hormone responsive element fused to the luciferase reporter was assessed in Chinese hamster ovary cells or 293 fibroblasts transiently transfected with PRLR cDNAs. The function of the short form was examined after cotransfection of both the long and short forms. These results clearly show that the short form acts as a dominant negative inhibitor through the formation of inactive heterodimers, resulting in an inhibition of Janus kinase 2 (JAK2) activation. The present study also investigates the possible participation of cytoplasmic receptors in the signal transduction pathway, using cotransfection experiments and a new approach that selectively determines the contribution of cytoplasmic receptors in the process of signal transduction. We cotransfected Chinese hamster ovary cells with two cDNA constructs: a cytoplasmic (soluble) form of the receptor with a deleted signal peptide (delta-19), which is unable to bind PRL, and a functionally inactive receptor mutant (lacking box 1), which is anchored in the plasma membrane and able to bind PRL. This approach has allowed us to show that delta-19, lacking expression at the plasma membrane, can transduce the hormonal message, at least to a limited extent (up to 30% of wild type efficiency), providing that association/activation occurs with a PRL-PRLR complex initiated at the cell surface level; box 1 of the cytoplasmic form is necessary to rescue this partial transcriptional activity of the inactive mutant. This partial recovery is also parallel to the partial activation of JAK2, indicating that the signal transduction pathway implicated JAK2. Our results provide evidence that heterodimerization of receptors can be implicated either in the positive or in negative activation of gene transcription.

Detection of glycosylated growth hormone-binding proteins in rat serum

Affinity cross-linking technique revealed the presence of three growth hormone-binding proteins (GHBP) in dealbuminized rat serum. The apparent molecular weights, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were 52,000, 44,000 and 39,000. By use of carbohydrate chain cleaving enzymes it was found that the binding proteins contain N-linked complex carbohydrate chains, representing 8000, 4000 and 5000 in apparent molecular weight, respectively. Gel permeation chromatography and sucrose density gradient centrifugation revealed a growth hormone-binding entity with Stoke's radius 94.5 A (+/- 2.8, n = 5) and an s-value of 10.8 S (+/- 0.31, n = 5). The molecular weight could be calculated to 413,000. Both for gel chromatography and sucrose density gradient experiments unlabelled growth hormone reduced the radioactive peaks from 0 to 50%. The relation of this binding entity to the GHBP's described with affinity cross-linking technique and the nature of this binding entity is at present unclear. No serum binding protein(s) for prolactin was detected by affinity cross-linking technique, gel permeation chromatography or sucrose density gradient analysis.

Binding proteins for growth hormone and prolactin in rabbit kidney cytosol

Two soluble, receptor-like binding proteins with apparent somatotrophic [growth hormone (GH)] and lactogenic [prolactin (PRL)] specificities, respectively, and that are present in rabbit kidney cytosol have now been examined in more detail using specific GH receptor and PRL receptor monoclonal antibodies (MAb). Gel chromatography of 125I-labeled human GH (125I-hGH) kidney cytosol complexes in the absence of these MAbs revealed two specifically bound regions of radioactivity at molecular weights (MW) of approximately 120,000 and approximately 60,000, which are similar in size to complexes formed by the native GH receptor of rabbit liver cytosol and the PRL receptor of mammary gland. Co-incubation with GH-receptor MAb inhibited 125I-hGH binding only to the higher MW (120,000) species, whereas the PRL-receptor MAb inhibited only the lower MW (60,000) species, thus establishing definitively the hormonal specificities of the two binding proteins. The presence of both GH- and PRL-specific binding subunits in cytosol was confirmed using covalent cross-linking techniques. No GH binding protein was detected in kidney membranes. The presence of naturally soluble, receptor-like binding proteins for GH and PRL in kidney cytosol preparations raises the possibility of their playing a role in the intracellular regulation of kidney function and/or metabolism.