Insulin Receptor Precursor Research Articles

Impaired SERCA2a phosphorylation causes diabetic cardiomyopathy through impinging on cardiac contractility and precursor protein processing

Abstract Diabetic cardiomyopathy (DCM) is currently a progressive and nonstoppable complication in type 2 diabetic patients. Metabolic insults and insulin resistance are involved in its pathogenesis; however, the underlying mechanisms are still not clearly understood. Here we show that calcium dysregulation can be both a cause and a consequence of cardiac insulin resistance that leads to DCM. A western diet induces the development of DCM through at least three phases in mice, among which an early phase depends on impaired Thr484-phosphorylation of sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a) elicited by insulin resistance. Mutation of SERCA2a-Thr484 to a nonphosphorylatable alanine delays calcium re-uptake into the sarcoplasmic reticulum in the cardiomyocytes and decreases cardiac function at the baseline. Importantly, this mutation blunts the early phase of DCM, but has no effect on disease progression in the following phases. Interestingly, impairment of sarcoplasmic reticulum calcium re-uptake caused by the SERCA2a-Thr484 mutation inhibited processing of insulin receptor precursor through FURIN convertase, resulting in cardiac insulin resistance. Collectively, these data reveal a bidirectional relationship between insulin resistance and impairment of calcium homeostasis, which may underlie the early pathogenesis of DCM. Our findings have therapeutic implications for early intervention of DCM.

A genome-wide association study to identify candidate genes for erectile dysfunction.

Erectile dysfunction (ED) can be caused by different diseases and controlled by several genetic networks. In this study, to identify the genes related to ED, the expression profiles of normal and ED samples were investigated by the Gene Expression Omnibus (GEO) database. Seventeen genes were identified as associated genes with ED. The protein and nucleic acid sequences of selected genes were retrieved from the UCSC database. Selected genes were diverse according to their physicochemical properties and functions. Category function revealed that selected genes are involved in pathways related to humans some diseases. Furthermore, based on protein interactions, genes associated with the insulin pathway had the greatest interaction with the studied genes. To identify the common cis-regulatory elements, the promoter site of the selected genes was retrieved from the UCSC database. The Gapped Local Alignment of Motifs tool was used for finding common conserved motifs into the promoter site of selected genes. Besides, INSR protein as an insulin receptor precursor showed a high potential site for posttranslation modifications, including phosphorylation and N-glycosylation. Also, in this study, two Guanine-Cytosine (GC)-rich regions were identified as conserved motifs in the upstream of studied genes which can be involved in regulating the expression of genes associated with ED. Also, the conserved binding site of miR-29-3p that is involved in various cancers was observed in the 3' untranslated region of genes associated with ED. Our study introduced new genes associated with ED, which can be good candidates for further analyzing related to human ED.

Overexpression of p53 Improves Blood Glucose Control in an Insulin Resistant Diabetic Mouse Model.

This paper aimed to assess the physiological effects of p53 on glucose homeostasis in vivo. A recombinant adenoviral p53 (rAd-p53) vector was administered to insulin-resistant diabetic mice. Intraperitoneal glucose tolerance test was performed in all groups of mice. Changes in fasting blood glucose, serum triglycerides, C-peptide, and insulin concentrations in treated and untreated mice were measured. Analyses of the target genes related to glucose metabolism were performed. Treatment with the rAd-p53 improved glucose control in a dose- and time-dependent manner and lowered significantly the fasting blood glucose, the serum triglycerides, and improved tolerance test of glucose as compared to control. Lowered blood glucose was associated with up-regulation of genes in the glycogenesis pathways, and down-regulation of genes in the gluconeogenesis pathways in the liver. Overexpressions of GLUT2, GK, PPAR-γ, and insulin receptor precursor were also observed in the liver and the pancreas of treated animals. Activation of p53-mediated glucose metabolism led to insulin-like antidiabetic effect in the mouse model especially by changing hepatic insulin sensitivity in the diabetic mouse model.

A New Highly Efficient Substrate-trapping Mutant of Protein Tyrosine Phosphatase 1B (PTP1B) Reveals Full Autoactivation of the Insulin Receptor Precursor

PTP1B is a protein tyrosine-phosphatase located on the cytosolic side of the endoplasmic reticulum that plays an important role in the regulation of the insulin receptor (IR). Replacement of the conserved Asp-181 by alanine is known to convert PTP1B into a substrate-trapping protein that binds to but cannot dephosphorylate its substrates. In this work, we have studied the effect of an additional mutation (Y46F) on the substrate-trapping efficiency of PTP1B-D181A. We observed that this mutation converts PTP1B-D181A into a highly efficient substrate-trapping mutant, resulting in much higher recovery of tyrosine-phosphorylated proteins coimmunoprecipitated with PTP1B. Bioluminescence resonance energy transfer (BRET) experiments were also performed to compare the dynamics of interaction of the IR with these mutants. Basal BRET, which mainly reflects the interaction of PTP1B with the IR precursor during its biosynthesis in the endoplasmic reticulum, was markedly increased with the PTP1B-D181A-Y46F mutant. In contrast, insulin-induced BRET was markedly reduced with PTP1B-D181A-Y46F. I(125) insulin binding experiments indicated that PTP1B-D181-Y46F reduced the expression of IR at the plasma membrane. Reduced expression at the cell surface was associated with higher amounts of the uncleaved IR precursor in the cell. Moreover, we observed that substantial amounts of the uncleaved IR precursor reached the Tris-phosphorylated, fully activated form in an insulin independent fashion. These results support the notion that PTP1B plays a crucial role in the control of the activity of the IR precursor during its biosynthesis. In addition, this new substrate-trapping mutant may be a valuable tool for the identification of new PTP1B substrates.

Glycogen synthase kinase‐3β: homologous regulation of cell surface insulin receptor level via controlling insulin receptor mRNA stability in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, 48 h-treatment with 20 mmol/L LiCl, 1 mmol/L valproic acid, 30 micromol/L SB216763, 30 micromol/L SB415286, or 100 nmol/L insulin, a condition that inhibits constitutive active glycogen synthase kinase-3 (GSK-3), decreased cell surface (125)I-insulin binding capacity by approximately 39%, without altering the K(d) value; LiCl, SB216763 or insulin decreased insulin receptor (IR) and IR precursor levels, attenuating insulin-induced Tyr-autophosphorylation of IR. LiCl increased inhibitory Ser9-phosphorylation of GSK-3beta at 6 h, decreasing (125)I-insulin binding at 24 h. SB216763-induced (125)I-insulin binding reduction (IC(50) = 3 micromol/L) was preceded by beta-catenin level increase by SB216763 (EC(50) = 11 micromol/L), a hallmark of GSK-3 inhibition. Insulin-induced rapid (> 1 min) Ser9-phosphorylation of GSK-3beta (Nemoto et al. 2006) was followed by approximately 48% decrease of IR level. LiCl did not stimulate endocytosis, nor proteolysis of IR. LiCl destabilized IR mRNA (t(1/2) = 9.3 vs. 6.5 h), decreasing IR mRNA level by approximately 47%, without altering IR gene transcription. Decreases of (125)I-insulin binding and IR level, as well as increased Ser9-phosphorylation of GSK-3beta were restored to the control levels by washing the test compound-treated cells. Thus, GSK-3beta regulates IR level via controlling IR mRNA stability.

Interaction of PTPB with the insulin receptor precursor during its biosynthesis in the endoplasmic reticulum

PTP1B is a protein tyrosine-phosphatase predominantly located on the cystosolic surface of the endoplasmic reticulum. This tyrosine-phosphatase plays a major role in the regulation of the activity of the insulin receptor (IR). We have studied the interaction of the IR with PTP1B in living cells using bioluminescence resonance energy transfer (BRET). The IR was fused to Renilla luciferase and a substrate-trapping mutant of PTP1B was fused to the yellow variant of the green fluorescent protein (YFP). When the two partners interacted, an energy transfer occurred between the luciferase and the YFP, and a fluorescent signal, emitted by the YFP, could be detected. The interaction of the IR with PTP1B could be monitored in real time for more than 30 min. Insulin rapidly and dose-dependently stimulated this interaction. The basal (insulin-independent) interaction of IR with PTP1B was much lower with a soluble form than with the endoplasmic reticulum-targeted form of PTP1B, indicating that this basal interaction mainly occurred in the endoplasmic reticulum. In the basal state, PTP1B and the IR indeed co-localized in the endoplasmic reticulum, as demonstrated by confocal microscopy and cell fractionation experiments. Moreover, inhibition of IR processing with tunicamycin indicated that the basal interaction of PTP1B with IR occurred during biosynthesis of the IR precursor in the endoplasmic reticulum. These results strongly suggest that PTP1B not only dephosphorylates the insulin receptor that has been activated by insulin, but also regulates the insulin receptor precursor during its biosynthesis. Localisation of PTP1B to the endoplasmic reticulum may be important to prevent insulin-independent autonomous activity of the immature insulin receptor precursor.

Distinct effects of ketone bodies on down-regulation of cell surface insulin receptor and insulin receptor substrate-1 phosphorylation in adrenal chromaffin cells.

Treatment (>/=24 h) of cultured bovine adrenal chromaffin cells with ketoacidosis-related concentrations (>/=3 mM) of acetoacetate (but not beta-hydroxybutyrate, acetone, and acidic medium) caused a time- and concentration-dependent reduction of cell surface (125)I-insulin binding by ~38%, with no change in the K(d) value. The reduction of (125)I-insulin binding returned to control nontreated level at 24 h after the washout of acetoacetate-treated cells. Acetoacetate did not increase the internalization rate of cell surface insulin receptor (IR), as measured in the presence of brefeldin A, an inhibitor of cell surface vesicular exit from the trans-Golgi network. Acetoacetate (10 mM for 24 h) lowered cellular levels of the immunoreactive IR precursor molecule (approximately 190 kDa) and IR by 22 and 28%, respectively. Acetoacetate decreased IR mRNA levels by approximately 23% as early as 6 h, producing their maximum plateau reduction at 12 and 24 h. The half-life of IR mRNA was shortened by acetoacetate from 13.6 to 9.5 h. Immunoprecipitation followed by immunoblot analysis revealed that insulin-induced (100 nM for 10 min) tyrosine-phosphorylation of insulin receptor substrate-1 (IRS-1) was attenuated by 56% in acetoacetate-treated cells, with no change in IRS-1 level. These results suggest that chronic treatment with acetoacetate selectively down-regulated the density of cell surface functional IR via lowering IR mRNA levels and IR synthesis, thereby retarding insulin-induced activation of IRS-1.

Down-regulation of cell surface insulin receptor and insulin receptor substrate-1 phosphorylation by inhibitor of 90-kDa heat-shock protein family: endoplasmic reticulum retention of monomeric insulin receptor precursor with calnexin in adrenal chromaffin cells.

Treatment (>/=6 h) of cultured bovine adrenal chromaffin cells with geldanamycin (GA) or herbimycin A (HA), an inhibitor of the 90-kDa heat-shock protein (Hsp90) family, decreased cell surface (125)I-insulin binding. The effect of GA was concentration (EC(50) = 84 nM)- and time (t(1/2) = 8.5 h)-dependent; GA (1 microM for 24 h) lowered the B(max) value of (125)I-insulin binding by 80%, without changing the K(d) value. Western blot analysis showed that GA (>/=3 h) lowered insulin receptor (IR) level by 83% (t(1/2) = 7.4 h; EC(50) = 74 nM), while raising IR precursor level by 100% (t(1/2) = 7.9 h; EC(50) = 300 nM). Pulse-label followed by reducing and nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that monomeric IR precursor (~190 kDa) developed into the homodimeric IR precursor (approximately 380 kDa) and the mature alpha(2)beta(2) IR (~410 kDa) in nontreated cells, but not in GA-treated cells; in GA-treated cells, the homodimerization-incompetent form of monomeric IR precursor was degraded via endoplasmic reticulum (ER)-associated protein degradation. Immunoprecipitation followed by immunoblot analysis showed that IR precursor was associated with calnexin (CNX) to a greater extent in GA-treated cells, compared with nontreated cells. GA had no effect on IR mRNA levels and internalization rate of cell surface IRs. In GA-treated cells, insulin-induced tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) was attenuated by 77%, with no change in IRS-1 level. Thus, inhibition of the Hsp90 family by GA or HA interrupts homodimerization of monomeric IR precursor in the ER and increases retention of monomeric IR precursor with CNX; this event retards cell surface expression of IR and attenuates insulin-induced activation of IRS-1.

Decreasing hypothalamic insulin receptors causes hyperphagia and insulin resistance in rats.

We investigated the role of hypothalamic insulin signaling in the regulation of energy balance and insulin action in rats through selective decreases in insulin receptor expression in discrete hypothalamic nuclei. We generated an antisense oligodeoxynucleotide directed against the insulin receptor precursor protein and administered this directly into the third cerebral ventricle. Immunostaining of rat brains after 7-day administration of the oligodeoxynucleotide showed a selective decrease of insulin receptor protein within cells in the medial portion of the arcuate nucleus (decreased by approximately 80% as compared to rats treated with a control oligodeoxynucleotide). Insulin receptors in other hypothalamic and extra-hypothalamic areas were not affected. This selective decrease in hypothalamic insulin receptor protein was accompanied by rapid onset of hyperphagia and increased fat mass. During insulin-clamp studies, physiological hyperinsulinemia decreased glucose production by 55% in rats treated with control oligodeoxynucleotides but by only 25% in rats treated with insulin receptor antisense oligodeoxynucleotides. Thus, insulin receptors in discrete areas of the hypothalamus have a physiological role in the control of food intake, fat mass and hepatic action of insulin.

Down-regulation of cell surface insulin receptors by sarco(endo)plasmic reticulum Ca 2+-ATPase inhibitor in adrenal chromaffin cells

Long-term (≥12 h) treatment of cultured bovine adrenal chromaffin cells with thapsigargin (TG), an inhibitor of sarco(endo)plasmic reticulum Ca 2+-ATPase (SERCA), caused a time ( t 1/2=16.3 h)- and concentration (IC 50=37.8 nM)-dependent decrease of cell surface 125I-insulin binding by 35%, but did not change the K d value. TG caused a sustained increase of cytoplasmic concentration of Ca 2+ ([Ca 2+] c) in a biphasic manner, and the effect of TG on 125I-insulin binding was abolished by BAPTA-AM. Western blot analysis showed that TG lowered insulin receptor (IR) β-subunit level in membrane, but did not alter total cellular levels of IR precursor and IR β-subunit. Internalization of cell surface IR, as measured by using brefeldin A, an inhibitor of vesicular exit from the trans-Golgi network (TGN), was not changed by TG. These results suggest that inhibition of SERCA by TG and the subsequent increase of [Ca 2+] c down-regulates cell surface IR by retarding externalization of IR from the TGN.

Post-translational reduction of cell surface expression of insulin receptors by cyclosporin A, FK506 and rapamycin in bovine adrenal chromaffin cells

Long-term (≥3 h) treatment of cultured bovine adrenal chromaffin cells with cyclosporin A (CsA) decreased cell surface 125I-insulin binding by 62% in a concentration (IC 50=18 μM)- and time ( t 1/2=16 h)-dependent manner, but did not change the K d value. FK506 (1 μM) or rapamycin (3 μM) treatment reduced 125I-insulin binding. Western blot analysis showed that CsA treatment decreased insulin receptor (IR) β-subunit level ( t 1/2=15 h) in membrane fraction, but did not alter total cellular levels of IR precursor and IR β-subunit. Internalization rate of cell surface IR measured by using brefeldin A, an inhibitor of vesicular exit from the trans-Golgi network, was comparable between non-treated and CsA-treated cells. Thus, CsA, FK506 and rapamycin inhibit peptidyl prolyl cis- trans isomerase activities of cyclophilin and FK506-binding protein, and down-regulate IR presumably by reducing cell surface externalization of IR.

Efficient translocation and processing with Xenopus egg extracts of proteins synthesized in rabbit reticulocyte lysate.

Cell-free translation/translocation systems are broadly applied to examine gene expression and characterize the structure-function relationship of gene products. We present the characterization of Xenopus egg extract (XEE) translocation and processing of proteins synthesized in rabbit reticulocyte lysate. The XEE was prepared from eggs laid by adult female frogs that received serial injections of gonadotropins. The eggs were then dejellied in 2% L-cysteine-HCl and the cytoplasm extracted by centrifugation at 10,000 rpm for 15 min. The in vitro translocation and processing of XEE was examined with a cell-free translation system containing reticulocyte lysate, and appropriate messenger ribonucleic acid (RNA) or complementary deoxyribonucleic acid plasmids with RNA polymerase. Cell-free production of the following proteins were used to assess posttranslational modifications: Escherichia coli beta-lactamase for signal sequence cleavage, Saccharomyces cerevisiae alpha-mating factor for translocation and N-linked glycosylation, the soluble protein luciferase for functional activity, and the membrane-bound human insulin receptor for translation efficiency. All translation products were identified by [35S]-methionine labeling, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. The results demonstrate that (1) XEE produces near-complete signal sequence and N-glycosylation processing of proteins synthesized in reticulocyte lysate, (2) XEE contains endoplasmic reticulum-equivalent microsomes, which allows for protein translocation and protease protection, (3) the addition of XEE in the translation reaction does not affect synthesis and chemiluminescence activity of luciferase, (4) XEE is efficient in processing the nascent 160-kDa human insulin receptor precursor, a transmembrane protein, and (5) as compared to canine pancreatic microsomes, XEE translocation efficiency is minimally decreased with the addition of dimethylsulfoxide. These results are the first description of the combined use of XEE with reticulocyte lysate and clearly demonstrate a higher efficiency of translocation and processing compared to canine pancreatic microsomes. This method of cell-free translation and processing allows for more extensive in vitro examination of posttranslational modifications of secretory and membrane-bound proteins.

The in vitro synthesized and processed human insulin receptor precursor binds insulin

The cell-free examination of the human insulin receptor during biogenesis may provide a greater understanding of the elements that contribute to the acquisition of receptor function. The insulin receptor precursor components were produced in a cell-free system and the insulin binding ability of the [ 35S]methionine-labeled translation products was determined. The processed proreceptor represented by a 190 kDa band was retained on insulin-linked biotin-streptavidin agarose or an insulin column. The insulin binding 190 kDa band migrated slower than the non-binding 190 kDa band on SDS-PAGE which suggests that covalent modifications account for these differences. The trypsin-digested product of the 190 kDa proreceptor was also retained on insulin-linked biotin-streptavidin agarose, however the α-subunit precursor was retained on insulin agarose to a much lesser degree. We conclude that a significant fraction of the processed, in vitro translated insulin proreceptor acquires insulin binding ability.

Role of proline 193 in the insulin receptor post-translational processing.

A point mutation, P193L, in the insulin receptor alpha subunit, has been previously identified in a patient affected by an extreme form of insulin resistance due to reduced insulin binding. In our study we investigated the cellular mechanisms by which P193L substitution causes a reduction of insulin receptor numbers on the cell surface. Mutated insulin receptors have been generated and expressed in COS1 cells. Transcription as well as translation of P193L insulin receptor have been measured and compared with wild type insulin receptor. P193L insulin receptor is normally transcribed and progresses to the step of insulin proreceptor, which does not proceed to dimerization, resulting in the accumulation of the 210 kDa form. These findings suggest that the P193L insulin proreceptor is retained in the endoplasmic reticulum, where several molecular chaperones drive the folding of protein precursors. Therefore the interaction between mutated insulin receptor precursor and two endoplasmic reticulum resident chaperones (GRP78 and calnexin) were investigated. P193L insulin proreceptor co-immunoprecipitates with greater amounts of GRP78 and its interaction with calnexin is greatly delayed compared with wild type insulin receptor precursor. Co-transfection of wild type and mutated insulin receptors causes a considerable reduction of cell surface wild type insulin receptors. P193 is critical for insulin proreceptor folding. The monomeric form of P193L insulin proreceptor is retained in the endoplasmic reticulum by a calnexin and GRP78 mediated mechanism that reduces mature insulin receptor expression on the cell surface.

Cleavage site mutants of the subtype B insulin receptor are uncleaved and fully functional

The insulin receptor (IR) is a membrane-bound glycoprotein composed of α and β subunits derived from a common precursor. This processing is observed for both subtypes A and B of the IR and its physiological importance is poorly understood. In order to investigate the functional consequences of the absence of IR precursor cleavage, using site-directed mutagenesis of the hIRB cDNA, we have produced two mutants replacing the sequence Arg-Lys-Arg-Arg by either His-Lys-His-Arg or Arg-Lys-Arg-Ser. These two mutants, stably expressed in CHO, were structurally and functionally characterized in comparison to the wild-type human IR. These mutations result in the production of uncleaved receptors which are expressed normally at the cell surface. These receptors bind insulin with a normal affinity and activate the tyrosine-kinase resulting in normal phosphorylation of the receptors. These uncleaved receptors can mediate both the metabolic and mitogenic effects of insulin. These results provide evidence for a fully functional uncleaved insulin receptor of the B subtype (exon 11+) in contrast to the uncleaved A subtype (exon 11−) described in the literature, which shows a reduced affinity for insulin and cannot therefore correctly transduce the insulin signal.

Processing of mutated proinsulin with tetrabasic cleavage sites to mature insulin reflects the expression of furin in nonendocrine cell lines

Furin is a mammalian propeptide-processing endoprotease in nonendocrine cells and has been demonstrated to be present in virtually all nonendocrine cells, including fibroblasts, epithelial cells, and hepatocytes. Furin cleaves the concensus processing site -Arg-4-X-3-Lys/Arg-2-Arg-1 decreases X+1-. Some subunit-containing precursor proteins, including an insulin receptor precursor, possess an additional basic residue at position -3, thus forming a tetrabasic processing site. This implies that a tetrabasic processing site must be easily cleavable in nonendocrine cells. We created a mutant proinsulin DNA with a peptide structure comprised of B- and A-chains linked to the C-peptide by a pair of tetrabasic residues, in the following order: B-chain-Arg-Arg-Lys-Arg-C peptide-Arg-Arg-Lys-Arg-A-chain. The native proinsulin structure was B-chain-Arg-Arg-C-peptide-Lys-Arg-A-chain. Both the native and mutant proinsulins were expressed in the following four cell lines: a monkey kidney-derived cell line (COS-7), a Chinese hamster ovary-derived cell line (CHO), a human liver cancer-derived cell line (HepG2), and a mouse fibroblast-like cell line (NIH3T3). We used these cell lines because they contain different quantities of furin mRNA, ranking as follows: NIH3T3 > HepG2 > COS > CHO. When mutant insulin was expressed in these cells, the conversion of proinsulin to mature insulin was approximately 85% in NIH3T3, 70% in HepG2, 60% in COS, and 50% in CHO. The conversion correlated well with the furin expression in each cell line as measured by the density of its Northern blot band. Moreover, in CHO, the cell line with the lowest furin expression, coexpression of mutant proinsulin with furin resulted in complete conversion of proinsulin to mature insulin.

Processing of mutated proinsulin with tetrabasic cleavage sites to mature insulin reflects the expression of furin in nonendocrine cell lines.

Furin is a mammalian propeptide-processing endoprotease in nonendocrine cells and has been demonstrated to be present in virtually all nonendocrine cells, including fibroblasts, epithelial cells, and hepatocytes. Furin cleaves the concensus processing site -Arg-4-X-3-Lys/Arg-2-Arg-1 decreases X+1-. Some subunit-containing precursor proteins, including an insulin receptor precursor, possess an additional basic residue at position -3, thus forming a tetrabasic processing site. This implies that a tetrabasic processing site must be easily cleavable in nonendocrine cells. We created a mutant proinsulin DNA with a peptide structure comprised of B- and A-chains linked to the C-peptide by a pair of tetrabasic residues, in the following order: B-chain-Arg-Arg-Lys-Arg-C peptide-Arg-Arg-Lys-Arg-A-chain. The native proinsulin structure was B-chain-Arg-Arg-C-peptide-Lys-Arg-A-chain. Both the native and mutant proinsulins were expressed in the following four cell lines: a monkey kidney-derived cell line (COS-7), a Chinese hamster ovary-derived cell line (CHO), a human liver cancer-derived cell line (HepG2), and a mouse fibroblast-like cell line (NIH3T3). We used these cell lines because they contain different quantities of furin mRNA, ranking as follows: NIH3T3 > HepG2 > COS > CHO. When mutant insulin was expressed in these cells, the conversion of proinsulin to mature insulin was approximately 85% in NIH3T3, 70% in HepG2, 60% in COS, and 50% in CHO. The conversion correlated well with the furin expression in each cell line as measured by the density of its Northern blot band. Moreover, in CHO, the cell line with the lowest furin expression, coexpression of mutant proinsulin with furin resulted in complete conversion of proinsulin to mature insulin.

Defective posttranslational processing activates the tyrosine kinase encoded by the MET proto-oncogene (hepatocyte growth factor receptor).

The MET proto-oncogene encodes a 190-kDa disulfide-linked heterodimeric receptor (p190 alpha beta) whose tyrosine kinase activity is triggered by the hepatocyte growth factor. The mature receptor is made of two subunits: an alpha chain of 50 kDa and a beta chain of 145 kDa, arising from proteolytic cleavage of a single-chain precursor of 170 kDa (pr170). In a colon carcinoma cell line (LoVo), the precursor is not cleaved and the Met protein is exposed at the cell surface as a single-chain polypeptide of 190 kDa (p190NC). The expression of the uncleaved Met protein is due to defective posttranslational processing, since in this cell line (i) the proteolytic cleavage site Lys-303-Arg-Lys-Lys-Arg-Ser-308 is present in the precursor, (ii) p190NC is sensitive to mild trypsin digestion of the cell surface, generating alpha and beta chains of the correct size, and (iii) the 205-kDa insulin receptor precursor is not cleaved as well. p190NC is a functional tyrosine kinase in vitro and is activated in vivo, as shown by constitutive autophosphorylation on tyrosine. The MET gene is neither amplified nor rearranged in LoVo cells. Overlapping cDNA clones selected from a library derived from LoVo mRNA were sequenced. No mutations were present in the MET-coding region. These data indicate that the tyrosine kinase encoded by the MET proto-oncogene can be activated as a consequence of a posttranslational defect.

Defective posttranslational processing activates the tyrosine kinase encoded by the MET proto-oncogene (hepatocyte growth factor receptor).

The MET proto-oncogene encodes a 190-kDa disulfide-linked heterodimeric receptor (p190 alpha beta) whose tyrosine kinase activity is triggered by the hepatocyte growth factor. The mature receptor is made of two subunits: an alpha chain of 50 kDa and a beta chain of 145 kDa, arising from proteolytic cleavage of a single-chain precursor of 170 kDa (pr170). In a colon carcinoma cell line (LoVo), the precursor is not cleaved and the Met protein is exposed at the cell surface as a single-chain polypeptide of 190 kDa (p190NC). The expression of the uncleaved Met protein is due to defective posttranslational processing, since in this cell line (i) the proteolytic cleavage site Lys-303-Arg-Lys-Lys-Arg-Ser-308 is present in the precursor, (ii) p190NC is sensitive to mild trypsin digestion of the cell surface, generating alpha and beta chains of the correct size, and (iii) the 205-kDa insulin receptor precursor is not cleaved as well. p190NC is a functional tyrosine kinase in vitro and is activated in vivo, as shown by constitutive autophosphorylation on tyrosine. The MET gene is neither amplified nor rearranged in LoVo cells. Overlapping cDNA clones selected from a library derived from LoVo mRNA were sequenced. No mutations were present in the MET-coding region. These data indicate that the tyrosine kinase encoded by the MET proto-oncogene can be activated as a consequence of a posttranslational defect.

Effects of amino acid replacements within the tetrabasic cleavage site on the processing of the human insulin receptor precursor expressed in Chinese hamster ovary cells.

We have studied the specificity requirements for processing of the human insulin proreceptor by successively replacing each basic amino acid in the tetrabasic cleavage site with alanine. These mutated receptor cDNAs have then been overexpressed in Chinese hamster ovary cells, using vectors containing the mouse dihydrofolate reductase gene to amplify the transfected cDNAs in the presence of increasing concentrations of methotrexate. High levels of expression, ranging up to 6 x 10(7) receptors/cell were achieved in these experiments. Replacement of the P1 arginine with alanine led to the complete suppression of processing, as occurs also in a naturally occurring serine mutation at this site (Yoshimasa, Y., Seino, S., Whittaker, J., Kakehi, T., Kosaki, A., Kuzuya, H., Imura, H., Bell, G. I., and Steiner, D. F. (1988) Science 240, 783-787). A small amount of cleavage at alternative sites was detected. Replacement of the P2 arginine or P3 lysine with alanine did not in either case affect conversion to mature alpha and beta subunits, while replacement of the P4 arginine significantly inhibited processing. The binding isotherms for the processed versions of the receptor were comparable to previously published normal values. The unprocessed proreceptor bound insulin normally but was autophosphorylated less efficiently than processed versions of the receptor expressed in the same cells. These results suggest that a single processing protease with trypsin-like specificity may be involved in processing both insulin and insulin-like growth factor-I receptor precursors as well as a variety of viral envelope glycoprotein precursors.