Increased phosphorylation of ribosomal protein S6 following microinjection of insulin receptor-kinase into Xenopus oocytes

Metabolic control and future opportunities for growth regulation

The last 10 years have seen a significant expansion in the scope of attempts to manipulate the growth of animals (Buttery, Lindsay and Haynes, 1986). The expansion of interest has been driven by a number of factors, both economic and theoretical. At the economic level the need to develop energetically and economically efficient strategies of animal production has been coupled with a renewed awareness of the implications for human health of excessive intakes of saturated fats. Emphasis then has switched from the maximization of weight gain as an end in itself towards a need to promote protein deposition at any given intake and, at the same time, to reduce the fat content of meat and meat products. These twin objectives might be achieved by one of three strategies: the promotion of protein deposition alone, because at any given rate of weight gain this will tend to minimize the rate of fat deposition (the so-called repartitioning effect); the reduction of fat gain (an approach that has received particularly close attention by those concerned primarily with human obesity); or ideally the simultaneous promotion of protein accretion and depression of that of fat.

Metformin (Glucophage) inhibits tyrosine phosphatase activity to stimulate the insulin receptor tyrosine kinase

Metformin is a commonly used anti-diabetic but whether its mechanism involves action on the insulin receptor or on downstream events is still controversial. With a time course that was slow compared with insulin action, metformin increased tyrosine phosphorylation of the regulatory domain of the insulin receptor (specifically, tyrosine residues 1150 and 1151). In a direct action, therapeutic levels of metformin stimulated the tyrosine kinase activity of the soluble intracellular portion of the beta subunit of the human insulin receptor toward a substrate derived from the insulin receptor regulatory domain. However, metformin did not alter the order of substrate phosphorylation by the insulin receptor kinase. Using a Xenopus oocyte preparation, we simultaneously recorded tyrosine kinase and phosphatase activities that regulate the insulin receptor by measuring the tyrosine phosphorylation and dephosphorylation of peptides derived from the regulatory domain of the human insulin receptor. In an indirect stimulation of the insulin receptor, metformin inhibited endogenous tyrosine phosphatases and purified human protein tyrosine phosphatase 1B that dephosphorylate and inhibit the insulin receptor kinase. Thus, there was evidence that metformin acted directly upon the insulin receptor and indirectly through inhibition of tyrosine phosphatases.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

Inhibitory effect of fluoride on insulin receptor autophosphorylation and tyrosine kinase activity

Fluoride is a nucleophilic reagent which has been reported to inhibit a variety of different enzymes such as esterases, asymmetrical hydrolases and phosphatases. In this report, we demonstrate that fluoride inhibits tyrosine kinase activity of insulin receptors partially purified from rat skeletal muscle and human placenta. Fluoride inhibited in a similar dose-dependent manner both beta-subunit autophosphorylation and tyrosine kinase activity for exogenous substrates. This inhibitory effect of fluoride was not due to the formation of complexes with aluminum and took place in the absence of modifications of insulin-binding properties of the insulin receptor. Fluoride did not complete with the binding site for ATP or Mn2+. Fluoride also inhibited the autophosphorylation and tyrosine kinase activity of receptors for insulin-like growth factor I from human placenta. Addition of fluoride to the pre-phosphorylated insulin receptor produced a slow (time range of minutes) inhibition of receptor kinase activity. Furthermore, fluoride inhibited tyrosine kinase activity in the absence of changes in the phosphorylation of prephosphorylated insulin receptors, and the sensitivity to fluoride was similar to the sensitivity of the unphosphorylated insulin receptor. The effect of fluoride-on tyrosine kinase activity was markedly decreased when insulin receptors were preincubated with the copolymer of glutamate/tyrosine. Prior exposure of receptors to free tyrosine or phosphotyrosine also prevented the inhibitory effect of fluoride. However, the protective effect of tyrosine or phosphotyrosine was maximal at low concentrations, suggesting the interaction of these compounds with the receptor itself rather than with fluoride. These data suggest: (i) that fluoride interacts directly and slowly with the insulin receptor, which causes inhibition of its phosphotransferase activity; (ii) that the binding site of fluoride is not structurally modified by receptor phosphorylation; and (iii) based on the fact that fluoride inhibits phosphotransferase activity in the absence of alterations in the binding of ATP, Mn2+ or insulin, we speculate that fluoride binding might affect the transfer of phosphate from ATP to the tyrosine residues of the beta-subunit of the insulin receptor and to the tyrosine residues of exogenous substrates.

Genes encoding receptors for insulin and insulin-like growth factor I are expressed in Xenopus oocytes and embryos

Insulin and insulin-like growth factor I (IGF-I) initiate their metabolic, growth, and differentiation effects through binding to the insulin receptor and the IGF-I receptor, two members of the tyrosine kinase family of receptors. To study the role of these peptides and receptors in early development, we used the polymerase chain reaction and embryo-derived RNA to generate partial cDNA sequences of the insulin receptor and IGF-I receptor from the amphibian Xenopus laevis. Three unique tyrosine kinase-related sequences were obtained. Two of the nucleotide sequences, XTK 1a and XTK 1b, corresponded to peptide that share 92% amino acid identity, and each is 89% identical to the human insulin receptor. The third sequence, XTK 2, corresponds to a peptide that has 92% amino acid identity with the human IGF-I receptor but only 80% identity with XTK 1a and XTK 1b. On the basis of these similarities, the pattern of conserved amino acids, and the tetraploid nature of the Xenopus genome, we suggest that XTK 1a and XTK 1b most likely represent the product of two different nonallelic insulin receptor genes, while XTK 2 may be one of the probable two Xenopus IGF-I receptor genes. By reverse transcription-polymerase chain reaction and gene-specific hybridization, expression of the three XTK sequences was detected in the oocyte, unfertilized egg, and embryos through gastrulation, neurulation, and tailbud stages. Competition binding assays with Xenopus membrane preparations demonstrated insulin receptors and IGF-I receptors in older tadpoles. IGF-I receptors were also present in oocytes, eggs, and gastrula embryos. By contrast, insulin binding was present but atypical in oocytes and was barely detected in eggs and gastrula embryos. The expression of receptors for insulin and IGF-I in early Xenopus embryos and their apparent distinct developmental regulation suggest that these molecules and their ligands may be important in early Xenopus development.

Microinjection of a protein-tyrosine-phosphatase inhibits insulin action in Xenopus oocytes

A protein-tyrosine-phosphatase (PTPase 1B; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48), specific for phosphotyrosyl residues, was microinjected into Xenopus oocytes. This resulted in a 3- to 5-fold increase in PTPase activity over endogenous levels. The PTPase blocked the insulin-stimulated phosphorylation of tyrosyl residues on endogenous proteins, including a protein having a molecular mass in the same range as the beta subunit of the insulin or insulin-like growth factor I receptor. PTPase 1B also blocked the activation of an S6 peptide kinase--i.e., an enzyme recognizing a peptide having the sequence RRLSSLRA found in a segment of ribosomal protein S6 and known to be activated early in response to insulin. On the other hand, the insulin stimulation of an S6 kinase, detected by using 40S ribosomes as substrate, was unaffected even though PTPase 1B partially prevented the phosphorylation of ribosomal protein S6 in vivo. Mono Q chromatography of insulin-treated oocyte extracts revealed two main peaks of S6 kinase activity. Fractions from the first peak displayed S6 peptide kinase activity that was essentially abolished in profiles from PTPase 1B-injected oocytes. Material from the second peak, which was best revealed by using 40S ribosomes as substrate and had comparatively little S6 peptide kinase activity, was minimally affected by PTPase 1B. These observations suggest that at least two distinct "S6 kinases" are involved in ribosomal protein S6 phosphorylation in vivo and that the activation pathways for these enzymes differ in their sensitivity to PTPase 1B.

Effect of microinjection of a low-Mr human placenta protein tyrosine phosphatase on induction of meiotic cell division in Xenopus oocytes

Homogeneous preparations of a protein phosphatase that is specific for phosphotyrosyl residues (protein tyrosine phosphatase [PTPase] 1B) were isolated from human placenta and microinjected into Xenopus oocytes. This resulted in an increase in activity of up to 10-fold over control levels, as measured in homogenates with use of an artificial substrate (reduced carboxamidomethylated and maleylated lysozyme). Microinjected PTPase was stable for at least 18 h. It is distributed within the oocyte in a manner similar to the endogenous activity and is suggestive of an interaction with cellular structures or molecules located predominantly in the animal hemisphere. The phosphatase markedly retarded (by up to 5 h) maturation induced by insulin. This, in conjunction with the demonstration that PTPase 1B abolished insulin stimulation of an S6 peptide (RRLSSLRA) kinase concomitant with a decrease in the phosphorylation of tyrosyl residues in a protein with the same apparent Mr as the beta subunit of the insulin and insulinlike growth factor 1 receptors (M. F. Cicirelli, N. K. Tonks, C. D. Diltz, E. H. Fischer, and E. G. Krebs, submitted for publication), provides further support for an essential role of protein tyrosine phosphorylation in insulin action. Furthermore, maturation was significantly retarded even when the PTPase was injected 2 to 4 h after exposure of the cells to insulin. PTPase 1B also retarded maturation induced by progesterone and maturation-promoting factor, which presumably do not act through the insulin receptor. These data point to a second site of action of the PTPase in the pathway of meiotic cell division, downstream of the insulin receptor and following the appearance of active maturation-promoting factor.

Effect of microinjection of a low-Mr human placenta protein tyrosine phosphatase on induction of meiotic cell division in Xenopus oocytes

Homogeneous preparations of a protein phosphatase that is specific for phosphotyrosyl residues (protein tyrosine phosphatase [PTPase] 1B) were isolated from human placenta and microinjected into Xenopus oocytes. This resulted in an increase in activity of up to 10-fold over control levels, as measured in homogenates with use of an artificial substrate (reduced carboxamidomethylated and maleylated lysozyme). Microinjected PTPase was stable for at least 18 h. It is distributed within the oocyte in a manner similar to the endogenous activity and is suggestive of an interaction with cellular structures or molecules located predominantly in the animal hemisphere. The phosphatase markedly retarded (by up to 5 h) maturation induced by insulin. This, in conjunction with the demonstration that PTPase 1B abolished insulin stimulation of an S6 peptide (RRLSSLRA) kinase concomitant with a decrease in the phosphorylation of tyrosyl residues in a protein with the same apparent Mr as the beta subunit of the insulin and insulinlike growth factor 1 receptors (M. F. Cicirelli, N. K. Tonks, C. D. Diltz, E. H. Fischer, and E. G. Krebs, submitted for publication), provides further support for an essential role of protein tyrosine phosphorylation in insulin action. Furthermore, maturation was significantly retarded even when the PTPase was injected 2 to 4 h after exposure of the cells to insulin. PTPase 1B also retarded maturation induced by progesterone and maturation-promoting factor, which presumably do not act through the insulin receptor. These data point to a second site of action of the PTPase in the pathway of meiotic cell division, downstream of the insulin receptor and following the appearance of active maturation-promoting factor.

Microinjection of antisense c-mos oligonucleotides prevents meiosis II in the maturing mouse egg

Injection of antisense oligonucleotides was used to investigate the function of c-mos in murine oocytes. Oocytes injected with antisense c-mos oligonucleotides completed the first meiotic division but failed to initiate meiosis II. Instead, loss of c-mos function led to chromosome decondensation, reformation of a nucleus after meiosis I, and cleavage to two cells. Therefore, c-mos is required for meiosis II during murine oocyte maturation.

Insulin and progesterone activate a common synthetic ribosomal protein S6 peptide kinase in Xenopus oocytes

A synthetic peptide Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala, the structure of which is based on that of a phosphorylated sequence in ribosomal protein S6, was employed as a probe for stimulated kinase activity in Xenopus laevis oocytes induced to mature with insulin or progesterone. Insulin elicited an early (20-30 min) 3-fold stimulation of S6 peptide phosphorylating activity that was not evident with progesterone. However, both hormones produced a delayed 7-12-fold stimulation of S6 peptide phosphorylating activity at the time of germinal vesicle breakdown. The results of DEAE-Sephacel, Sephacryl S-200, TSK-400, and heparin-Sepharose chromatographic fractionation experiments imply that a common S6 peptide kinase is activated as a consequence of short and long term insulin exposure, as well as in long term progesterone treatment of oocytes. Omission of potassium from the oocyte culture medium greatly facilitated insulin-induced meiotic maturation.

Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II

Ribosomal protein S6 is a component of the eukaryotic 40S ribosomal subunit that becomes phosphorylated on multiple serine residues in response to a variety of mitogens, including insulin, growth factors, and transforming proteins of many oncogenic viruses. Recently, an activated S6 kinase (S6 K II) has been purified to homogeneity from Xenopus eggs, and characterized immunologically and at the molecular level. Purified S6 K II can be deactivated in vitro by incubation with either protein phosphatase 1 or protein phosphatase 2A. Reactivation and phosphorylation of S6 K II occurs in vitro with an insulin-stimulated microtubule-associated protein-2 (MAP-2) protein kinase which is itself a phosphoprotein that can be deactivated by protein phosphatase 2A. These studies suggest that a step in insulin signalling involves sequential activation by phosphorylation of at least two serine/threonine protein kinases.

Antibodies to Xenopus egg S6 kinase II recognize S6 kinase from progesterone- and insulin-stimulated Xenopus oocytes and from proliferating chicken embryo fibroblasts

Ribosomal protein S6 becomes highly phosphorylated during progesterone- or insulin-induced maturation of Xenopus laevis oocytes. We have previously purified an Mr 92,000 protein as one of the major S6 kinases from Xenopus unfertilized eggs. In this paper we confirm by renaturation of activity from a sodium dodecyl sulfate-polyacrylamide gel that this protein is an S6 kinase. This enzyme, termed S6 kinase II (S6 K II), was used for the preparation of polyclonal antiserum. Immunocomplexes formed with the antiserum and purified S6 K II were able to express kinase activity with the same substrate specificity as that of the purified enzyme, including autophosphorylation of S6 K II itself. The antiserum did not react with S6 kinase I, another major S6 kinase present in Xenopus eggs, which is chromatographically distinct from S6 K II. The administration of progesterone to oocytes resulted in a 20- to 25-fold increase in S6 kinase activity in extracts of these cells. Immunocomplex kinase assays done on extracts revealed that anti-S6 K II serum reacted with S6 kinase from progesterone-treated oocytes. This antiserum also reacted with the activated S6 kinase from insulin-stimulated oocytes. In addition, anti-S6 K II serum reacted with activated S6 kinase from chicken embryo fibroblasts stimulated with serum or transformed by Rous sarcoma virus. These results indicate that S6 K II or an antigenically related S6 kinase(s) is subject to regulation by mitogenic stimuli in various cell types.

After insulin binds

Three recent advances pertinent to the mechanism of insulin action include (i) the discovery that the insulin receptor is an insulin-dependent protein tyrosine kinase, functionally related to certain growth factor receptors and oncogene-encoded proteins, (ii) the molecular cloning of the insulin proreceptor complementary DNA, and (iii) evidence that the protein tyrosine kinase activity of the receptor is essential for insulin action. Efforts are now focusing on the physiological substrates for the receptor kinase. Experience to date suggests that they will be rare proteins whose phosphorylation in intact cells may be transient. The advantages of attempting to dissect the initial biochemical pathway of insulin action include the wealth of information about the metabolic consequences of insulin action and the potential for genetic analysis in Drosophila and in man.

Regulation of insulin receptor-associated tyrosine kinase by a polyclonal IgG

The effect of a polyclonal anti-insulin receptor antibody (pIgG) on the insulin receptor tyrosine kinase (IRTK) activity toward poly-(Glu-Tyr) was examined using wheat germ agglutinin agarose-purified insulin receptors from rat liver membranes. The main effect of pIgG was a reduction of Vmax (from 60.8 to 31.8 pmol/min/mg), without changes of Km, when IRTK was activated by insulin. In contrast, when IRTK was activated by ATP preincubation, pIgG was unable to affect the reaction, suggesting that IRTK possesses at least two regulatory mechanisms, one of which can be affected by pIgG.

Antibodies to Xenopus egg S6 kinase II recognize S6 kinase from progesterone- and insulin-stimulated Xenopus oocytes and from proliferating chicken embryo fibroblasts

Ribosomal protein S6 becomes highly phosphorylated during progesterone- or insulin-induced maturation of Xenopus laevis oocytes. We have previously purified an Mr 92,000 protein as one of the major S6 kinases from Xenopus unfertilized eggs. In this paper we confirm by renaturation of activity from a sodium dodecyl sulfate-polyacrylamide gel that this protein is an S6 kinase. This enzyme, termed S6 kinase II (S6 K II), was used for the preparation of polyclonal antiserum. Immunocomplexes formed with the antiserum and purified S6 K II were able to express kinase activity with the same substrate specificity as that of the purified enzyme, including autophosphorylation of S6 K II itself. The antiserum did not react with S6 kinase I, another major S6 kinase present in Xenopus eggs, which is chromatographically distinct from S6 K II. The administration of progesterone to oocytes resulted in a 20- to 25-fold increase in S6 kinase activity in extracts of these cells. Immunocomplex kinase assays done on extracts revealed that anti-S6 K II serum reacted with S6 kinase from progesterone-treated oocytes. This antiserum also reacted with the activated S6 kinase from insulin-stimulated oocytes. In addition, anti-S6 K II serum reacted with activated S6 kinase from chicken embryo fibroblasts stimulated with serum or transformed by Rous sarcoma virus. These results indicate that S6 K II or an antigenically related S6 kinase(s) is subject to regulation by mitogenic stimuli in various cell types.

Identification and characterization of a hexameric form of nucleolar phosphoprotein B23

Under native purification conditions, an oligomeric form (Mr = 230,000) and monomeric form (37,000) of protein B23 were purified by affinity chromatography. Both forms were identified by Western blot immunoassay and ELISA. The molecular weight of the oligomeric form of protein B23 was estimated to be 230,000 with a Stoke's radius and a sedimentation coefficient of 51 A and 10 S, respectively. The oligomer (230 kDa) of protein B23 was dissociated into monomers (37 kDa) by treatment with 7 M urea. Quantitation of the monomer by gel scanning densitometry indicated that the oligomeric form of protein B23 is a hexamer containing four alpha and two beta monomers (37 kDa). A trace amount of nucleic acids (amounting to less than 3% of the total mass) was detected in the affinity-purified oligomers of protein B23. Protein B23 may be a structural element which is involved in ribosome transport or assembly in the nucleus.

Insulin stimulates a membrane-bound serine kinase that may be phosphorylated on tyrosine

Triton X-100-solubilized high-density microsomes from insulin-treated rat adipocytes exhibit a marked increase in serine/threonine and tyrosine kinase activities toward exogenous histone when compared to controls. The insulin-dependent activation of microsomal histone kinase activities occurs within the physiological range of hormone concentrations (ED50 = 0.6 nM). The hormone-enhanced histone phosphorylation by the high-density microsomes appears to be catalyzed by two distinct kinases, based on their differential interaction with wheat germ agglutinin-agarose. The insulin-sensitive serine/threonine kinase is not retained by The insulin-sensitive serine/threonine kinase is not retained by the lectin column, whereas the tyrosine kinase appears to be a glycoprotein as evidenced by its adsorption to the immobilized lectin. The insulin-stimulated serine/threonine kinase exhibits preferential phosphorylation of histone and Kemptide (synthetic Leu-Arg-Arg-Ala-Ser-Leu-Gly) compared to a number of other peptide substrates. The substrate specificity of this serine/threonine kinase shows that it is distinct from the kinases that phosphorylate ribosomal protein S6, casein, phosvitin, ATP citrate lyase, and glycogen synthase and from multifunctional calmodulin-dependent, cAMP- and cGMP-dependent, and Ca2+/phospholipid-dependent protein kinases. Furthermore, 22% of the insulin-sensitive serine/threonine kinase activity can be adsorbed by monoclonal anti-phosphotyrosine antibodies immobilized on agarose. Its adsorption is specifically inhibited by excess free phosphotyrosine but not phosphoserine or phosphothreonine. The data suggest that this insulin-stimulated serine/threonine kinase in adipocyte high-density microsomes is tyrosine-phosphorylated, consistent with the hypothesis that the stimulatory action of insulin on this kinase may be mediated by tyrosine phosphorylation.

Purification of a bovine liver S6 kinase

A bovine liver protein serine kinase that catalyzes the multisite phosphorylation of ribosomal protein S6 has been purified to near homogeneity. The enzyme has an Mr of 67,000 on SDS-polyacrylamide gel electrophoresis and an apparent molecular weight of 55,000 on glycerol gradient sedimentation. Its enzymic properties, substrate specificity, molecular size and chromatographic behaviour are similar to those of the principal growth factor--and phorbol 12-myristate 13-acetate-stimulated S6 kinase of cultured cells.

Estrogen stimulates growth of mammary tumor cells ZR-75 without activation of S6 kinase and S6 phosphorylation. Difference from epidermal growth factor and alpha-transforming growth-factor-induced proliferation

Growth of the human mammary tumor cell line ZR-75-1 is stimulated by epidermal growth factor (EGF) and alpha-type transforming growth factor (alpha TGF), as well as by estradiol (E2). The role of activation of S6 kinase and S6 phosphorylation in the EGF(alpha TGF)-induced and E2-induced growth was investigated. Maximal effects on growth are observed at 10 nM EGF or alpha TGF. EGF as well as alpha TGF treatment of serum-starved cells leads to rapid activation of S6 kinase; the activity is increased about tenfold after 30 min of EGF treatment and declines with the time reaching about 25% of the maximal activity after 2 h of EGF treatment. Similar to the growth response, S6 kinase is activated at lower doses of EGF than alpha TGF and shows a maximal response at 10 nM for both growth factors. In contrast to this finding the incubation of serum-starved cells with E2 over a concentration range between 1 pM and 10 nM and times from 30 min to 4 h does not lead to increased S6 kinase activity. On investigating whether this lack of response to E2 is due to desensitization of the system by induction of alpha TGF it was found that preincubation of cells with alpha TGF for 2-6 h desensitizes them to reactivation of S6 kinase by alpha TGF, whereas preincubation with E2 does not. When S6 phosphorylation is monitored over times from 1 h to 6 h, it is observed that EGF leads to increased S6 phosphorylation, whereas E2 does not. The rate of onset of protein synthesis in the first 2 h of stimulation, when EGF-induced S6 phosphorylation is maximal, is more rapid with EGF than with E2. The results suggest that different pathway are involved in E2-induced and EGF(alpha TGF)-induced proliferation.

Decreased kinase activity of insulin receptors from adipocytes of non-insulin-dependent diabetic subjects

The tyrosine kinase activity of the insulin receptor was examined with partially-purified insulin receptors from adipocytes obtained from 13 lean nondiabetics, 14 obese nondiabetics, and 13 obese subjects with non-insulin-dependent diabetes (NIDDM). Incubation of receptors at 4 degrees C with [gamma-32P]ATP and insulin resulted in a maximal 10-12-fold increase in autophosphorylation of the 92-kDa beta-subunit of the receptor with a half maximal effect at 1-3 ng/ml free insulin. Insulin receptor kinase activity in the three experimental groups was measured by means of both autophosphorylation and phosphorylation of the exogenous substrate Glu4:Tyr1. In the absence of insulin, autophosphorylation and Glu4:Tyr1 phosphorylation activities, measured with equal numbers of insulin receptors, were comparable among the three groups. In contrast, insulin-stimulated kinase activity was comparable in the control and obese subjects, but was reduced by approximately 50% in the NIDDM group. These findings indicate that the decrease in kinase activity in NIDDM resulted from a reduction in coupling efficiency between insulin binding and activation of the receptor kinase. The insulin receptor kinase defects observed in NIDDM could be etiologically related to insulin resistance in NIDDM and the pathogenesis of the diabetic state.