Tunicamycin sensitivity of prolactin, insulin and epidermal growth factor receptors in rat liver plasmalemma

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells. [Media: see text].

Age-related changes in insulin receptor mRNA and protein expression in genetically obese Zucker rats

AIM

The mechanisms underlying the age-related decrease in insulin-receptor (IR) binding in genetically obese Zucker rats are not well understood. For this reason, the present study analyzed the expression of IR mRNA and protein in selected tissues from 1- to 4-month-old obese (fa/fa) Zucker rats and lean (Fa/-) age-matched controls.

METHODS

The following parameters were evaluated: (1) IR mRNA level, and proportion of isotypes A (exon 11-) and B (exon 11+) of IR mRNA in liver, brain and kidney; (2) level, molecular size and tyrosine phosphorylation of IR-beta subunit in liver subcellular fractions; and (3) stability of liver IR based on sensitivity in vivo of insulin-binding activity and IR-beta levels in response to tunicamycin, a glycosylation inhibitor.

RESULTS

At one month, IR mRNA level was increased in liver and brain, but decreased in kidneys and, at four months, both mRNA level and isotype B proportion were decreased in liver. From age two months, the following changes in liver IR protein expression were observed: (1) decreased IR-beta level in whole homogenates, but increased IR-beta levels in endosomal fractions; (2) increased IR-beta tyrosine phosphorylation; and (3) at four months, increased levels of both intact IR-beta (95 kDa) and IR-beta fragments (72 and 52 kDa) in lysosomal fractions, along with decreased stability in vivo of the IR.

CONCLUSION

These data show that obese Zucker rats display age-related alterations of IR gene expression at both pre- and post-translational stages and, in particular, increased endocytosis and degradation of IR protein.

Cloning and functional characterization of allelic variation in the porcine prolactin receptor

Prolactin (PRL) regulates various functions in pigs including reproduction, mammary development and lactation. We used 5'-rapid amplification of cDNA ends (5'-RACE) to clone three full-length alleles of the porcine PRL receptor (pPRLR) from Landrace (alleles LR2 and LR4) and Yucatan miniature (MP) pigs, corresponding to the A and B alleles previously reported to be associated with reproductive traits. When expressed in Chinese hamster ovary (CHO-K1) cells, all three pPRLRs transduced differentiation signals to a beta-casein promoter with the same effectiveness, where human growth hormone (hGH) and porcine PRL (pPRL) were more effective ligands than ovine PRL (oPRL). The pPRLR had a lower binding affinity for oPRL than pPRL while binding affinity for hGH was not different between the three pPRLR variants. The pPRLRs primarily localized to the cytoplasm with perinuclear concentration. In conclusion, we have cloned three allelic variants of the pPRLR and have functionally characterized these as different from the hPRLR. However, our data do not support the proposal that allelic variation of the pPRLR confers functional differences in vivo.

Characterization of a monoclonal antibody against human prolactin receptors

Tamoxifen is the first line of therapy for most human breast cancers. It not only works through the estrogen receptor but also can directly affect the binding of prolactin to its receptor. To define this latter mechanism, the nature of the prolactin receptor needs to be clearly defined. Monoclonal antibody (MAb) B6.2, and IgG1 raised against a membrane-enriched fraction from metastatic human breast cancer cells, was as effective as polyclonal anti-prolactin receptor antibody in inhibiting the binding of prolactin to membranes from human tissue and to T47D human breast cancer cells. Control MAbs, MOPC-2I and the anti-NCA B1.1 MAb, had no effect on binding. Epidermal growth-factor receptors on these same cells were unaffected by B6.2. Prolactin-induced growth of the T47D cells was blocked by addition of B6.2 to the media while the control antibodies were without effect. Specific binding of B6.2 to the cells was completely inhibited by prolactin. Binding of both prolactin and B6.2 was inhibited by growing the T47D cells in the presence of tunicamycin A1 under conditions where protein synthesis was not affected but glycosylation of proteins was. An affinity column of B6.2 was used to purify its antigen from T47D cells. The primary purification product, a M(r) 90,000 protein, specifically bound the lactogenic hormones human prolactin, human growth hormone and ovine prolactin but not the somatogenic hormone, bovine growth hormone and was precipitated by the polyclonal anti-prolactin receptor antibody but not by control MAbs. When tryptic and V8 digests of the B6.2 antigen and purified prolactin receptors were compared, identical electrophoretic profiles were obtained. Mouse 3T3 cells, when stably transfected with the gene for the long form of the human prolactin receptor, reacted with B6.2 and polyclonal anti-prolactin receptor antibody. Parental 3T3 cells, devoid of prolactin receptors, were negative for all antibodies tested. Thus, MAb B6.2 provides a useful tool for further studies on purification and characterization of these receptors from human tissues and may provide new insights into treatment for breast cancer.

Localization of epidermal growth factor receptors in cells of the enamel organ of the rat incisor

Epidermal growth factor (EGF) is a peptide shown to effect precocious incisor tooth eruption in rat pups. Binding sites for EGF were visualized in the continuously erupting adult rat incisor by light and electron microscope radioautography after in vivo injection of 125I-EGF. These binding sites represented EGF receptors because of (i) competition between 125I-EGF binding at 2 min after injection and a coinjected excess of unlabeled EGF; (ii) the receptor-mediated endocytosis of 125I-EGF at 15 and 30 min after injection; and (iii) the demonstration of EGF receptor kinase activation in vivo. The stem and the mitotic cells in the epithelial odontogenic organ at the growing end of the tooth develop into two nondividing layers of the enamel organ: (i) ameloblasts which secrete enamel and are subsequently involved in the enamel maturation process, and (ii) papillary layer cells situated between the blood supply and the ameloblasts. Although few EGF receptors were present at the mitotic end, receptor density was highest at the mature end of the enamel organ. High levels of 125I-EGF binding were found on papillary layer cells and ruffle-ended, but not smooth-ended, ameloblasts. This implies a cyclical exteriorization and internalization of receptors during modulations between the two cell types. These data suggest that the EGF receptor mediates a major function of the enamel organ in the formation of enamel.

Selective degradation of insulin within rat liver endosomes

To characterize the role of the endosome in the degradation of insulin in liver, we employed a cell-free system in which the degradation of internalized 125I-insulin within isolated intact endosomes was evaluated. Incubation of endosomes containing internalized 125I-insulin in the cell-free system resulted in a rapid generation of TCA soluble radiolabeled products (t1/2, 6 min). Sephadex G-50 chromatography of radioactivity extracted from endosomes during the incubation showed a time dependent increase in material eluting as radioiodotyrosine. The apparent Vmax of the insulin degrading activity was 4 ng insulin degraded.min-1.mg cell fraction protein-1 and the apparent Km was 60 ng insulin.mg cell fraction protein-1. The endosomal protease(s) was insulin-specific since neither internalized 125I-epidermal growth factor (EGF) nor 125I-prolactin was degraded within isolated endosomes as assessed by TCA precipitation and Sephadex G-50 chromatography. Significant inhibition of degradation was observed after inclusion of p-chloromercuribenzoic acid (PCMB), 1,10-phenanthroline, bacitracin, or 0.1% Triton X-100 into the system. Maximal insulin degradation required the addition of ATP to the cell-free system that resulted in acidification as measured by acridine orange accumulation. Endosomal insulin degradation was inhibited markedly in the presence of pH dissipating agents such as nigericin, monensin, and chloroquine or the proton translocase inhibitors N-ethylmaleimide (NEM) and dicyclohexylcarbodiimide (DCCD). Polyethylene glycol (PEG) precipitation of insulin-receptor complexes revealed that endosomal degradation augmented the dissociation of insulin from its receptor and that dissociated insulin was serving as substrate to the endosomal protease(s). The results suggest that as insulin is internalized it rapidly but incompletely dissociates from its receptor. Dissociated insulin is then degraded by an insulin specific protease(s) leading to further dissociation and degradation.

Characterization of hepatic lactogen receptor. Subcellular distribution and characterization of N-linked carbohydrate chains

The types of carbohydrate chains present in a rat liver lactogenic hormone-binding receptor species with an Mr of 82,000, and in its hormone-binding subunits with Mr values of 40,000 and 35,000, were characterized using carbohydrate-chain-cleaving enzymes and affinity cross-linking. The subcellular distribution of lactogenic hormone-binding species was studied in organelle-enriched fractions. The monomeric Mr-40,000 and Mr-35,000 species contain N-linked tri- or tetra-antennary complex and high-mannose chains respectively. The Mr-82,000 species exists in two forms, where the Mr-40,000 and Mr-35,000 subunits are each combined with unglycosylated and, with the technique used, unlabelled subunit(s). Studies with organelle-enriched fractions revealed that the Mr-35,000 species was found in an endoplasmic reticulum-enriched fraction. The Mr-40,000 species was the predominant monomeric binding species in Golgi/endosome- and plasma membrane-enriched fractions. It is suggested that the Mr-35,000 species is a precursor to the Mr-40,000 species. In lysosome/endosome- or lysosome-enriched fractions, a broad distribution in Mr (35,000-40,000) was characteristic of the hormone-binding species. The Mr-82,000 species was only found in a Golgi/endosome-enriched fraction. Labelling of endosome lactogen receptor by injection of 125I-labelled ovine prolactin in vivo and cross-linking yielded only the Mr-40,000 species. Thus, the Mr-40,000 and Mr-35,000 lactogenic hormone-binding species each appear to be combined with the unglycosylated receptor subunit(s) in the Golgi complex to form Mr-82,000 heterodimeric complexes.

Hepatic prolactin binding is rapidly altered by endotoxin in lactating mice

Endotoxin or lipopolysaccharide (LPS), a component of the outer membrane of gram-negative bacteria, produces profound physiologic changes in most mammals. The effects of LPS on ovine prolactin (oPRL) binding by hepatic membranes of lactating mice is explored in this report. Specific 125I-oPRL binding by liver membranes from LPS-responder C3HfB/HeN mice increased two-fold within fifteen minutes of the injection of LPS, while no change was observed in the non-responder C3H/HeJ mice. Specific 125I-insulin binding did not change. Scatchard analysis of equilibrium binding of oPRL to C3HfB/HeN liver membranes indicated that within fifteen minutes of LPS injection, a receptor of differing binding affinity appears and then disappears by one hour post-injection. We propose that these rapid alterations in the specific binding of oPRL by liver membranes from LPS-injected, lactating C3HfB/HeN mice are due to the transient creation or unmasking of a novel class of PRL receptor.