Insulin receptors and the site of action of insulin

Insulin: its binding to specific receptors and its stimulation of DNA synthesis and 2',3'-cyclic nucleotide phosphohydrolase activity in cerebral cells cultured from embryonic mouse brain

The presence and specificity of insulin receptors was investigated in culture cells obtained from 15-16 days old embryonic mouse cerebra. Developmental studies suggested that the maximum insulin binding occurred at about 11 days in vitro (DIV). Scatchard analysis of binding data revealed two types of binding sites. One type of receptor was the high affinity type (Kd = 7.77 x 10(-9) M; number of receptor sites, Bmax = 350 fmol/mg protein) while the other type was of low affinity type (Kd = 5.75 X 10(-8)M; Bmax = 1150 fmol/mg protein). The specificity of receptors for insulin was also confirmed by showing that [125I]insulin was displaced by non-radioactive insulin but not by glucagon or growth hormone. Insulin displayed a clear dose-dependent stimulation of thymidine incorporation. It also stimulated the activity of the enzyme 2', 3'-cyclic nucleotide phosphohydrolase (CNPase), which is specifically associated with myelin produced from oligodendroglia. Thus insulin has a positive influence on the proliferation and differentiation of brain cells.

The effect of insulin on progesterone production and cellular growth in long-term cultures of human granulosa lutein cells

The direct action of insulin on human granulosa lutein cells (GLCs) in long-term cultures obtained from in vitro fertilization (IVF) cycles was investigated. Progesterone (P) secretion by GLC increased progressively in both basal and human chorionic gonadotropin (hCG; 100 mIU/ml) stimulated conditions up to 4 days in culture, and plateaued thereafter. Insulin (0.0025 mU/ml to 2500 mU/ml) had no effect on either basal or hCG stimulation during the culture period. GLC in culture formed a monolayer and multiplied at a rate of approximately once every 3 days. Neither morphology nor cell division was affected by insulin in supraphysiologic levels (25 mU/ml). These results suggest that GLC obtained from preovulatory follicles in an IVF program are already stimulated maximally by in vivo exposure to high doses of human menopausal gonadotropin (hMG)/hCG administered to the women. Contrary to its stimulatory effect on early preovulatory granulosa cells, insulin dose not affect P production, cellular morphology, or growth rate of luteinized granulosa cells.

Insulin binding and effects on pyrimidine nucleoside uptake and incorporation in cultured mouse astrocytes

Binding of 125I-insulin to primary cultures of differentiated mouse astrocytes was time-dependent, reaching equilibrium after 2 h at 22 degrees C, with equilibrium binding corresponding to 20.79 fmol/mg of protein, representing approximately 5,000 occupied binding sites/cell. The half-life of 125I-insulin dissociation at 22 degrees C was 2 min, with an initial dissociation rate constant of 4.12 X 10(-2) s-1. Dissociation of bound 125I-insulin was not accelerated significantly in the presence of unlabeled insulin (16.7 microM). Porcine and desoctapeptide insulins competed for specific 125I-insulin binding in a dose-dependent manner, whereas growth hormone, glucagon, and somatostatin did not. For porcine insulin, Scatchard analysis suggested multiple-affinity binding sites (high-affinity Ka = 4.92 X 10(8) M-1; low-affinity Ka = 0.95 X 10(7) M-1). After incubation with insulin (0.5 microM) for 2 h at 37 degrees C, increases above basal values of 254 +/- 23 and 189 +/- 34% for [3H]uridine uptake and incorporation, respectively, were observed. After incubation with insulin (0.5 microM) for 24 h at 37 degrees C, there were increases of 145 +/- 6% for [3H]thymidine uptake and 166 +/- 11% for thymidine incorporation. Basal and stimulated uridine and thymidine uptake and incorporation were inhibited by 50 microM dipyridamole. These studies confirm that mouse astrocytes in vitro possess specific insulin receptors and demonstrate an effect of insulin on pyrimidine nucleoside uptake and incorporation.

Ultrastructural evidence for the accumulation of insulin in nuclei of intact 3T3-L1 adipocytes by an insulin-receptor mediated process

Monomeric ferritin-labeled insulin (Fm-Ins), a biologically active, electron-dense marker of occupied insulin receptors, was used to characterize the internalization of insulin in 3T3-L1 adipocytes. Fm-Ins bound specifically to insulin receptors and was internalized in a time- and temperature-dependent manner. Fm-Ins was found in cytoplasmic vesicles within 5-10 min at 37 degrees C and subsequently was observed in multivesicular bodies and lysosomes. In addition, small amounts of Fm-Ins were associated with nuclei after 30 min. The number of Fm-Ins particles observed in nuclei continued to increase in a time-dependent manner until at least 90 min. In the nucleus, several Fm-Ins particles usually were found in the same general location--near nuclear pores, associated with the periphery of the condensed chromatin. Addition of a 250-fold excess of unlabeled insulin or incubation at 15 degrees C reduced the number of Fm-Ins particles found in nuclei after 90 min by 99% or 92%, respectively. Nuclear accumulation of unlabeled ferritin was only 2% of that found with Fm-Ins after 90 min at 37 degrees C. Biochemical experiments utilizing 125I-labeled insulin and subcellular fractionation indicated that intact 3T3-L1 adipocytes internalized insulin rapidly and that approximately equal to 3% of the internalized ligand accumulated in nuclei after 1 hr. These data provide biochemical and high-resolution ultrastructural evidence that 3T3-L1 adipocytes accumulate potentially significant amounts of insulin in nuclei by an insulin receptor-mediated process. The transport of insulin or the insulin-receptor complex to nuclei in this cell or in others may be directly involved in the long-term biological effects of insulin--in particular, in the control of DNA and RNA synthesis.

Correlation between the rhythms of blood insulin levels and the stimulation of spleen lymphocytes by concanavalin-A in mice

An ultradian rhythm of DNA synthesis and an in-vitro response to Concanavalin-A (Con-A) stimulation has been found in splenocytes of DBA/2 male mice, living under conditions standardized for light-synchronized periodicity. It was characterized by two peaks during the nycthemeral cycle. One of them was very sharp. In spite of a constant mode of synchronization of the animals, there were seasonal variations of the rhythm. Such variations did not, however, affect the position of the sharp peak, which occurred always at the same hour. The rhythm of incorporation of [3H]-thymidine by splenocytes, whether or not stimulated by Con-A, seemed correlated with that of blood insulin levels in mice. The possible effect of the physiological increase in blood insulin during the circadian cycle has been confirmed experimentally. Thus it seems probable that insulin would be one of the internal synchronizers .

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.

The degradation of [125I]iodoinsulin by cytosol from rat liver and the formation of high-molecular-weight products. A study by gel chromatography

Degradation of [125I]iodoinsulin by cytosol from rat liver (as judged by the appearance of perchloric acid soluble counts) occurs rapidly and generates both high- and low-molecular-weight labelled products on chromatography on AcA54 or AcA202. One of the low-molecular-weight products can be converted to a high-molecular-weight product by further incubation with fresh cytosol. Degradation is not inhibited by chloroquine, but is affected by bacitracin. In the presence of bacitracin most of the radioactivity elutes in the void volume and possibly represents interaction of undegraded insulin with other cytoplasmic components.

Erythrocyte insulin receptors remain intact through three days of storage

The status of the erythrocyte insulin receptor was investigated prior to and during storage at 4C in acid-citrate dextrose (ACD) solution. The receptors on cells that were obtained from both resting and exercised fasting subjects and stored for up to three days in ACD were unchanged as evaluated by both maximal specific hormone binding and the concentration of insulin required to one half maximally inhibit specific binding. These findings indicate, therefore, that the binding of insulin to its receptor on erythrocytes can be assayed in samples of stored blood and that this assay reflects the status of the receptor at the time of sampling.

Action of insulin at the nuclear envelope

Insulin binding sites are present on purified nuclear envelopes from liver and other tissues, and EM autoradiographs and other types of studies indicate that insulin can enter intact target cells and interact with several types of intracellular membranes, including the nuclear envelope. More recent studies indicate that insulin has direct effects on both mRNA efflux from isolated nuclei and nuclear envelope NTPase, the enzyme that regulates mRNA efflux. These studies raise the possibility, therefore, that insulin regulates mRNA levels in target cells by directly influencing nuclear membrane functions as NTPase. Since insulin does not dramatically elevate mRNA levels for all proteins, the question arises as to how insulin selectively increases mRNA for specific mRNAs. One possibility is that there is targeting of specific mRNA molecules for specific pore complexes and that insulin may only influence a certain fraction of the nuclear pores. Thus, continued investigation is needed concerning the role of polypeptide hormones such as insulin in nucleocytoplasmic exchange.

Binding, degradation, and bioactivity of insulin in primary cultures of rat hepatocytes

Hepatocytes from fasted rats, previously maintained as a monolayer in a serum-free glucagon-containing culture medium are demonstrated to provide a useful model system for the study of receptor-mediated mechanisms of insulin action. The cultured liver cells show glucagon- and insulin-responsive biological effects. These cultures show the long-term effects of insulin on the syntheses of protein ([3H]leucine incorporation), glycogen, and lipids (conversion of [3H]glucose) in a dose-dependent manner in the physiological range of insulin concentrations. The order of the effects of different analogues of insulin on each of the bioactivities studied at 37 degrees C is the same as their order to compete at 25 degrees C for binding to insulin-specific receptors. The characterization of radioactive products of 125I-insulin using gel filtration and anti-insulin A chain antibody has shown a transient accumulation of insulin A chain, indicating that the sequential pathway of insulin degradation is operative in the anchored, cultured hepatocytes. Whereas the synthesis of proteins proceeds linearly after insulin inoculation, the syntheses of glycogen and lipids first occur after a lag period of about 10 and 12 h, respectively. None of the three 125I-labeled fragments or products of 125I-insulin released in the culture mediums showed any biological activity (glycogenesis) in cultured hepatocytes although a high-molecular-weight 125I-product isolated from cells could not be tested because of its insolubility. The possibility that the insulin effects might have been mediated via a nonradioactive fragment of insulin or another chemical agent remains open.

Effect of insulin at dilution endpoint concentrations on macromolecular synthesis in normal mouse mammary and human breast cancer epithelial cells

Employing defined media conditions, the insulin sensitivities of mouse mammary gland epithelial cells in primary culture and MCF-7 human mammary epithelial cells were determined. Insulin stimulated the rates of (3H] uridine incorporation into RNA and [3H] leucine incorporation into protein in both primary mouse mammary gland epithelial cell cultures and MCF-7 cell cultures at concentrations approximating the dilution endpoint of the hormone (10-21 M). Insulin stimulated the rate of [3H] thymidine incorporation into DNA in primary mouse mammary gland epithelial cells at the dilution endpoint concentrations. However, MCF-7 cells required insulin concentrations 100-1000-times that necessary in mouse mammary epithelial cultures to elicit an increased rate of [3H] thymidine incorporation into DNA. Evidence is presented which suggests that the increased rates of uptake of 3H- uridine, [3H] thymidine and [3H] leucine into their respective precursor pools is not responsible for the apparent stimulation of RNA, DNA and protein synthesis.

The comparison of vanadyl (IV) and insulin-induced hyperpolarization of the mammalian muscle cell

Extracellularly applied vanadyl (IV) hyperpolarized the membrane potential of mouse diaphragm muscle from about -74.0 mV up to -81.7 mV. The hyperpolarizing effect of 10(-4) mol.I-1 vanadyl (IV) is comparable with hyperpolarization induced by 100 mU.ml-1 insulin. Both compounds increased the intracellular K+ concentration, the hyperpolarizing effect of vanadyl (IV) and insulin is blocked by ouabain and is unaffected by removal of K+ from the external medium. Triggering of the release of intracellular K+ associated with cellular proteins is proposed as the mechanism of vanadyl (IV) and insulin-induced hyperpolarization.

Insulin binding to the HL-60 human promyelocytic cell line

The HL-60 cell is a human leukemic promyelocyte that can be grown in liquid suspension culture. This cell line binds radiolabeled insulin in a rapid, reversible specific manner indicating that it contains cells surface insulin-binding sites. these sites show a pH optimum of 7.8-8.0. Maximal binding was achieved in approx. 60 min at 22 degrees C and was linear with cell number between 5 X 10(6) and 10(8) cells per ml. Displacement of labeled insulin by unlabeled hormone was consistent with a 'two site' model with a high affinity site of 2.71 +/- 0.55 X 10(8) M-1 and 8700 sites per cell. Release of labeled insulin from its binding site was accelerated by unlabeled insulin at temperatures above 22 degrees C, but not at 4 or 15 degrees C. We conclude that the HL-60 cell contains cell surface binding sites for insulin with properties similar to other well-described insulin receptors of other tissues.

Insulin binding sites on the nuclear envelope: potential relationship to mRNA metabolism

Insulin regulates the growth and metabolism of most tissues. The hormonal potency of insulin results, to a large extent, from its ability to regulate target cells at a variety of subcellular sites. For many years, the effects of insulin on membrane transport, enzyme activity, and protein synthesis have been studied extensively. Less attention, however, was given to how insulin regulates nuclear functions. Recently the presence of specific binding sites for insulin on nuclei and nuclear envelopes have been documented and characterized. These binding sites have biochemical characteristics that are different from insulin binding sites on the plasma membrane. Moreover, direct in vitro effects of insulin on messenger RNA (mRNA) metabolism have recently been reported. These effects include the stimulation of mRNA efflux from intact nuclei, and stimulation of nucleoside triphosphatase activity (NTPase), the enzyme that regulates mRNA efflux. Thus, significant insight is now being gained concerning the action of insulin on the cell nucleus.

The effects of insulin-glucose on microappendages of the plasmalemma in the early chick embryo

Chick embryos incubated for 18-24 hours (stages 4-6) were immersed for 3-5 minutes in Hanks' Balanced Salt Solution containing either insulin (10(-9) M) or D-glucose (10(-1) M) or a mixture of both. The ventral and dorsal surfaces of the entoderm and the ventral aspect of the mesoblast were examined in a scanning electron microscope. D-glucose alone or insulin alone produced negligible change. When combined these ingredients generated highly pleomorphic microappendages of the plasmalemma. The ventral surface of the entoderm was characterized by a large number of microvilli, both long and short. Concentrations of microvilli and blebs appeared in recesses at cell margins. Some of these represented mesoblastic protrusions through intercellular apertures in the entoderm. The dorsal entodermal surface developed stellate clusters of microvilli and blebs. The ventral aspect of the mesoblast became the most pleomorphic area of all. It possessed a variety of microappendages including large populations of microvilli and blebs that resembled the protrusions observed from the subblastodermic cavity. There were also stellate clusters similar to those on the dorsal entodermal surface. The same mesoblastic area in controls was virtually free of microappendages. The mesoblastic protrusions, occurring as they do prior to an established circulation, are interpreted as a "feeding" phenomenon.

Thyroid hormone metabolism in primary cultured rat hepatocytes. Effects of glucose, glucagon, and insulin

Primary cultured adult rat hepatocytes were used to study regulation of thyroid hormone deiodination. Control studies showed that these cells survived for at leas 4 d, during which time they actively deiodinated both the phenolic (5'-) and non-phenolic (5-) rings of L-thyroxine (T4),3,5,3'-triiodo-L-thyronine, and 3,3',5'-triiodothyronine. Increasing the substate concentration caused a decrease in fractional iodide release and a corresponding increase in conjugation with sulfate and glucuronide. Propylthiouracil strongly inhibited the 5'-deiodinase activity and caused only a slight decrease in 5-deiodinase activity. Thus, these monolayer-cultured cells preserved many of the properties of normal hepatocytes. Incubation with combinations of insulin, glucagon, and/or glucose for 5 h showed that insulin stimulated T4 5'-deiodination, whereas glucagon inhibited the insulin stimulation but had no effect in the absence of insulin. Glucose had no effect and did not alter the effect of the hormones. The insulin-enhanced deiodination increased between 1 and 5 h, which suggests that the previous inability to demonstrate an insulin effect was due to the short survival of the in vitro liver systems used in those studies. The present data suggest that the inhibition of T4 5'-deiodination observed during fasting, and its restoration by refeeding, may be related to the effects of feeding on insulin and glucagon release rather than on glucose per se.

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.

Binding of [125I]insulin to specific receptors and stimulation of nucleotide incorporation in cells cultured from rat brain

The occurrence of insulin receptors and biological responses to insulin has been investigated in trypsin-dissociated fetal rat brain cells maintained in culture for 8 days. Binding of [125I]insulin to brain cells in culture was time- and pH-dependent and 85--90% specific. Porcine insulin competed for [125I]insulin binding in a dose-dependent manner. Unrelated polypeptides, including angiotensin II, glucagon, bovine growth hormone, and bovine prolactin did not compete for [125I]insulin binding. The half-life of [125I]insulin dissociation from receptors at 24 degrees C was 15 min and a plot of In[B/Bo] vs time suggested two dissociated rate constants of 2.7 X 10(-4) sec-1 and 5.0 X 10(-5) sec-1. Scatchard analysis of the binding data gave a curvilinear plot which may indicate negative cooperativity or the occurrence of both high affinity (Ka = 2 X 10(11) M-1) and low affinity (Ka = 4 X 10(10) M-1) sites. Of the estimated total of 4.9 X 10(4) binding sites per cell, 28--30% appear to be high affinity sites. Incubation of cultures with insulin caused a time- and dose-dependent stimulation of [3H]thymidine and [3H]uridine incorporation into TCA-precipitable material. Maximum stimulation of thymidine incorporation (2--5-fold) occurred 11 h after incubation with 167 nM insulin. The same concentration of insulin caused a 2.2-fold increase in [3H]uridine incorporation in 2 h. These results indicate that cells cultured from rat brain contain specific insulin receptors capable of mediating effects of insulin on macromolecular synthesis in the central nervous system.

Lysosomal enzymes in experimental diabetic cardiomyopathy

The present study examines the role of cardiac lysosomal enzymes in the pathogenesis of the cardiomyopathy that develops in genetically diabetic C57BL/KsJ db+/db+ mice. Db+/db+ mice and littermate controls were sacrificed as age-matched pairs between 5-26 weeks of age. C57BL/6J ob/ob mice and littermates served as other controls. The hearts were excised, homogenized, and the following enzymatic activities measured: N-Acetyl-beta-glucosaminidase, N-acetyl-beta-galactosaminidase, beta-glucosaminidase, aryl sulphatase, alpha-mannosidase, alpha-glucosidase, beta-galactosidase, beta glucosidase, total p-nitrophenyl phosphatase, acid phosphatase and 5'-phosphodiesterase type IV. There is a progressive decrease in cardiac lysosomal enzyme activities of db+/db+ mice for the period 5-21 weeks of age. All enzyme activity is depressed significantly during the 9-21 week interval with beta-glucuronidase, aryl sulphatase and beta-glucosidase decreased about 40-50%. The decrease in lysosomal enzyme activity can explain the accumulation of large residual bodies and interstitial material in the myocardium of the db+/db+ animals

Rapid regulation of plasma membrane insulin receptors

Injection IP of insulin at a dosage of 1 microgram/g body weight into normal rats produced a rapid rise in serum insulin levels from < 1 to 298 ng/ml, and a rapid decrease in specific 125I-insulin binding to its receptors in purified liver plasma membranes. A fall in binding was seen as early as 10 minutes after injection and binding remained decreased for up to 60 min. At 10 min, 125I-insulin binding had fallen to 59% of controls; in contrast, 125I-glucagon binding remained unchanged. Extraction of these plasma membrane followed by radioimmunoassay for insulin did not reveal appreciable amounts of exogenous insulin. The 125I-insulin dissociation rate from plasma membranes of control and insulin treated rats was the same, also indicating a lack of exogenous insulin. Scatchard analyses indicated that the decreased binding seen after insulin injection was due primarily to a change in the number of insulin receptors and not their affinity. These studies suggest, therefore, that high doses of insulin in vivo can rapidly regulate the number of plasma membrane insulin receptors in liver.

Anti-insulin receptor antibodies mimic the effects of insulin on the activities of pyruvate dehydrogenase and acetylCoA carboxylase and on specific protein phosphorylation in rat epididymal fat cells

Previous studies have shown that autoantibodies against insulin receptors found in certain patients with severe insulin resistance stimulate glucose transport and metabolism in fat cell and muscle preparations. The present studies show that preincubation of rat epididymal adipose tissue with 1 : 1000 dilution on one such serum results in a two fold increase in the initial activities of pyruvate dehydrogenase and acetylCoA carboxylase. These increases are similar to the maximum effects of insulin. Incubation of isolated fat cells with the serum at the same concentration also resulted in the increased phosphorylation of three intracellular proteins with subunit molecular weights of 130,000, 35,000 and 22,000 to the same extent as observed with insulin. These findings lend further support to the view that the short term effects of insulin do not involve the entry of the insulin molecule (or part thereof) into cells of target tissues.

Prolonged effect of insulin on glucose uptake by rat skeletal muscle

The fate of insulin, as it relates to its action on skeletal-muscle glucose uptake, was studied in non-cyclically perfused rat hindlimbs. Insulin (1m-i.u./ml) with and without (125)I-labelled insulin was infused intra-arterially for 5 or 6min. Net glucose uptake and the release of (125)I-labelled insulin into the venous effluent were evaluated by arteriovenous-difference measurements for an additional 24-32min. The infusion of insulin for 5min promoted glucose uptake, an effect that persisted throughout a subsequent 25min of perfusion in the absence of insulin. The addition of insulin antibody to the perfusate in the presence of insulin blocked the action of insulin on glucose uptake, but it failed to alter insulin action if the muscle tissue had been exposed to insulin before addition of antibody. When (125)I-labelled albumin was infused for 6min, venous effluent radioactivity decayed rapidly and remained HClO(4)-insoluble and there was no significant tissue retension of radioactivity. Comparable experiments in which (125)I-labelled insulin was infused for 6min revealed that the venous effluent radioactivity decayed more slowly, a significant amount of the (125)I-labelled insulin appeared as fragments (HClO(4)-soluble) and there was a significant retention of radioactivity in the tissue. Radioactivity in muscle tissue biopsies obtained 28min after infusion of (125)I-labelled insulin was associated largely with intact insulin and a peptide of mol.wt. 2400. The total radioactivity retained in the muscle at this time was 7% of the amount infused. An insulin bolus (1i.u.) failed to increase the discharge of this tissue-associated radioactivity. These results suggest that insulin and a product of insulin metabolism persists in muscle tissue long after the arterial presence of insulin ends. This tissue residence and processing of insulin may be important components of insulin's prolonged action on glucose uptake by skeletal muscle.

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.

Entry of insulin into human cultured lymphocytes: electron microscope autoradiographic analysis

Electron microscope autoradiographs were prepared of IM-9 human cultured lymphocytes incubated with iodine-125-labeled insulin. With the use of [125I]insulin and Ilford L-4 emulsion, the technique had a resolution half-distance of approximately 0.085 micrometer. Autoradiographs revealed a time-dependent entry of insulin into the cell interior that was maximal after 30 minutes of incubation. At this time point nearly 40 percent of the [125I]insulin was in the interior of the cell at a distance 1 micrometer or greater from the plasma membrane. Grain distribution and volume density analyses revealed that the intracellular insulin was concentrated in the endoplasmic reticulum and nuclear membrane.