The presence of a soluble, truncated form of the IGF-II/Man-6-P receptor in serum has suggested that cleavage from the cell surface may be an initial step in the degradation of this protein (MacDonald, R. G., Tepper, M. A., Clairmont, K. B., Perregaux, S. B., and Czech, M. P. (1989) J. Biol. Chem. 264, 3256-3261). In order to test this hypothesis, we pulse-labeled cultured BRL-3A rat liver cells with [35S]methionine and [35S]cysteine and measured the fate of labeled receptor at various times after incubation with unlabeled amino acids. It was found that the appearance of labeled IGF-II/Man-6-P receptor in the medium accounts quantitatively for the loss of labeled receptor from the BRL-3A cells. In similar experiments with Chinese hamster ovary cells, L6 rat myoblasts, and chick embryo fibroblasts, labeled receptor from the cell membranes decreases with a time course corresponding to the appearance of soluble receptor in the medium. The release of labeled receptor into the medium can be blocked by the addition of the protease inhibitors aprotinin, chymostatin, or phenylmethylsulfonyl fluoride, but not antipain, leupeptin, and benzamidine. The results are consistent with the hypothesis that the degradation and loss of cellular IGF-II/Man-6-P receptors occurs by a nonlysosomal mechanism involving their proteolysis and removal into the extracellular fluid.

In mammals a single receptor protein binds both insulin-like growth factor II (IGF-II) and mannose 6-phosphate (Man 6-P) containing ligands, most notably lysosomal enzymes. However, in chick embryo fibroblasts IGF-II binds predominantly to a type 1 IGF receptor, and no IGF-II/Man 6-P receptor has been identified in this species. In order to determine if chickens possess an IGF-II/Man 6-P receptor, an affinity resin (pentamannosyl 6-phosphate (PMP) Sepharose) was used to purify receptors from chicken membrane extracts by their ability to bind mannose 6-phosphate. Then 125I-IGF-II was used to evaluate their ability to bind IGF-II. These experiments demonstrate that nonmammalian Man 6-P receptors lack the ability to bind IGF-II, suggesting that the ability to bind IGF-II has been gained recently in evolution by the mammalian Man 6-P receptor. The second area of study involves the serum form of the IGF-II/Man 6-P receptor. This receptor had been detected in the serum of a number of mammalian species, yet its structure, function, regulation, and origin were unknown. Initial studies, done with Dr. R. G. MacDonald, showed that the serum receptor is truncated such that the C-terminal cytoplasmic domain of the cellular receptor is removed. These studies also demonstrate a regulation of serum receptor levels with age, similar to that seen for the cellular receptor, and that the serum form of the receptor existed in several forms which appeared intact under nonreducing conditions, but as multiple proteolytic products upon reduction. Finally, these studies demonstrated that both the cellular and serum IGF-II/Man 6-P receptors are capable of binding IGF-II and Man 6-P simultaneously. In studies on the serum form of the IGF-II/Man 6-P receptor that I have conducted independently, the regulation of the serum IGF-II/Man 6-P and transferrin receptors by insulin has been demonstrated. In these studies, insulin injected into rats subcutaneously resulted in a time and dose dependent increase in serum receptor levels. Finally, to investigate the relationship of the serum IGF- II/Man 6-P receptor to the cellular form of the receptor, pulse chase experiments were performed. These experiments demonstrate that the soluble (serum form released into the medium) receptor is a major degradation product of the cellular receptor. Furthermore, the lack of detectable amounts of the lower Mr soluble receptor intracellularly and the parallel relationship of cell surface and soluble receptor suggest that the proteolysis is occurring from the cell surface. Finally, a number of experiments suggest that the degradation rate depends upon the conformation state of the receptor: binding of IGF-II or Man 6-P makes the receptor more susceptible to proteolysis while the presence of lysosomal enzymes prevents receptor proteolysis. In summary, the serum form of the IGF-II receptor is a proteolytic product of the cellular form of the receptor. The rate of release depends upon the number of receptors at the cell surface and the binding state of the receptor. In circulation, the receptor retains the ability to bind both types of ligands, it thus may serve as an IGF binding protein and/or a lysosomal enzyme binding protein. These results suggest a model whereby the cellular receptor is proteolytically cleaved by a plasma membrane protease to produce a short membrane anchored fragment and the serum receptor. In vivo this pathway serves as the major degradative pathway of the IGF-II/Man 6-P receptor, with the serum form being cleared from circulation by further degradation and reuptake.

Previous work indicates that a serum form of the insulin-like growth factor-II/mannose-6-phosphate (IGF-II/Man-6-P) receptor that circulates in mammals is a truncated form of the cellular receptor which lacks its cytoplasmic domain. In this study the effects of insulin administration on the levels of the serum forms of IGF-II/Man-6-P and transferrin receptors were measured. After sc injection of insulin into rats fasted overnight, the amount of IGF-II/Man-6-P receptor in serum increases to approximately twice that observed in untreated animals within 30 min, then decreases to a level lower than the initial level by 60 min before returning to control values by 90 min. The time course of the initial increase in serum receptor levels in response to insulin is similar to that observed for the decrease in blood glucose concentrations. Measurements of serum enzyme activities (creatine kinase and lactate dehydrogenase) during this time show no significant increase in response to insulin treatment. Furthermore, the increase in serum IGF-II/Man-6-P receptors is proportional to the amount of insulin injected over the range tested. In diabetic rats the serum IGF-II/Man-6-P receptor concentration is decreased to approximately 80% as much serum receptor as that in normal age-matched rats. The acute response of serum IGF-II/Man-6-P receptor levels to insulin administration is similar in both time course and extent of change to the increase in isolated fat cells of cell surface receptor levels due to insulin action. These results suggest the hypothesis that insulin stimulates the movement of cellular IGF-II/Man-6-P receptors to the cell surface, where they are proteolytically cleaved and released into serum.

The recent demonstration that a single mammalian receptor protein binds both mannose 6-phosphate (Man-6-P) and insulin-like growth factor II (IGF-II) with high affinity has suggested a multifunctional physiological role for this receptor, possibly including signal transduction. In order to better understand the functions of this receptor, we have investigated the properties of Man-6-P receptors from non-mammalian species. Receptors were affinity-purified from Triton X-100 extracts of total membranes from Xenopus and chicken liver as well as rat placenta using pentamannosyl 6-phosphate-Sepharose. The Man-6-P receptor was adsorbed to the pentamannosyl 6-phosphate-Sepharose and specifically eluted by Man-6-P in all three species, as evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining. When the purified receptors from these three species were cross-linked to 125I-IGF-II with disuccinimidyl suberate, only receptors isolated from rat membranes were affinity-labeled. To further evaluate the properties of these Man-6-P receptors, binding of 125I-rat-IGF-II and 125I-chicken Tyr-Gly-Thr-Ala-IGF-II to purified receptors from Xenopus, chicken, and rat was evaluated by polyethylene glycol precipitation. Only the rat Man-6-P receptor exhibited detectable binding of 125I-IGF-II. These data suggest that the emergence of a high affinity IGF-II binding site on the Man-6-P receptor occurred in evolution after the divergence of mammals from other vertebrates. Thus, the biological actions of IGF-II in chickens and frogs appear to be initiated by the type I IGF receptor.

The insulin-like growth factor (IGF)-II/mannose 6-phosphate (Man-6-P) receptor present in mammalian tissues as an apparent molecular mass = 250 kDa glycoprotein has recently been detected in fetal rat serum in a lower molecular mass form (240 kDa). In the present studies the serum receptor was affinity labeled with 125I-IGF-II after its adsorption onto pentamannosyl 6-phosphate-Sepharose, demonstrating that it can also bind both ligands simultaneously. The receptors in both serum and fresh plasma exhibited the lower molecular mass compared to tissue receptors, indicating this form circulates in vivo. In order to probe the structural basis of the serum receptor's lower mass, we raised antipeptide antibodies against cytoplasmic and extracellular domains of the tissue form of the rat receptor deduced from complementary DNA clones (MacDonald, R. G., Pfeffer, S. R., Coussens, L., Tepper, M. A., Brocklebank, C. M., Mole, J. E., Anderson, J. K., Chen, E., Czech, M. P., and Ullrich, A. (1988) Science 239, 1134-1137). Peptide 22C, Glu-Glu-Glu-Thr-Asp-Glu-Asn-Glu-Thr-Glu-Trp-Leu-Met-Glu-Glu-Ile-Gln-Val- Pro-Ala - Pro-Arg, located in the cytoplasmic domain 32 residues carboxyl-terminal to the transmembrane region, and peptide 13D, Tyr-Tyr-Leu-Asn-Val-Cys-Arg-Pro-Leu-Asn-Pro-Val-Pro-Gly-Cys-Asp, located 1476 residues amino-terminal to the transmembrane domain were synthesized and used as immunogens in rabbits. IGF-II/Man-6-P receptors were first immunoprecipitated from either rat serum or a Triton X-100 extract of rat placental plasma membranes using a polyclonal antireceptor antibody. The immunoadsorbed receptors were then reduced, alkylated, electrophoresed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, blotted onto nitrocellulose, and probed with antipeptide antibodies. Anti-13D revealed the major receptor band in all the membrane and serum samples tested as well as several minor species of lower apparent mass in serum. Fetal and neonatal rat sera contained 3-4 times as much of the receptor as adult serum. In contrast, anti-22C recognized the membrane IGF-II/Man-6-P receptor but failed to recognize any of the serum receptor species. These results indicate that the serum IGF-II/Man-6-P receptor is truncated or altered in its cytoplasmic domain, consistent with the hypothesis that it is derived from cells by proteolytic cleavage.

Insulin-like growth factor II (IGF-II)/mannose 6-phosphate (Man-6-P) receptors immunoprecipitated from purified plasma membranes of 32P-labeled rat adipocytes are markedly heterogenous in their phosphorylation state. Approximately 80% of the plasma membrane receptors are solubilized in 1% (vol/vol) Triton X-100 and are phosphorylated on serine residues at a stoichiometry of approximately 0.1-0.2 mol of phosphate per mol of receptor. In contrast, 15-20% of the receptors are Triton X-100-insoluble and are phosphorylated on serine and threonine residues at approximately 4 or 5 mol of phosphate per mol of receptor. This Triton X-100-insoluble membrane subfraction contains only 5% of the total plasma membrane protein and yet contains all of the clathrin heavy chain associated with plasma membrane, as detected by immunoblotting with a monoclonal antibody. Based on the relative yields of protein in the detergent-insoluble material, IGF-II/Man-6-P receptors are concentrated approximately equal to 3-fold in this clathrin-enriched subfraction. Insulin treatment of intact cells increased the total IGF-II/Man-6-P receptors in the Triton X-100-soluble fraction of the plasma membrane, whereas no change in receptor number in the detergent-insoluble fraction was seen. However, insulin markedly decreased the phosphorylation stoichiometry of the Triton X-100-insoluble receptors. Taken together, these results indicate that insulin decreases the phosphorylation state of a highly phosphorylated subpopulation of IGF-II/Man-6-P receptors on the plasma membrane. In addition, insulin action may prevent the concentration of these receptors in a clathrin-enriched membrane subfraction.