Analysis of the human type I insulin-like growth factor receptor promoter region

We isolated genomic fragments containing the 5' region of the human type I insulin-like growth factor receptor gene. A unique transcription start site was identified, defining a 1038 bp 5'-untranslated region. No TATA or CCAAT elements were identified in the proximal 480 nucleotides of 5'-flanking region. The region surrounding the transcription start site was similar to a recently described "initiator" sequence. The 5'-flanking and 5'-untranslated regions were highly GC-rich, with numerous potential Sp1 binding sites. A potential AP-2 binding site was identified in the 5'-flanking region and a potential thyroid response element was identified in the 5'-untranslated region. The 5' region of the human gene was very similar to that of the rat gene, with conservation of many of the potential regulatory elements.

Identification of multiple transcription start sites in the human insulin-like growth factor-I gene

We have localized four transcription initiation sites in the human insulin-like growth factor-I (IGF-I) gene. Two transcription start sites were identified which result in a longer and shorter version of the leader derived from the known exon 1 of the IGF-I gene. Transcription starting at the upstream transcription initiation site results in a leader exon 1 of about 1155 nucleotides (nt), whereas transcription starting at the downstream initiation site results in a leader of about 240 nt. The majority of the transcripts initiate at the latter site. We further identified a region in the human IGF-I gene between exons 1 and 2, which shows a high degree of homology with the rat IGF-I leader exon 1B. By means of the polymerase chain reaction (PCR) we detected human IGF-I mRNAs containing this novel leader. The corresponding exon was designated exon 1B according to the rat IGF-I gene terminology. PCR and RNase protection analyses identified two transcription start sites within this alternative leader exon 1B. Transcription initiated at the most upstream start site results in a leader of about 750 nt, whereas transcription starting at the downstream site is heterogeneous, resulting in leaders of 65-75 nt long. No consensus TATA-box or AT-rich regions are present immediately upstream of all four transcription start sites identified, nor are these regions particularly GC-rich. The IGF-I gene is known to be expressed differentially in a tissue- and development-specific fashion. Differential activation of multiple promoters could very well play a crucial role in IGF-I gene regulation.

Renal IGF-1 mRNA levels are enhanced following unilateral nephrectomy in immature but not adult rats

The increase in IGF-1 gene expression following unilateral nephrectomy (UNX) in adult rats is controversial. In this study we have examined whether developmental differences exist in the effect of UNX on IGF-1 gene expression. Immature (23 days) and adult (4 months) Wistar rats underwent a sham operation or left UNX, and were sacrificed 24 or 48 hrs later. IGF-1 mRNA levels were determined in left (control) and right (compensated) kidneys using solution hybridization/RNase protection assays. By 48 hrs post-UNX, remnant kidneys had grown 20 +/- 1% in adult rats (P less than 0.05), and 69 +/- 5% in immature rats (P less than 0.05). IGF-1 mRNA levels were not increased in the adult compensated kidneys at either 24 or 48 hrs post-UNX. In contrast, kidneys from immature rats 24 and 48 hrs post-UNX had an average 4-fold increase (P less than 0.05) in exon 1 IGF-1 mRNA levels, and an average 3-fold increase (P less than 0.05) in exon 2 mRNA levels. Thus, these findings suggest that there is an age-dependent difference in the effects of UnX on IGF-1 gene expression, and provide the first evidence that IGF-1 gene expression increases following unilateral nephrectomy in immature rats.

Developmental regulation of insulin-like growth factor-I-stimulated glucose transporter in rat brain astrocytes

Astrocytic glial cells from 1- and 21-day-old rat brains were established in primary culture to study the expression of insulin-like growth factor-I (IGF-I) receptors and IGF-I-stimulated glucose transporter (Glut-1). Astrocytes from both age groups expressed specific high affinity IGF-I receptors, whose relative affinities for IGF-I, IGF-II, and insulin were comparable. However, the total number of binding sites and IGF-I receptor mRNA levels were 148% and 240% higher in astrocytes from 21-day-old compared with 1-day-old brains. IGF-I caused a dose-dependent stimulation of [3H]2-deoxy-D-glucose [( 3H]dGlc) uptake in astrocytes from 1-day-old brains. This was associated with increases in Glut-1 protein and mRNA levels. In contrast, astrocytes from 21-day-old brains exhibited a 58% decrease in the binding capacity and a 77% decrease in the steady state levels of Glut-1 protein and its mRNA. In addition, IGF-I failed to stimulate the Glut-1 system in these cells. This lack of IGF-I effect is not due to an alteration inherent to the Glut-1 system, since 12-O-tetradecanoyl-phorbol-13-acetate stimulated [3H]dGlc uptake and Glut-1 protein and its mRNA levels. These observations suggest that changes in basal and IGF-I-stimulated Glut-1 system in brain astrocytes may be developmentally regulated.

Transcription initiation in the two leader exons of the rat IGF-I gene occurs from disperse versus localized sites

Rat IGF-I mRNAs contain two different 5'-UTR sequences as a result of alternate splicing of leader exons. Using a combination of solution hybridization/RNase protection and primer extension assays, we have mapped the transcriptional start sites in these leader exons. There appear to be three putative transcription start sites in exon 1 spread over a 140-bp region, the most upstream of which defines a 381 bp-long exon 1. There appear to be three distinct start sites in exon 2, the most upstream of which defines a greater than 770 bp-long exon 2. The two downstream start sites in exon 1 together account for approximately 70% of IGF-I gene expression in adult rat liver. Essentially all of the remaining IGF-I gene expression comes from the second start site in exon 2. Rat IGF-I gene transcription may therefore be regulated by two distinct promoter regions, a disperse promoter for exon 1, with several transcription initiation sites, and a more typical promoter region for exon 2, which controls transcription initiation from a discrete region.

Insulin-like growth factor I (IGF-I) receptors and IGF-I action in oligodendrocytes from rat brains

Oligodendrocyte progenitor cells were prepared by mechanical dissociation of 1-day-old rat brain cultures. These cells undergo proliferation and differentiation into oligodendrocytes as demonstrated by the expression of proliferation and differentiation-related specific antigens. We have used this unique culture system to characterize insulin-like growth factor I (IGF-I) receptors and their action in the central nervous system (CNS). 125I-IGF-I specifically binds to these cultures with high affinity. Competition-inhibition data suggest that IGF-I is most potent in competing for 125I-IGF-I binding, followed by IGF-II and insulin. Scatchard analyses of the binding data indicate a curvilinear plot with a Kd for high affinity of 0.2 nM, and a Bmax of 247 fmol/mg, and a Kd for low affinity of 3.2 nM and Bmax of 1213 fmol/mg protein. Covalent cross-linking followed by SDS-PAGE analysis demonstrated a radioactive band of Mr 135,000 which corresponds to the alpha subunit of the IGF-I receptor. Solution hybridization/RNase protection assay produced a single protected band corresponding to IGF-I receptor messenger RNA, further confirming the presence of these receptors. Incubation of progenitor cells with IGF-I resulted in a time- and concentration-dependent increase in [3H]thymidine incorporation and cell numbers. This effect appears to be mediated by IGF-I receptors since IGF-II and insulin were proportionately less potent. In addition to its effect on proliferation, IGF-I also increased the number of 4E7- and GC-antigen positive cells. These observations indicate that oligodendrocytes in primary culture express specific IGF-I receptors and that the interaction of IGF-I with these receptors results in the proliferation as well as differentiation of oligodendrocytes.

Differential association of insulin-like growth factor I mRNA variants with polysomes in vivo

Expression of the rat insulin-like growth factor I (IGF-I) gene results in a number of mature mRNA species that differ in size primarily at the 3' end due to differential polyadenylation site usage. Additionally, alternate splicing in both 5' and 3' regions produces RNAs which have the capacity to encode different IGF-I precursor peptides. We have analyzed total and polysomal RNAs using Northern blot analyses and solution hybridization/RNase protection assays to assess the in vivo translatability of these various IGF-I mRNA species. The results suggest that all of the known splicing variants are found on polysomes and may, therefore, be translated into a number of IGF-I precursors in vivo. One particular 5'-untranslated (UTR) variant is relatively enriched in polysomal RNA, a finding which suggests that removal of some of the 5'-UTR sequences encoded by exon 1 may enhance translatability. Of the IGF-I mRNAs with different lengths of 3'-UTR, only the shorter species were found on polysomes, suggesting that some aspect of the long 3'-UTR may prevent translation. Thus, differential processing of the primary transcript of the IGF-I gene may serve to generate IGF-I mRNA species which specify different precursors as well as to control their relative translatability.

Insulin-like growth factor I mRNA levels are developmentally regulated in specific regions of the rat brain

The expression of mRNAs encoding insulin-like growth factor I (IGF-I) and the IGF-I receptor in the developing rat brain from embryonic day 16 to postnatal day 82 was analyzed using solution hybridization-RNase protection assays. Four distinct developmental patterns in the steady-state levels of IGF-I mRNA were seen. Specifically, the olfactory bulb showed a high perinatal level of IGF-I mRNA which declined dramatically by postnatal day 8. In contrast, cerebral cortex displayed maximal levels of IGF-I mRNA at postnatal day 8 and 13, which subsequently declined to adult levels (P82). A third developmental pattern was seen in the hypothalamus, where IGF-I mRNA increased from E16 up to postnatal day 3 and remained elevated thereafter. Finally, IGF-I mRNA levels in brainstem and cerebellum remained unchanged throughout the time period studied. We conclude that there are specific regional patterns of IGF-I gene expression in the developing rat brain. In contrast, IGF-I receptor gene expression did not exhibit any region-specific developmental changes. The developmental patterns of IGF-I gene expression seen in this study further substantiate the potential role of IGF-I in normal brain development.

Hormonal regulation of rat hypothalamic neuropeptide mRNAs: effect of hypophysectomy and hormone replacement on growth-hormone-releasing factor, somatostatin and the insulin-like growth factors

Hormonal feedback regulation of hypothalamic peptides putatively involved in growth hormone (GH) regulation has been studied by measurement of steady-state mRNA levels in male hypophysectomized rats with or without thyroid hormone, corticosterone, testosterone or GH replacement. Hypothalamic GH-releasing factor (GRF) mRNA levels increased progressively following hypophysectomy to 420% of sham levels after 15 days while hypothalamic insulin-like growth factor I (IGF-I) and insulin-like growth factor II (IGF-II) mRNA levels decreased to less than 40% of sham levels. Whole hypothalamic somatostatin mRNA levels were not significantly different from sham. One week of continuous GH infusion restored hypothalamic IGF-I mRNA to levels (95%) indistinguishable from those in sham-operated controls but had no effect on either IGF-II or GRF mRNA. Thyroid hormone, corticosterone and testosterone treatment without GH had no effect on the hypophysectomy-induced reduction of either IGF-I or IGF-II mRNA levels but reversed the elevation of GRF mRNA. We conclude that hypothalamic IGF-I may be involved in GH feedback regulation and thus may function as a hypothalamic modulator of GH. In contrast, IGF-II may be regulated by one of the pituitary trophic hormones but not by GH or the target hormones tested. Finally, hypothalamic GRF mRNA regulation appears to be complex and may include target hormone feedback.

Retinoid modulation of insulin-like growth factor-binding proteins and inhibition of breast carcinoma proliferation

Retinoids induce cellular differentiation and inhibit cellular proliferation. Proliferation of human breast carcinoma cells in vitro is markedly inhibited by these compounds. On the other hand, insulin-like growth factors (IGFs) and their receptors seem to be involved in the growth of certain breast carcinoma cells by autocrine or paracrine effects. Since the effects of both IGF-I and IGF-II may be modulated by specific binding proteins (IGF-BPs) we examined the possibility that one mechanism by which retinoic acid may inhibit cancer growth is by an alteration in these BPs, thereby blocking IGF's growth effect. Retinoic acid (RA; 1 microM) completely blocked the effect of IGF-I (50 ng/ml) on enhancing proliferation of MCF-7 cells in culture. This effect of RA was not associated with any significant change in specific IGF-I-binding sites on these cells. RA induced a 3-fold increase in IGF-binding activity in conditioned medium, measured using a polyethylene glycol-immunoglobulin precipitation assay and a charcoal absorption assay. This increase was associated with the appearance of 42- and 46-kDa IGF-BPs on ligand blotting. The effect of RA on these IGF-BPs was time and concentration dependent. In contrast, during some experiments the 27- and 36-kDa BPs actually decreased. These findings support the hypothesis that RA may inhibit the growth of certain breast carcinoma cells by increasing the secretion of certain IGF-BPs, which could directly modulate the growth effect of IGFs.

Rat ovarian insulin-like growth factor II gene expression is theca-interstitial cell-exclusive: hormonal regulation and receptor distribution

While the potential role of insulin-like growth factor (IGF)-I in ovarian physiology has been extensively studied, relatively limited attention has been paid to IGF-II the very presence of which in the mature rat ovary has been questioned. In the present study, we have reevaluated rat ovarian IGF-II gene expression, its cellular localization, hormonal regulation, and site(s) of receptor interaction. IGF-II mRNA was detected in whole ovaries from immature as well as mature intact rats. Cellular localization studies revealed IGF-II transcripts in theca-interstitial but not granulosa cells (a site of IGF-I gene expression). In contrast, no cellular selectivity was noted for Type I and Type II IGF receptor gene expression, both of which were clearly detectable in both granulosa and theca-interstitial cells. In vivo treatment of immature hypophysectomized rats with diethylstilbestrol reduced ovarian IGF-II mRNA levels while increasing IGF-I mRNA levels. Taken together, these and previous observations reveal fundamental differences in the cellular localization and hormonal regulation of ovarian IGF gene expression in that IGF-II gene expression (unlike IGF-I) is theca-interstitial (rather than granulosa) cell-specific, and is subject to down (as opposed to up) regulation in response to estrogenic stimulation. In contrast, Type I and Type II IGF receptors exist on both somatic cell types of the rat ovary. These observations are consistent with the view that IGF-II of theca-interstitial cell origin may not only play an autocrine role but may also serve as one of several signals through which this androgen-producing cell may communicate in a paracrine fashion with the adjacent granulosa cell compartment.

Experimental diabetes increases insulinlike growth factor I and II receptor concentration and gene expression in kidney

Insulinlike growth factor I (IGF-I) is a mitogenic hormone with important regulatory roles in growth and development. One of the target organs for IGF-I action is the kidney, which synthesizes abundant IGF-I receptors and IGF-I itself. To study the involvement of IGF-I and the IGF-I receptor in the development of nephropathy, one of the major complications of diabetes mellitus, we measured the expression of these genes in the kidney and in other tissues of the streptozocin-induced diabetic rat. The binding of 125I-labeled IGF-I to crude membranes was measured in the same tissues. We observed a 2.5-fold increase in the steady-state level of IGF-I-receptor mRNA in the diabetic kidney, which was accompanied by a 2.3-fold increase in IGF-I binding. In addition to this increase in IGF-I binding to the IGF-I receptor, there was also binding to a lower-molecular-weight material that may represent an IGF-binding protein. No change was detected in the level of IGF-I-peptide mRNA. Similarly, IGF-II-receptor mRNA levels and IGF-II binding were significantly increased in the diabetic kidney. IGF-I- and IGF-II-receptor mRNA levels and IGF-I and IGF-II binding returned to control values after insulin treatment. Because the IGF-I receptor is able to transduce mitogenic signals on activation of its tyrosine kinase domain, we hypothesize that, among other factors, high levels of receptor in the diabetic kidney may also be involved in the development of diabetic nephropathy. Increased IGF-II-receptor expression in the diabetic kidney may be important for the intracellular transport and packaging of lysosomal enzymes, although a role for this receptor in signal transduction cannot be excluded. Finally, the possible role of IGF-binding proteins requires further study.

Liver regeneration is associated with increased expression of the insulin-like growth factor-II/mannose-6-phosphate receptor

The process of liver regeneration involves the concerted action of certain growth factors, which stimulate hepatocyte proliferation, and other antiproliferative factors, which prevent uncontrolled growth of this organ. Some of the biological actions of insulin-like growth factor-II (IGF-II), a mitogenic polypeptide closely related to insulin, may be mediated by the IGF-II receptor. This receptor consists of a single chain extracellular domain and a very small cytoplasmic domain, and can bind lysosomal enzymes that contain mannose-6-phosphate (M-6-P) residues. Since these enzymes may be involved in remodelling processes in certain tissues, we measured the expression of the IGF-II/M-6-P receptor in the liver after subtotal hepatectomy. Binding of [125I]IGF-II to crude plasma membranes from regenerating liver was maximal 2 days after hepatectomy (4.9% specific binding/60 micrograms protein) and subsequently decreased. Both control livers (livers removed at the time of operation) and sham-operated control livers demonstrated specific [125I]IGF-II binding of 1.1% throughout the experimental period. This increase in binding in regenerating liver was shown to be associated with an increase in the concentration of IGF-II receptor protein by means of Western blot analysis using a polyclonal anti-IGF-II/M-6-P receptor antiserum (3637). Similarly, steady state levels of IGF-II/M-6-P receptor mRNA, measured by solution hybridization/RNase protection assays, were significantly increased in the regenerating liver (2.0-fold over the control value 2 days after hepatectomy). Five and 10 days postsurgery, the levels of IGF-II receptor mRNA were markedly reduced, and they were even lower than the levels in control livers.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular pattern of insulin-like growth factor-I (IGF-I) and type I IGF receptor gene expression in early organogenesis: comparison with IGF-II gene expression

To investigate the potential role(s) of the insulin-like growth factors (IGFs) in embryogenesis, we have used in situ hybridization histochemistry to localize mRNAs for IGF-I, IGF-II, and the type I IGF receptor during an early period in rat embryonic development (embryonic days 14 and 15). IGF-I and IGF-II mRNAs were found in distinctly different patterns of cellular distribution. IGF-I mRNA was particularly abundant in undifferentiated mesenchymal tissue in the vicinity of sprouting nerves and spinal ganglia, and in circumscribed regions of the developing face that corresponded to the target zones of the trigeminal nerve. IGF-I mRNA was also found in aggregations of mesenchyme surrounding, but not in developing muscle and cartilage. IGF-I mRNA was selectively concentrated in areas of active tissue remodeling, such as the cardiac outflow tract, and was undetectable in liver, pituitary, and nervous system at this early stage of organogenesis. IGF-II mRNA was abundant in developing muscle, cartilage, and vascular tissue, and in the embryonic liver and pituitary. IGF-II mRNA was also conspicuous in areas of vascular interface with the brain, such as the choroid plexus and the organum vasculosum of the lamina terminalis. Messenger RNA for the type I IGF receptor was widely distributed in embryonic tissues, but the highest level were seen in the ventral floorplate of the hindbrain, where specialized neuroepithelial cells act as guides for axonal targeting. In conclusion, the different cellular patterns of expression of genes for IGF-I and IGF-II indicate that these two IGFs are differently regulated and, thus, may have significantly different roles in the process of embryonic development. Furthermore, the early and widespread expression of the type-I IGF receptor gene, in contrast to the relatively limited and localized pattern of IGF-I gene expression, is consistent with the view that this receptor may mediate the effects of IGF-II as well as IGF-I during embryogenesis.

The insulin-like growth factor I (IGF-I) gene is expressed in chick embryos during early organogenesis

A definition of the role of IGF-I in differentiation and development requires a detailed understanding of its expression and tissue-specific regulation in embryogenesis. Standard techniques for analysis of IGF-I gene expression are not sufficiently sensitive for studies in early embryos. We have used the highly sensitive polymerase chain reaction (PCR) to study IGF-I gene expression in whole chick embryos from the late blastula stage (E0 = laying) through the end of organogenesis (day 8), and in liver, brain and pancreas during mid-late embryogenesis and perinatally (hatching = day 21). Although at low levels in the blastoderm and gastrula, IGF-I mRNA was detectable in the whole embryo in all stages studied, with a tendency of the signal to increase with age during the first week of embryogenesis. In mid- and late embryogenesis, we easily detected IGF-I mRNA transcripts in pancreas and brain while the levels in the liver were barely detectable. Liver IGF-I mRNA increased markedly at the peak of postnatal growth (day 50). These studies suggest that while the major source of postnatal IGF-I may be the liver, extrahepatic tissues may be the predominant source of IGF-I during prenatal chicken development.

Cloning and characterization of the proximal promoter region of the rat insulin-like growth factor I (IGF-I) receptor gene

We have isolated genomic clones that contain the promoter region of the rat IGF-I receptor gene. A unique transcriptional start site was suggested by the results of primer extension and RNase protection assays, which also defined a 940-base 5'-untranslated region. Despite the single start site, the proximal 415 base pairs of 5'-flanking region were devoid of TATA or CCAAT elements. The region surrounding the start site was, however, similar to a recently described "initiator" sequence that can direct specific transcription initiation in the absence of a TATA element. The 5'-flanking region was GC-rich and contained several possible SP1 sites, but also included potential ETF and AP-2 binding sites. The rat IGF-I receptor gene promoter region appears to have some sequences similar to both "housekeeping" and highly regulated promoters and may be an example of an intermediary class of regulatory region.

Alternative splicing produces messenger RNAs encoding insulin-like growth factor-I prohormones that are differentially glycosylated in vitro

Rat insulin-like growth factor-I (IGF-I) cDNA sequences predict two prohormones that differ in the carboxy-terminal extension peptide (E-peptide) as a result of the inclusion or exclusion of the 52-basepair exon 4 sequence. In the absence of exon 4, the sequence codes for the IGF-Ia prohormone, whose E region contains two potential N-glycosylation sites. With differential splicing and the inclusion of exon 4, the resultant mRNA codes for IGF-Ib, with a longer E-region sequence. In addition, as a consequence of a frame shift, both potential glycosylation sites are lost in the IGF-Ib peptide. We used an in vitro translation system supplemented with canine pancreatic microsomal membranes to analyze cotranslational processing of the IGF-I propeptides. We have demonstrated that IGF-Ia prohormone, which contains two potential N-glycosylation sites in the E region, can be N-glycosylated in vitro, and that both glycosylation sites are probably used. As expected, the IGF-Ib preprohormone is processed by microsomes, but is not glycosylated.

Regulation of rat brain/HepG2 glucose transporter gene expression by phorbol esters in primary cultures of neuronal and astrocytic glial cells

We have demonstrated regulation of the rat brain/Hep G2 glucose transporter gene (GT1) by Northern blot analysis with a rat brain glucose transporter cDNA probe. Incubation of both neuronal and glial cells derived from neonatal rats with 12-O-tetradecanoyl-phorbol-13-acetate induced a time- and dose-dependent increase in the steady state levels of GT1 mRNA. In glial cells, this corresponded to an increase in both the level of GT1 protein and glucose transporter activity, as demonstrated by Western blot analysis and [3H]2-deoxyglucose (dGlc) uptake studies. In contrast, in neuronal cells 12-O-tetradecanoyl-phorbol-13-acetate had no effect on either the concentration/level of the GT or [3H]dGlc uptake. These results suggest that phorbol esters regulate dGlc uptake at the transcriptional level in both neuronal and glial cells, but that the increase in expression of the GT1 gene is dissociated from posttranscriptional events involved in dGlc uptake in neuronal cells.

Insulin-like growth factor-II in nonislet cell tumors associated with hypoglycemia: increased levels of messenger ribonucleic acid

The role of insulin-like growth factor-II (IGF-II) in the hypoglycemia associated with nonislet cell tumors is controversial. In this study we have addressed this question by measuring the IGF-II mRNA levels in extracts of these tumors. Hybridization of a 32P-labeled IGF-II cDNA to a Northern blot of RNA from three nonislet cell tumors associated with hypoglycemia (a hemangiopericytoma, fibrosarcoma, and malignant mesenchymal tumor) demonstrated six hybridizing bands, 6.8, 5.6, 4.7, 3.6, 2.6, and 2.1 kilobases in length. These bands were similar to those described by others in a range of tumors and normal tissues. Tissue IGF-II mRNA levels were quantitated using a solution hybridization/RNase protection assay. IGF-II mRNA levels in the tumors were similar to the level present in one line of human hepatoblastoma-derived Hep G2 cells, 5- to 6-fold higher than that in another line of Hep G2 cells, and 2- to 3-fold higher than that in term placenta. In contrast, little or no IGF-II mRNA was detected in a nonfunctioning islet cell adenoma or normal spleen. There was no evidence for amplification of the IGF-II gene in the one tumor in which it was sought. These data suggest that nonislet cell tumors associated with hypoglycemia produce large amounts of IGF-II mRNA and that this IGF-II mRNA appears to be the product of an IGF-II gene, which is apparently normal in the region encoding mature IGF-II peptide.

Mechanism of insulin resistance induced by sustained levels of cytosolic free calcium in rat adipocytes

We have recently provided evidence that elevated levels of cytosolic free Ca2+ ([Ca2+]i) decreased insulin-stimulated glucose uptake in isolated rat adipocytes. To investigate the mechanism of Ca2+ action, we examined the effects of elevated levels of [Ca2+]i on insulin binding, autophosphorylation, and tyrosine kinase activity (TKA) of insulin receptors as well as basal and insulin-stimulated cellular distribution of glucose transporters. The latter was assessed by cytochalasin-B binding to plasma membrane and cytosolic fractions. Elevated concentrations of [Ca2+]i were maintained by incubating adipocytes with a depolarizing concentration of K+ (40 mM). Basal nonstimulated glucose uptake was not altered by increased levels of [Ca2+]i. Adipocytes with higher [Ca2+]i (220 +/- 15 nM) showed 30% reduction in insulin-stimulated 2-deoxyglucose uptake compared with control cells ([Ca2+]i, 140 +/- 18 nM). Moreover, adipocytes with higher levels of [Ca2+]i demonstrated an approximately 10% reduction in autophosphorylation and TKA of insulin receptors without a change in insulin binding. Both basal and insulin-stimulated distributions of glucose transporters were unaffected by sustained levels of [Ca2+]i. The effects of elevated [Ca2+]i were not mimicked by protein kinase-C activation. These observations suggest that 1) elevated or sustained levels of [Ca2+]i impair insulin-stimulated glucose uptake; and 2) Ca2+-induced impairment appears to reside at the postbinding steps of insulin action and probably interferes with the TKA of insulin receptors and the intrinsic activity of glucose transporters.

Insulin and IGF-I receptors in neuroblastoma cells: increases in mRNA and binding produced by glyburide

Insulin and IGF-I binding to neuroblastoma cells (SK-N-MC) was increased by 13% and 7% respectively following a 24hr, incubation with the sulphonylurea glyburide. This increase in binding was associated with increased steady-state levels of insulin receptor and IGF-I receptor mRNA levels. Though insulin and IGF-I both stimulate glucose uptake into these cells, the increased binding following glyburide treatment was not associated with any change in glucose uptake.

Developmental regulation of the rat insulin-like growth factor I receptor gene

We have investigated the developmental regulation of the rat insulin-like growth factor I (IGF-I) receptor gene in various tissues using a sensitive and specific solution hybridization/RNase protection assay. For this purpose we characterized rat IGF-I receptor cDNAs that were cloned from a simian virus 40-transformed rat granulosa cell cDNA library. The specific cDNA clone used in these studies encoded the putative signal peptide and the first 53 amino acids of the alpha subunit and was approximately 94% homologous to its human counterpart. IGF-I receptor gene expression was studied during the perinatal period and at various intervals until early adulthood. Overall, steady-state IGF-I receptor mRNA levels decreased dramatically during postnatal development; however, the extent of the decrease differed among the various tissues studied. In contrast to receptor mRNA levels, IGF-I mRNA levels increased in some of the same tissues. The molecular mechanisms underlying this apparent divergent transcriptional control of the IGF-I and IGF-I receptor genes warrant further study.