Intracellular polypeptide hormone receptors. The demonstration of specific binding sites for insulin and human growth hormone in Golgi fractions isolated from the liver of female rats

Transmembrane delivery of polypeptide hormones bypassing the intrinsic cell surface receptors: a conjugate of insulin with alpha 2-macroglobulin (alpha 2M) recognizing both insulin and alpha 2M receptors and its biological activity in relation to endocytic pathways

125I-labeled insulin has been cross-linked to alpha 2-macroglobulin (alpha 2M) via a disulfide bond. The resulting insulin-alpha 2M conjugate carried 2.2 insulin moieties per mole of alpha 2M and was able to deliver insulin into rat hepatoma cells H35 and HTC. The insulin delivery was mediated predominantly through alpha 2M receptors and 2 h after binding it was found in the lyposomal fractions in the form of conjugate. When the conjugate was applied to rat hepatoma cells it stimulated activity of tyrosine aminotransferase (TAT) with a potency one-half that of native insulin. Hepatoma cells which were treated with conjugate in the presence of bacitracin were also stimulated for TAT activity. Since bacitracin completely inhibited the alpha 2M binding to its receptors, but inhibited conjugate binding by only 80%, this stimulation must have resulted from the remaining binding of conjugate. These results indicate that the insulin-alpha 2M conjugate was biologically active if it bound to insulin receptors, but that the conjugate bound and internalized through alpha 2M receptors did not act as a mediator for TAT activation. Our results using Percoll density gradients indicate a difference in intracellular processing between insulin, alpha 2M and the conjugate. Mechanisms of action of the conjugate are discussed in relation to the receptor-mediated endocytic pathways.

Intracellular transformation of prolactin following internalization into rat liver

We investigated prolactin (PRL) degradation in rat liver lysosomes both in vivo and in vitro. In previous studies we showed that, in addition to the Golgi apparatus, PRL is internalized towards lysosomes and light, lysosome-like vesicles which we identified as 'prelysosomes'. Injected [125I]oPRL that localized in lysosomes and prelysosomes at times varying from 0 to 45 min showed significant differences from fresh and plasma membrane- (PM) or Golgi-bound hormone. First, it was more easily dissociable by 3 M MgCl2 than Golgi- but less than PM-bound [125I]oPRL. Second, it was only in lysosomal fractions that, as time following injection increased, a significant part of dissociable radioactivity became non-TAC-precipitable. When MgCl2-extracted [125I]oPRL was subjected to gel filtration on a Sephadex G-75 fine column, some of the radioactivity, and especially that extracted from prelysosomal or lysosomal fractions, eluted as a high molecular weight (HMW) entity, most co-migrated with fresh [125I]oPRL, and a little was found in small fragments. Only the central peak had any rebinding activity, which was comparable to that of fresh hormone. In an in vitro study we incubated [125I]hGH with lysosomal fractions for 16 h at 25 degrees C. After centrifugation, an aliquot of supernatant hormone was assayed for its binding capacity to standard receptor preparations and the rest subjected to gel filtration. Peak fractions were also tested in binding assay. [125I]hGH that had been in contact with prelysosomes lost almost all of its ability to bind to standard receptors and totally migrated in the HMW peak, at the void volume of the column.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of epidermal growth factor (EGF) and insulin to human liver microsomes and Golgi fractions

Microsomes and Golgi fractions were isolated from 13 human liver samples without local malignancy. Binding of insulin to microsomes (per cent per 0.5 mg protein) was 14.4 +/- 7.9% with two classes of receptors: K1 = 1.4 nM, R1 = 0.28 pmol/mg; K2 = 8.1 nM, R2 = 0.62 pmol/mg. The binding was insignificantly lower than in rats. Binding of EGF was only 3.4 +/- 1.7% with two classes of receptors: K1 = 1.4 nM, R1 = 0.06 pmol/mg; K2 = 10.8 nM, R2 = 0.22 pmol/mg; the binding was much lower than in rats (26.3 +/- 5.8%). Binding of insulin to Golgi fraction (per cent per 0.1 mg protein) was 5.5 +/- 0.4% with straight line Scatchard plot; Kd = 5.6 nM, Ro = 3.06 pmol/mg; it was only half of that found in rats. In one case of hepatoma, the binding of insulin to microsomes was normal but that of EGF very low.

Effect of angiotensin II on RNA synthesis by isolated nuclei

Peptide hormones are known to bind to cell surface receptors as the first step in the generation of their effects on target tissues. However, it remains uncertain whether internalized hormone might also play a role in the production of longterm or trophic effects of peptide hormones. Because the peptide hormone angiotensin II appears to be internalized by target cells, we studied the effect of this peptide on isolated hepatic nuclei. At both 5 X 10(-7)M and 5 X 10(-9)M, angiotensin II significantly increased RNA synthesis. This effect was not mimicked by Sar1-Ala8-angiotensin II (saralasin) or the unrelated nonapeptide teprotide.

Down-regulation of prolactin receptors in the liver, mammary gland and kidney of female virgin rat, infused with ovine prolactin or human growth hormone

Down regulation of prolactin (PRL) receptors resulting from i.v. infusion of oPRL or human growth hormone (hGH) into female virgin rats was demonstrated. A decrease of over 85% in the number of free receptors was achieved within 15 - 30 min using infusion of oPRL or hGH at 25 micrograms/h and remained at this level until the end of infusion. Ovine growth hormone or recombinant bovine growth hormone at ten-fold higher concentration had no effect at all. The decrease in the specific binding resulted from a lower number of binding sites and not from change in the dissociation constants. The decrease in the total receptors in the liver was more gradual and leveled off at 40 - 50% of the initial value. Our results suggest that a change in blood PRL or hGH level may lead to a new steady state in the number, occupancy and distribution of prolactin receptors.

Solubilization, gel filtration and sedimentation behaviour of prolactin receptors from human ovarian tissue

Receptors for 125I-labelled human prolactin have been identified in the crude membrane fraction isolated from human ovarian tissue. The non-ionic detergent Triton X-100, has been used to solubilize the membrane fraction. The presence of the receptor in the detergent extract was demonstrated by gel filtration and sucrose density gradient centrifugation. The binding was time-temperature dependent, being maximal at 23 degrees C after 15 h of incubation. Large amounts of other peptide hormones did not inhibit the binding of 125I-labelled human prolactin. The binding Scatchard analysis demonstrated that the affinity of the soluble receptor (Ka 1.13 +/- 0.15 X 10(10) M-1) for the labelled hormone was slightly greater than that of the crude membrane fraction (Ka 0.91 +/- 0.12 X 10(10) M-1). The binding capacity of the solubilized receptor was also significantly greater than that seen in the particulate before solubilization. The apparent Stokes radius of the solubilized receptor was estimated to be 57 A and that the hormone-receptor complex 60 A. The sedimentation coefficient of the solubilized receptor was 7.0 +/- 0.1 s, whereas that of the hormone-receptor complex was 7.5 +/- 0.2 s.

Effects of microtubule-disrupting agents on insulin binding and degradation in isolated cardiocytes from adult rat

Isolated muscle cells from adult rat heart have been used to study the effects of microtubule disruptive drugs on the maintenance of steady-state insulin binding to cardiac insulin receptors. Vinblastine, vincristine and podophyllotoxin significantly inhibited insulin binding (25-50%) in the presence of insulin (10(-8) mol/l). The effect of vinblastine was found to be time- and temperature-dependent and to be dependent on the amount of insulin bound to the cell. In the presence of cycloheximide (0.1 mmol/l) insulin binding decreased by 30%; this effect was found to be additive to the action of vinblastine. Treatment of cells with vinblastine significantly reduced the low-molecular mass material produced by receptor-mediated degradation of insulin. This effect was not additive to that of the lysosomotropic agent chloroquine. The results suggest involvement of microtubules in the intracellular transfer of insulin receptors from and to the plasma membrane.

Isolation of mitochondria, lysosomes, and microsomes from the rat ventral prostate with a note on inverted microsomal vesicles

A procedure is presented for the isolation of lysosomes, mitochondria, and microsomes from the rat ventral prostate with relatively good yield. Homogenization was performed with a Polytron homogenizer or in combination with the Potter-Elvehjem device. Reasonably pure mitochondria and lysosomes could only be obtained using a Metrizamide gradient, whereas it was possible to prepare pure microsomal fractions by differential centrifugation in sucrose. The purity of the lysosomes and mitochondria was 90 and 85%, respectively, as judged by the presence of different marker enzymes. These findings were confirmed by ultrastructural analyses. Electron micrographs of the isolated lysosomes showed intact lysosomes surrounded by a single membrane. The lysosomes contained intramatrical vesicles with lipid-like material. Vesicles derived from the endoplasmic reticulum in the microsomal fraction ranged from 70 to 90% depending on the centrifugal force used to sediment the mitochondrial fraction. Electron micrographs of the microsomal fraction showed that about 40% of the vesicles were inverted and turned "inside-out", i.e., having their ribosomes attached to the inside of the vesicles. By fractionation of ethylenediaminetetraacetate treated microsomes on a sucrose gradient a partially purified fraction was isolated which consisted of 65% of inverted microsomes.

Evidence for the existence of gonadotropin receptors in the nuclei isolated from rat ovary

Specific binding of radiolabeled human chorionic gonadotropin (hCG) to nuclei isolated from pseudopregnant rat ovaries was studied. Incubation of cultured luteal cells or isolated nuclei with fluorescein isothiocyanate conjugated hCG showed concentration of fluorescence in the nuclear region. Isolated nuclei exhibited saturable high affinity binding of radiolabeled hCG with an apparent Kd of 3.42 X 10(-10) M. The binding was inhibited by increasing concentrations of unlabeled hCG. Under dissociating conditions, the bound hCG was dissociated from the nuclei. However, unlike the plasma membranes, the hCG bound to nuclei was not degraded before dissociation. Radiolabeled hCG bound to the nuclei could also be dissociated by brief exposure to MgCl2 or acidic incubation medium. The bound hCG was not extractable with 4M KCl or 2% Triton X-100. The available evidence suggest that nuclear receptors are distinct from plasma membrane receptors for hCG.

Microsomal opiate receptors differ from synaptic membrane receptors in proteolytic sensitivity

We have found that opiate receptors in smooth microsomal fractions differ from synaptic membrane-associated receptors in proteolytic sensitivity. With 3 proteases of different substrate specificities (trypsin, chymotrypsin and S. griseus protease) smooth microsomal opiate receptors from rat brain were consistently less sensitive to limited proteolysis than were synaptic membrane receptors. Thiamine pyrophosphatase, a luminal Golgi membrane marker enzyme, exhibited a similar resistance to S. griseus protease in microsomal preparations, while microsomal Na+/K+-ATPase (ouabain-sensitive) was readily destroyed by trypsin. We also discovered that smooth microsomal opiate receptors co-migrate with both Golgi membrane and endoplasmic reticulum marker proteins on equilibrium density gradients under isopycnic conditions. Electron microscopic examination of the Golgi-enriched fraction showed the typical cisternae frequently associated with isolated Golgi membranes. Synaptic junctions, presynaptic membranes, myelin and mitochondria were conspicuously absent from this fraction. Since the microsomes isolated in vitro showed similar topography to those in vivo, the binding sites for opiates could be localized on the luminal surface membranes of the microsomal fractions. The exquisite sensitivity of synaptic membrane opiate receptors to proteolysis suggests that these receptors are found on the extracellular surface of the synaptic junction.

Receptor mediated endocytosis of polypeptide hormones: mechanism and significance

Polypeptide hormones bind to specific receptors on the surface of cells. Under certain conditions they localize to specific microdomains of the membrane, i.e., microvilli and coated pits. At physiologic temperature the ligand is internalized by a process of adsorptive endocytosis. This process involves several intracellular membrane bounded structures including coated vesicles, non-coated vesicles and lysosomal structure. These events provide a simple and general mechanism for removal of the ligand from the cell surface in order to terminate its signal. Linked to this process is a mechanism for surface receptor regulation. Thus, the concentration of hormone receptors on the cell surface is a function of the synthetic rate, internalization rate and the rate of recycling of surface membrane.

Insulin degradation by insulin target cells

Recent findings illustrate the complexities associated with the interaction between insulin and its target cells. These results suggest that the processes involved in insulin action and those involved in insulin degradation may have certain steps in common. Both apparently begin when insulin binds to the insulin receptor. The next step is unknown but it ultimately leads to the internalization of the hormone before insulin dissociates from the cell surface. Furthermore, internalization appears to be a requirement for efficient degradation of insulin since the vast majority (perhaps all in certain cells) of the degrading activity is intracellular. Internalization may not be required to produce certain actions of the hormone, however, and the two processes may diverge at the point. It is not clear how insulin enters the target cell other than the process appears to be receptor-mediated. Also, further work is needed to more fully characterize the vesicles that contain internalized insulin. Finally, the actual location of insulin degradation and the enzyme(s) involved need further study, especially to clarify the relative contributions of lysosomes, cytosolic protease, and GIT to physiological insulin destruction. An understanding of the overall process of insulin degradation is required for a complete description of the physiologic disposition of the hormone at the target cell. Moreover, this system has subtle control mechanisms that may have important implications for the management of diabetes and other endocrine and metabolic disorders.

The presence of gonadotropin binding sites in the intracellular organelles of human ovaries

The nuclei (N), plasma membranes (PM), mitochondria-lysosomes, rough endoplasmic reticulum, and combined (light, medium, and heavy) Golgi (G) fractions were isolated from human ovaries. The purities of these fractions were evaluated by assays of appropriate marker enzymes, which revealed that some fractions were very pure but that others had minor contamination. When tested, all of the fractions exhibited 125I-labeled human chorionic gonadotropin (125I-hCG)-specific binding. This intracellular 125I-hCG binding was not due to PM contamination because: (1) N, which had no detectable 5'-nucleotidase (5'-NE) activity, a marker for PM, exhibited 125I-hCG-specific binding; (2) the G, which had only a fraction of the 5'-NE activity of PM, exhibited as much binding as PM; and (3) the ratios between specific 125I-hCG binding and 5'-NE activity in other fractions were not the same as for PM. They should have been the same if PM contamination was responsible for the 125I-hCG binding observed in other organelles. In conclusion, our results demonstrate that gonadotropin-binding sites are present in various intracellular organelles as well as in PM of human ovaries.

Water soluble receptors for human growth hormone from rabbit liver

Soluble receptors that bind human growth hormone have been prepared by incubation of liver membranes form pregnant female rabbits in 1 mM Tris buffer (ph 7.5 or 9.0) at 4 degrees C. Up to 29% of the growth hormone binding sites could be solubilized within 48 hours. The kinetics of binding of human growth hormone to the soluble receptor, the hormonal specificity and the binding parameters calculated by Scatchard analysis (Ka 2.2 x 10(9) M-1, capacity 409 fmole/mg) were essentially unchanged compared with those for the parent membrane-associated (particulate) receptor. Gel filtration on Ultrogel AcA22 indicated that the major binding peak eluted at a molecular weight of 300,000 daltons. Specificity studies showed that the soluble binding sites had a moderately high affinity for ovine prolactin (Ka integral of 1 x 10(8) M-1), but negligible affinity for insulin. Although aqueous extraction gives a lower yield of binding sites for human growth hormone than detergent extraction, it nevertheless avoids some of the problems associated with use of detergents and should facilitate the subsequent purification of the receptor in a relatively unaltered state. It may also have applicability for solubilization of other hormone receptor systems.

Insulin action

Our understanding of the molecular basis of insulin actin remains incomplete, but important new insights have been achieved recently. All available evidence to date indicates that intracellular signalling by the hormone results from its initial interaction with specific cell surface receptors. Insulin receptors from all tissues studied to date appear to be minimally composed of two Mr 125,000 subunits denoted as alpha and two Mr 90,000 subunits denoted as beta. The beta subunit is extremely sensitive to proteolytic cleavage near the center of its amino acid chain. The four subunits are linked together by disulfide bonds to give a symmetrical configuration with a stoichiometry of (alpha-s-s-beta)-s-s-(alpha-s-s-beta). This structure is remarkably similar to the general subunit composition of immunoglobulin G molecules and provides a structural basis for the postulate that this minimum insulin receptor structure may be divalent for binding hormone. A second area of recent progress involves the successful generation of a soluble factor or factors by insulin that are capable of modulating the activity of insulin-sensitive enzymes such as pyruvate dehydrogenase, glycogen synthase and cyclic adenosine monophosphate (cyclic AMP)-dependent protein kinase in cell-free systems. Indirect evidence indicates that the putative mediator or mediators of insulin action exhibits properties expected of a low molecular weight peptide, including destruction by proteases. The data available are consistent with the hypothesis that insulin-receptor interaction leads to the activation of a membrane protease that catalyzes the release of a peptide mediator or mediators of insulin action.

Effects of lysosomotropic agents, cytochalasin B and colchicine on the "down-regulation" of prolactin receptors in mammary gland explants

Effects of lysosomotropic agents and drugs which disrupt the cytoskeleton on prolactin receptor levels were studied in organ culture of rabbit mammary glands. As we have shown previously, prolactin is able to induce a down-regulation of its own receptor levels. In the present experiments, lysosomotropic agents, chloroquine, ammonium chloride and methylamine, in the presence of prolactin are capable of almost completely preventing this down-regulation. Interestingly, these lysosomotropic agents alone could increase the level of prolactin receptors, which confirms that the degradation of prolactin receptors occurs in the lysosomes and that this is a rapid process which can proceed in the absence of prolactin. The almost complete lack of effect of microfilament (cytochalasin B) or microtubule (colchicine) disrupting drugs seems to indicate that the cytoskeleton is not involved in the down-regulation process.

Comparative localization of prolactin-binding sites in different rat tissues by immunohistochemistry, radioautography and radioreceptor assay

Prolactin was localized in various rat tissues, using two different histological approaches: immunohistochemistry and autoradiography. Following injection of ovine prolactin (oPRL) to intact rats, oPRL was localized in the proximal convoluted tubules (PCT) of the kidney using an immunoperoxidase technique. Diffuse and weak staining was observed in the liver while no staining was observed on sections of adrenal, testis and brain. Incubation of [125I]oPRL on frozen cryostat tissue sections followed by radioautographic processing revealed specific radioautographic grains on cells of convoluted tubules of the kidney. Specific grains were also seen on hepatocytes of the liver, cells of the adrenal cortex, interstitial cells of the testis and epithelial cells of the choroid plexus of the brain. The amount of specific radioautographic grains seen per tissue agreed well with in vitro binding values of [125I]oPRL to crude membrane preparations in the same tissues. Our observations clearly show that in the rat specific binding sites for prolactin are present on cells of the proximal convoluted tubules of the kidney, hepatocytes of the liver, glandular cells of the zona reticularis, zona fasiculata and zona glomerulosa of the adrenal, interstitial cells of the testis and epithelial cells of the choroid plexus. These studies demonstrate the necessity of using more than one morphological approach in the identification of prolactin-binding sites in different tissues at the cellular level.

Lactogenic receptor regulation in hormone-stimulated steroidogenic cells

Receptor sites for lactogenic hormones such as prolactin (PRL), human growth hormone (HGH), and placental lactogens, are widely distributed in mammalian tissues, including mammary glands, steroid-secreting cells of the adrenal, testis, and ovary, and target cells of steroid hormone action such as liver, prostrate, and kidney. Although the biological functions of lactogen binding sites remain uncertain, a relationship between prolactin and lipoprotein metabolism is implied by the occurrence of prolactin receptors in steroidogenic cells of the gonads and adrenal, and by the ability of prolactin to increase esterified cholesterol in the testis. Recently, loss of testicular prolactin receptors has been observed following elevation of circulating luteinising hormone (LH) concentrations by the gonadotropin releasing hormone (GnRH) and its agonist analogues. The hormone dependence of lactogen receptor sites in steroid-secreting cells was further analysed in rat testis, ovary, and adrenal glands after treatment with the respective trophic hormones, gonadotropin and ACTH. In each of these tissues, rapid and transient loss of lactogen receptors was observed after trophic hormone stimulation. These findings indicate that increased turnover of lactogen receptors is an important component of the target-cell response, and suggest that prolactin receptors might be involved in the transport of lipoprotein precursors for steroid biosynthesis.

Studies on the nature of interaction between follicle-stimulating hormone and the testicular receptors

Two previous reports from this laboratory showed that the binding of 125I-labeled human choriogonadotropin and 125I-labeled human luteinizing hormone to rat testicular receptors is partially irreversible and the binding parameters obtained from Scatchard analysis of the data can be drastically altered simply by changing the reaction volume of the binding assays (Chen, C.J.H., Lindeman, J.G., Trowbridge, C.G. and Bhalla, V.K. (1979) Biochim, Biophys. Acta 584, 407--435; Bhalla, V.K., Trowbridge, C.G., Chen, C.J.H., Lindeman, J.G. and Rojas, F.J. (1979) Biochim, Biophys. Acta 584, 436--453). It is reported herein that the binding reaction between follicle-stimulating hormone and testicular receptors displays very similar characteristics. The results support the previous conclusion that receptor concentrations fluctuate in the membranes and that the extent of their loss from tissue membranes in vitro is dependent upon time and temperature of incubation, the volume of buffer present, and the quantity of hormone used.

Effects of lysomotropic agents, and of microfilament- and microtubule-disrupting drugs on the activation of casein-gene expression by prolactin in the mammary gland

The organ-culture technique was used to investigate the effects of lysomotropic agents (NH4Cl and chloroquine) and of modifiers of microfilaments (cytochalasin B) and microtubules (colchicine) on the induction of casein synthesis and the accumulation of casein mRNA by prolactin in the rabbit mammary gland. Neither chloroquine nor NH4Cl altered the lactogenic action of prolactin. Cytochalasin B attenuated the response to prolactin in terms of casein synthesis. However, this drug did not hamper the accumulation of casein mRNA. Colchicine exhibited a marked specific inhibitory effect on the induction of casein synthesis. It also prevented the accumulation of casein mRNA. These results suggest that a putative degradation of the internalized prolactin--receptor complex by lysosomes is not strictly involved in prolactin action. In addition, the integrity of the microfilaments seems unnecessary in the process of casein-gene activation by prolactin. By contrast, the integrity of the microtubule network seems absolutely necessary to ensure the transmission of prolactin information to the nucleus.

Inhibition of hepatic chylomicron remnant uptake in the cholestatic rat

[3H]retinal- or [3h]cholesterol- and [14C]fatty acid-labeled chylomicrons and chylomicron remnants were injected intravenously into normal rats and rats subjected to partial or total obstruction of the bile duct 8 or 48 h earlier. The clearance from plasma of the [3H]retinyl and the [3H]cholesteryl esters in 30-120 min was markedly decreased in rats with total obstruction, and the uptake of 3H by the liver was significantly less than in control rats. The elimination of the [14C]triacylglycerol of the native chyle lipoprotein was not significantly affected, and the delayed plasma clearance was seen also when the [3H]lipid ester radioactivity was injected as chylomicron remnants prepared in vitro. If only one of the two main bile duct branches was ligated, the uptake of radioactivity did not differ markedly in the obstructed and unobstructed part of the liver and only small effects on the plasma disappearance were seen. The defective clearance of the chylomicron remnants was thus not related to the obstructed bile flow per se but rather to the consequences of the total obstruction, such as the high plasma bile acid level or the accumulation of abnormal lipoproteins in plasma.

Polypeptide growth factors: some structural and mechanistic considerations

Polypeptide growth factors ae substances that stimulate an increase in cell size and/or cell number during embryonic development. In some cases, they have a similar effect on tissues in the mature organism where they function as "maintenance" factors to sustain cell viability. While their profound impact on cell behavior is well recognized, their relationship to other regulators of cell function has remained generally ill-defined. However, the developing appreciation of their hormone-like behavior suggests that they may be conveniently grouped with many other endocrine agents to form a broader group of secondary hormones. The utility of the classification is illustrated by the insulin-related family of molecules. It also serves to emphasize the similarities in function shared by many of these substances including trophic stimulation and modulation of gene expression. Internalization, though, appears to be another common feature. However, whether the uptake of the growth factor mediates an intracellular action or is designed solely to regulate responsiveness at the cell surface and/or degradation remains an important unanswered question. A brief review of two growth factors (nerve growth factor and epidermal growth factor) serves to outline the possible functions that may be served by this endocytotic process.