Characterization of the chicken muscle insulin receptor

Insulin receptors are present in chicken skeletal muscle. Crude membrane preparations demonstrated specific 125I-insulin binding. The nonspecific binding was high (36-55% of total binding) and slightly lower affinity receptors were found than are typically observed for crude membrane insulin binding in other chicken tissues. Affinity crosslinking of 125I-insulin to crude membranes revealed insulin receptor alpha-subunits of Mr 128K, intermediate between those of liver (134K) and brain (124K). When solubilized and partially purified on wheat germ agglutinin (WGA) affinity columns, chicken muscle insulin receptors exhibited typical high affinity binding, with approximately 10(-10) M unlabeled insulin producing 50% inhibition of the specific 125I-insulin binding. WGA purified chicken muscle insulin receptors also exhibited insulin-stimulated autophosphorylation of the beta-subunit, which appeared as phosphorylated bands of 92- and 81K. Both bands were immunoprecipitated by anti-receptor antiserum (B10). WGA purified membranes also demonstrated dose-dependent insulin-stimulated phosphorylation of the exogenous substrate poly(Glu,Tyr)4:1. However, unlike chicken liver, chicken muscle insulin receptor number and tyrosine kinase activity were unaltered by 48 hr of fasting or 48 hr of fasting and 24 hr of refeeding. Thus, despite the presence of insulin receptors in chicken muscle showing normal coupling to receptor tyrosine kinase activity, nutritional alterations modulate these parameters in a tissue-specific manner in chickens.

Differential expression of alternative 5' untranslated regions in mRNAs encoding rat insulin-like growth factor I

Rat insulin-like growth factor I (IGF-I) cDNAs contain three alternative 5' untranslated sequences (termed class A, B, and C), which are associated with an identical coding region for the mature IGF-I peptide. A solution hybridization/RNase protection assay was used to simultaneously quantitate the relative abundance of IGF-I transcripts with the different 5' untranslated regions. In all the tissues studied, transcripts with the class C 5' untranslated region were most abundant. In contrast, both class A and B transcripts were tissue specific. Class A transcripts were present in moderate abundance in liver; in low abundance in kidney, lung, testes, and stomach; and were undetectable in muscle, heart, and brain; whereas class B transcripts were detected only in liver. These three classes of 5' untranslated region were also regulated independently by growth hormone. In liver, heart, kidney, and lung, growth hormone increased the abundance of class C transcripts 2- to 3-fold. In liver, growth hormone increased the abundance of the class A and B transcripts 6- to 7-fold. In lung and kidney, on the other hand, the abundance of class A transcripts was not affected by growth hormone. Thus, rat IGF-I gene transcripts contain one of three alternative 5' untranslated regions, which are expressed in a tissue-specific manner and are differentially regulated by growth hormone. Finally, cDNA probes unique to two of the three 5' untranslated regions hybridized to all three major species of IGF-I mRNA typically seen on RNA blots with a coding region probe.

Retinal insulin receptors. 2. Characterization and insulin-induced tyrosine kinase activity in bovine retinal rod outer segments

Bovine retinal rod outer segments (ROS) possess specific, high-affinity receptors for insulin. These receptors exhibit an insulin-stimulatable tyrosine-specific activity that is capable of phosphorylating the receptor's own beta-subunit and exogenous substrate. ROS insulin receptors exhibit heterogeneity in the apparent molecular weight of the receptor's alpha-subunit. In this regard, insulin receptors from this single cell type resemble insulin receptors obtained from whole retina, but are unlike receptors from brain and liver.

Insulin and insulin-like growth factor-I stimulate a common endogenous phosphoprotein substrate (pp185) in intact neuroblastoma cells

Mouse neuroblastoma N18 cells contain specific high affinity insulin and insulin-like growth factor-I (IGF-I) receptors. Insulin and IGF-I induce phosphorylation, in intact cells, of their respective receptor beta subunits. The insulin receptor beta subunit is represented by a 95-kDa phosphoprotein that is recognized by a specific antiserum (B10). The IGF-I receptor beta subunit is represented by two phosphoproteins of molecular mass 95 and 105 kDa. The hormone-induced phosphorylation was rapid and dose-dependent occurring on both phosphoserine and phosphotyrosine residues. In addition, both insulin and IGF-I induced phosphorylation of an endogenous protein of molecular mass 185 kDa (pp185). The rapidity and dose dependency of the phosphorylation of pp185 suggested that it may represent a common endogenous substrate for the insulin and IGF-I receptors in these neural-derived cells. Phosphorylation was primarily on phosphoserine and phosphotyrosine residues. pp185 did not absorb to wheat germ agglutinin-agarose and was not stimulated by either epidermal growth factor or platelet-derived growth factor. The finding of pp185 in these neural-related cells as well as in non-neural tissues suggests that it may represent a ubiquitous endogenous substrate for both the insulin and IGF-I receptor kinases.

Frog brain and liver show evolutionary conservation of tissue-specific differences among insulin receptors

The insulin receptors of frog brain and liver show features typical of other insulin receptors with regard to affinity and specificity of binding to insulins and proinsulin, solubility in Triton X-100, binding to and elution from wheat germ agglutinin, and insulin-sensitive tyrosine kinase activity. Likewise, the brain and liver receptors differ from one another in electrophoretic mobility and susceptibility to treatment with neuraminidase, analogous to brain and liver receptors of reptiles, birds, and mammals; while the functional implications of these differences are unknown, their evolutionary conservation for 400-500 million years suggests the possibility that they might have importance.

Rat IGF-I cDNA's contain multiple 5'-untranslated regions

DNA sequencing of several independent rat IGF-I cDNA clones has revealed three different 5'-untranslated region sequences which contain multiple, upstream, in-frame initiation codons. Use of these codons could generate N-terminal heterogeneity in IGF-I precursor proteins. One of these 5'-untranslated region sequences contains a 40-bp segment which is an inverted repeat of a region in the common 3'-untranslated region. The ends of the IGF-I mRNA corresponding to this cDNA could form a stable duplex structure. Such a complex could prevent ribosomal access to the AUG codons preceding the coding region for the pre-pro-IGF-I peptide, suggesting the possibility of translational regulation of this form of IGF-I mRNA. The 3'-untranslated region inverted repeat sequence also is present in human and mouse IGF-I cDNA's, and, intriguingly, is more highly conserved than the rest of the 3'-untranslated region.

Insulin-like growth factor I receptors in neuronal and glial cells. Characterization and biological effects in primary culture

Primary cultures of neuronal and glial cells from 1-day-old neonatal rats contain high affinity receptors for insulin-like growth factor I (IGF-I). The IC50 for displacement of 125I-IGF-I binding by unlabeled IGF-I was 3 nM for neuronal cells and 4 nM for glial cells. Unlabeled insulin was 20-50 times less potent. Apparent molecular mass of the alpha subunits of the IGF-I receptor was 125 kDa in neuronal and 135 kDa in glial cells. IGF-I induced autophosphorylation of the IGF-I receptor beta subunit in lectin-purified membrane preparations in a dose-dependent manner. The major phosphoamino acid of the beta subunit in both cell types was tyrosine in the IGF-I-stimulated state and serine in the basal state. Apparent molecular mass of the beta subunits of the IGF-I receptors was 91 kDa for neuronal and 95 kDa for glial cells. Tyrosine kinase activity of the IGF-I receptors was demonstrated by IGF-I-induced phosphorylation of the exogenous substrate poly(Glu, Tyr) 4:1 in both cell types. IGF-I had no effect on 2-deoxyglucose uptake in neuronal cells. In contrast, in glial cells, IGF-I stimulated 2-deoxyglucose uptake at very high doses, presumably acting via the insulin receptor. The effect of IGF-I as a neurotrophic growth factor in both neuronal and glial cells was demonstrated by its stimulation of [3H]thymidine incorporation. These findings suggest the IGF-I is an important growth factor in nervous tissue-derived cells.

Insulin receptors in the peripheral nervous system: a structural and functional analysis

Although the brain is known to contain specific insulin receptors, there is no information on whether these receptors are also present in the peripheral nervous system (PNS). The present studies sought to provide this information by characterizing insulin binding in bovine autonomic (superior cervical) and sensory (trigeminal) ganglia. It was found that both ganglia contain specific, high-affinity receptors for insulin. Like insulin receptors in other tissue, these receptors could be solubilized and purified on wheat germ agarose columns and were found to have tyrosine-specific kinase activity. SDS-PAGE and autoradiography revealed that the apparent molecular weight (Mr) of the PNS insulin receptor was approximately 133 kDa which is similar to the Mr of hepatic receptors, but is approximately 10 kDa larger than the insulin receptor found in the brain. Because the vasculature of autonomic and sensory ganglia is fenestrated, it is possible that PNS insulin receptors are exposed to blood-borne insulin.

Characterization of an endogenous substrate related to insulin and insulin-like growth factor-I receptors in lizard brain

Lizard insulin receptors are evolutionarily highly conserved. Wheat germ agglutinin-purified brain membranes demonstrate the presence of an endogenous substrate (pp 105) for both the insulin and insulin-like growth factor-I receptors. Both insulin and I-insulin-like growth factor-I stimulate the phosphorylation of this endogenous substrate in a dose-dependent manner. Following insulin-stimulated autophosphorylation of the beta subunit, there is a lag period of about 5 min prior to observable phosphorylation of the endogenous substrate. Phosphoamino acid analysis of both the beta subunit as well as pp 105 reveal primarily phosphotyrosine in both the basal as well as the stimulated state.

Molecular cloning of rat insulin-like growth factor I complementary deoxyribonucleic acids: differential messenger ribonucleic acid processing and regulation by growth hormone in extrahepatic tissues

Two classes of insulin-like growth factor I (IGF-I) cDNAs were isolated from an adult rat liver library using a human IGF-I cDNA probe. The two types of rat IGF-I cDNA differed by the presence or absence of a 52-base pair insert which altered the derived C-terminal amino acid sequence of the E peptide, but not the 3'-untranslated region or the sequence coding for the mature IGF-I protein. When probes derived from these cDNA clones were hybridized to Northern blots of rat mRNA, specific bands of 8.6, 2.1, and 1.0-1.4 kilobases were seen. Hybridization to poly(A)+ RNA from various tissues from GH-treated and control rats demonstrated an increase in IGF-I mRNA due to GH treatment in all tissues examined.

Developmental regulation of somatostatin gene expression in the brain is region specific

Developmental regulation of somatostatin (SRIF) gene expression was studied in five regions of rat brain and in rat stomach. Total RNA was isolated from hypothalamus, cortex, brainstem, cerebellum, and olfactory bulb, as well as stomach at eight stages of development from prenatal day 16 to postnatal day 82. Hybridization of a 32P-labeled rat SRIF cDNA probe to Northern blots of total RNA from the above tissues during development demonstrated a single hybridizing band approximately 670 base pairs in length. When SRIF mRNA levels from each stage of development were quantified and normalized by the amount of poly (A)+ RNA present at that stage of development, a unique pattern of SRIF gene expression was seen in each region. In brainstem and cerebellum, SRIF mRNA levels peaked early in development between prenatal day 21 and postnatal day 8 and then declined until postnatal day 82. Hypothalamus and cortex, on the other hand, showed a progressive increase during development with peak levels occurring between postnatal days 13 and 82. In contrast, stomach and olfactory bulb showed SRIF mRNA levels which were low during early development and which rose late in development (postnatal days 13 to 82). Marked differences in the amount of SRIF mRNA within each region were present as well. These data suggest that there is differential expression of the SRIF gene in different regions of the brain and in the stomach during development. Further study of this phenomenon may provide insight into the in vivo control of SRIF gene expression and the role of SRIF in the developing brain.

Structural and functional studies on insulin receptors from alligator brain and liver

Insulin receptors are present in membranes prepared from Alligator mississippiensis brain and liver. The apparent molecular weight (MW) of the alpha subunits are 132 kDa and 118 kDa in liver and brain respectively. Apparent MW of the beta subunit is 92 kDa in both brain and liver receptors. Despite the structural differences between brain and liver alpha subunits, brain insulin receptors demonstrate the normal coupling between alpha and beta subunits, i.e. following binding of insulin to the alpha subunit the beta subunit undergoes autophophorylation and stimulates tyrosine specific phosphorylation of exogenously added substrates. These findings suggest that functional insulin receptors are evolutionarily well conserved.

The interaction of brain insulin receptors with wheat germ agglutinin

Brain insulin receptors adsorb to and are recoverable from wheat germ agglutinin-agarose (WGA) columns. Similar results are obtained using dissuccinimidyl suberate (DSS)-crosslinked receptors or photo-affinity labeled receptors. WGA can be used for partial purification of brain insulin receptors provided the appropriate WGA preparation is chosen and the optimal ratio of receptor protein to lectin is achieved.

Fasting and refeeding alter the insulin receptor tyrosine kinase in chicken liver but fail to affect brain insulin receptors

Insulin receptors from chicken liver and brain were studied following alterations in the nutritional state. Chickens were either fasted for 48 h, fasted for 48 h and then refed for 24 h, or fed a regular diet ad libitum. 125I-Porcine insulin binding was significantly elevated in liver membranes from the fasted animals and lowered in refed chickens when compared to preparations from ad libitum fed chickens. These changes in 125I-insulin binding were inversely related to the levels of plasma insulin and since receptor affinities for insulin were similar in each group, they probably represent alterations in receptor number. Apparent Mr of alpha subunits of the insulin receptors was unaffected by alterations in the nutritional states. The presence of ATPase-like activities that co-eluted with liver insulin receptors from wheat germ agglutinin lectin columns but not from pea lectin columns necessitated the use of both pea and wheat germ agglutinin for liver insulin receptor purification. The insulin receptors purified from both lectin columns were recognized by anti-insulin receptor antiserum and had similar affinities for insulin which were unaltered by the nutritional state. Insulin-stimulatable autophosphorylation of the beta subunit of the insulin receptor was lower in livers from fasted chickens and intermediate in refed chickens. Furthermore, basal and insulin-induced phosphorylation of the artificial substrate poly(Glu,Tyr) 4:1 was significantly less in the fasting state and intermediate in the refed state compared to the ad libitum fed state. Insulin sensitivity (measured as the dose of insulin required for 50% maximal stimulation of kinase activity) was similar in all three states suggesting that the differences in insulin-induced phosphorylation are due to a change in maximal stimulation and not a change in insulin sensitivity. In contrast to the alterations seen with liver receptors, brain insulin receptors were unaffected by these alterations in nutritional state. These findings suggest that: liver insulin receptors are affected by altering the nutritional state; insulin binding to liver membranes is inversely related to plasma insulin levels; and tyrosine kinase is decreased both in fasted and refed animals suggesting an uncoupling of the normal interaction between alpha subunit and beta subunit in liver insulin receptors.

Development of brain insulin receptors: structural and functional studies of insulin receptors from whole brain and primary cell cultures

We studied the structural and functional characteristics of insulin receptors from rat brain and liver from late gestation through adulthood as well as from cultured neuronal and glial cells from neonatal rats. Specific insulin binding was present on membrane preparations from brain and liver at all stages of development studied, with maximal binding in neonates greater than 19-day-old fetuses greater than adults for both brain and liver. Maximal specific binding to cultured neuronal and glial cell membranes was similar (6.2% vs. 7.1%, respectively). [125I]Iodoinsulin cross-linking to the insulin receptor demonstrated that the mol wt (Mr) of the brain alpha-subunit was less than that of the liver alpha-subunit at all stages. [125I]Iodoinsulin cross-linking also demonstrated that the glial cell alpha-subunit (Mr, 130,000) migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis to a position intermediate between the liver (Mr, 135,000) and brain (Mr, 119,000), whereas the neuronal cell alpha-subunit (Mr, 118,000) comigrated with the brain alpha-subunit. In solubilized lectin-purified preparations from brain and liver during development as well as from neuronal and glial cells, insulin stimulated phosphorylation of the beta-subunit. The Mr of the brain beta-subunit, as determined by migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was less than that of the liver beta-subunit. The neuronal cell beta-subunit comigrated with the brain beta-subunit while the glial cell beta-subunit migrated to a position intermediate between the brain and liver beta-subunit. Solubilized lectin-purified preparations from all tissues demonstrated insulin-stimulable phosphorylation of exogenous substrates. From these studies we conclude that 1) functional insulin receptors are present in the brain during development in the rat; and 2) the structural differences demonstrated between neuronal and glial cell and between brain and nonneuronal insulin receptors taken together with previously demonstrated functional differences of the insulin receptor on these tissues suggest a unique function for insulin receptors on neuronal tissues.

Insulin receptors in lizard brain and liver: structural and functional studies of alpha and beta subunits demonstrate evolutionary conservation

Specific insulin receptors are present in the liver and brain of the lizard Anolis carolinesis. In this study, the specific binding of 125I-insulin to the receptors showed time, temperature and pH dependency. Specific binding to crude membranes prepared from brain was 1-2% of the total radioactivity added compared to 4-5% in the crude membranes prepared from liver. Solubilization and wheat germ agglutinin purification of the membranes resulted in an increase in the specific binding (per mg of protein) between 6 and 32 times for liver membranes and 13-186 for brain membranes. Binding inhibition of tracer insulin by unlabeled porcine insulin was characteristic for insulin receptors with 50% inhibition for liver crude membranes at 60 ng/ml of porcine insulin and 0.7 ng/ml for purified brain insulin receptors. Chicken insulin was 2- to 3-fold more potent and proinsulin about 100 times less potent than porcine insulin. The alpha-subunits of liver and brain had apparent molecular weights on sodium dodecyl sulfate polyacrylamide gel electrophoresis of 135 kDa and 120 kDa respectively. Apparent molecular weights of beta subunits were 92 kDa for both tissues. Insulin stimulated phosphorylation of the beta subunit of both brain and liver receptors. Both tissues demonstrated tyrosine-specific phosphorylation, which was stimulated by insulin, of exogenously added artificial substrates. In addition, purified brain insulin receptor preparations contained an endogenous protein with apparent molecular weight of 105 kDa, whose phosphorylation was stimulated by insulin (10(-7) mol/l). This phosphoprotein was not immunoprecipitated by anti-insulin receptor antibodies. These studies suggest that the structural differences between brain and liver receptors previously demonstrated in the rat are also present in the lizard, which is about 300,000,000 years older than the mammalian species. Thus, there is strong evolutionary conservation of the brain insulin receptor.

Insulin receptors from guinea pig liver and brain: structural and functional studies

We studied the structural and functional characteristics of insulin receptors from guinea pig liver and brain. Binding to crude membrane preparations of liver and brain was time, temperature, and pH dependent. Maximal specific binding to liver crude membrane preparations was 16.4 +/- 0.5%, and that to brain crude membrane preparations was 10.4 +/- 1.8%. Specificity studies demonstrated typical affinities for insulin receptors with chicken insulin greater than porcine insulin greater than human proinsulin greater than desoctapeptide insulin in both liver and brain. Antiinsulin receptor antiserum inhibited binding of [125I]insulin to both liver and brain crude membrane preparations. Electrophoresis performed under reducing conditions after affinity cross-linking of liver and brain insulin receptors with [125I]insulin revealed labeled proteins (alpha-subunit) with apparent mol wt of 136,000 in liver and 121,000 in brain. Treatment of liver and brain receptors with both endoglycosidase H and endoglycosidase F increased the electrophoretic mobility of the alpha-subunit. [125I]Insulin cross-linked receptors from both liver and brain adsorbed to and eluted from wheat germ agglutinin columns in a similar manner, as demonstrated by binding and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In solubilized lectin-purified receptor preparations from liver and brain, insulin stimulated the phosphorylation of the beta-subunit and exogenous substrates. When the delta-kinase activity, as measured by exogenous substrate phosphorylation, of the brain and liver preparations was normalized to the maximal bound to free ratio of the preparations, the delta-kinase activity was about 4-fold greater in brain than in liver. These studies suggest that the differences between brain and liver insulin receptor alpha-subunits previously demonstrated in rats are also present in guinea pigs.

Extracts of protozoa contain materials that react specifically in the immunoassay for guinea pig insulin

Extracts of protozoa contain materials that resemble guinea pig insulin, which is noted for its unusual structure and properties. The protozoan derived materials react in the radioimmunoassay for guinea pig insulin; some but not all of these immunoreactive materials migrate on gel filtration in the position of authentic guinea pig insulin. Experiments were done to exclude artifacts in the assay as well as inadvertent contamination by guinea pig insulin. By immunological methods, we segregated the guinea pig type immunoactivity from that which has rat/pork type immunoactivity. These findings extend our studies of extracts of guinea pig tissues which also have these two types of insulin immunoactivities.

Tyrosine kinase activity of brain insulin and IGF-1 receptors

Lectin-purified rat brain preparations demonstrate specific [125I]insulin and [125I]-IGF-1 binding. Insulin-stimulable tyrosine kinase activity as measured by exogenous substrate phosphorylation was present in brain and liver lectin purified preparations with the delta kinase activity/B/F of brain approximately 2.5 fold greater than that of liver. Insulin-stimulable tyrosine kinase activity was abolished in liver but decreased by only approximately 50 percent in brain after immuno-depletion with antiserum which recognizes insulin but not IGF-1 receptors. Insulin and IGF-1 dose responses for phosphorylation of the immunodepleted brain preparations suggested that the remaining tyrosine kinase activity was IGF-1 receptor mediated. Thus, functional IGF-1 receptors are present in rat brain, and the doses of insulin typically used to evaluate insulin receptor tyrosine kinase activity will stimulate IGF-1 receptor tyrosine kinase activity as well.