Growth-promoting Effect Of Insulin Research Articles

Role of insulin and insulin resistance in androgen excess disorders.

Insulin has complex effects on cell growth, metabolism and differentiation, and these effects are mediated by a cell-surface bound receptor and eventually a cascade of intracellular signaling events. Among the several metabolic and growth-promoting effects of insulin, insulin resistance is defined as an attenuated effect of insulin on glucose metabolism, primarily the limited export of blood glucose into skeletal muscle and adipose tissue. On the other hand, not all the signaling pathways and insulin-responsive tissues are equally affected, and some effects other than the metabolic actions of insulin are overexpressed. Ovaries and the adrenal glands are two examples of tissues remaining sensitive to insulin actions where insulin may contribute to increased androgen secretion. Polycystic ovary syndrome (PCOS) is the most common form of androgen excess disorder (AED), and its pathogenesis is closely associated with insulin resistance. Patients with idiopathic hirsutism also exhibit insulin resistance, albeit lower than patients with PCOS. Although it is not as evident as in PCOS, patients with congenital adrenal hyperplasia may have insulin resistance, which may be further exacerbated with glucocorticoid overtreatment and obesity. Among patients with severe insulin resistance syndromes, irrespective of the type of disease, hyperinsulinemia promotes ovarian androgen synthesis independently of gonadotropins. It is highly debated in whom and how insulin resistance should be diagnosed and treated among patients with AEDs, including PCOS. It is not suitable to administer an insulin sensitizer relying on only some mathematical models used for estimating insulin resistance. Instead, the treatment decision should be based on the constellation of the signs, symptoms and presence of obesity; acanthosis nigricans; and some laboratory abnormalities such as impaired glucose tolerance and impaired fasting glucose.

Protein-tyrosine Phosphatase 1B Deficiency Reduces Insulin Resistance and the Diabetic Phenotype in Mice with Polygenic Insulin Resistance

Mice heterozygous for insulin receptor (IR) and IR substrate (IRS)-1 deficiency provide a model of polygenic type 2 diabetes in which early-onset, genetically programmed insulin resistance leads to diabetes. Protein-tyrosine phosphatase 1B (PTP1B) dephosphorylates tyrosine residues in IR and possibly IRS proteins, thereby inhibiting insulin signaling. Mice lacking PTP1B are lean and have increased insulin sensitivity. To determine whether PTP1B can modify polygenic insulin resistance, we crossed PTP1B-/- mice with mice with a double heterozygous deficiency of IR and IRS-1 alleles (DHet). DHet mice weighed slightly less than wild-type mice and exhibited severe insulin resistance and hyperglycemia, with approximately 35% of DHet males developing diabetes by 9-10 weeks of age. Body weight in DHet mice with PTP1B deficiency was similar to that in DHet mice. However, absence of PTP1B in DHet mice markedly improved glucose tolerance and insulin sensitivity at 10-11 weeks of age and reduced the incidence of diabetes and hyperplastic pancreatic islets at 6 months of age. Insulin-stimulated phosphorylation of IR, IRS proteins, Akt/protein kinase B, glycogen synthase kinase 3beta, and p70(S6K) was impaired in DHet mouse muscle and liver and was differentially improved by PTP1B deficiency. In addition, increased phosphoenolpyruvate carboxykinase expression in DHet mouse liver was reversed by PTP1B deficiency. In summary, PTP1B deficiency reduces insulin resistance and hyperglycemia without altering body weight in a model of polygenic type 2 diabetes. Thus, even in the setting of high genetic risk for diabetes, reducing PTP1B is partially protective, further demonstrating its attractiveness as a target for prevention and treatment of type 2 diabetes.

Insulin-enhanced liposome-mediated gene transfer into a gastric carcinoma cell line.

1. Liposome-mediated transfection is useful due to no DNA constraints, lower immunogenicity and easy preparation. However, it has the disadvantage of low transfection efficiency. We aimed to test whether lipofection efficiency could be enhanced in gastrointestinal cell lines by the growth-promoting effect of insulin. 2. To assess the effect of insulin on lipofection efficiency and the cell cycle, expression of green fluorescent protein (GFP) and DNA distribution in gastric (MKN1), colonic (HT29) and pancreatic (BxPC3) carcinoma cell lines was analysed using flow cytometry. 3. The percentage of positive cells with GFP was significantly higher in MKN1 cells in culture medium with 5 mg/mL insulin than without insulin, whereas the percentage was the same in HT29 and BxPC3 cells with insulin as without insulin. The percentage of S phase fraction MKN1 cells with insulin was greater than without insulin, whereas the percentage of S phase fractions of HT29 and BxPC3 cells was the same with or without insulin. Lipofection efficiency correlated with the percentage of S phase fraction. 4. Insulin has the potential to enhance efficiency of lipofection into a sensitive cell line by increasing cellular proliferation.

Increased IGFR activity and glucose transport in cultured skeletal muscle from insulin receptor null mice.

We have studied the role of the insulin receptor (IR) in metabolic and growth-promoting effects of insulin on primary cultures of skeletal muscle derived from the limb muscle of IR null mice. Cultures of IR null skeletal muscle displayed normal morphology and spontaneous contractile activity. Expression of muscle-differentiating proteins was slightly reduced in myoblasts and myotubes of the IR null skeletal muscle cells, whereas that of the Na+/K+ pump appeared to be unchanged. Insulin-like growth factor receptor (IGFR) expression was higher in myoblasts from IR knockout (IRKO) than from IR wild-type (IRWT) mice but was essentially unchanged in myotubes. Expression of the GLUT-1 and GLUT-4 transporters appeared to be higher in IRKO than in IRWT myoblasts and was significantly greater in myotubes from IRKO than from IRWT cultures. Consistent with GLUT expression, both basal and insulin or insulin-like growth factor I (IGF-I)-stimulated glucose uptakes were higher in IR null skeletal myotubes than in wild-type skeletal myotubes. Interestingly, autophosphorylation of IGFR induced by insulin and IGF-I was markedly increased in IR null skeletal myotubes. These results indicate that, in the absence of IR, there is a compensatory increase in basal as well as in insulin- and IGF-I-induced glucose transport, the former being mediated via increased activation of the IGF-I receptor.

3.P.256 Growth-promoting effect of insulin on aortic smooth muscle cells from insulin resistant Goto-Kakizaki rats: Role of protein kinase C

3.P.256 Growth-promoting effect of insulin on aortic smooth muscle cells from insulin resistant Goto-Kakizaki rats: Role of protein kinase C

Does Mitogen-Activated-Protein Kinase have a Role in Insulin Action?. The Cases for and Against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Does Mitogen‐Activated‐Protein Kinase have a Role in Insulin Action?

The discovery of the mitogen‐activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk‐1 and Erk‐2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth‐promoting effects of insulin and the regulation of Glut‐1 and c‐fos expression, and AP‐1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut‐4 translocation), glycogen synthesis, acetyl‐CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Relaxin stimulation of porcine granulosa cell deoxyribonucleic acid synthesis in vitro: interactions with insulin and insulin-like growth factor I.

Relaxin (RLX) is a product of the theca interna layer of the developing pig follicle. However, its physiological role within the ovary is unknown. The purpose of this study was to determine the effects of RLX alone or in conjunction with insulin and insulin-like growth factor-I (IGF-I) on porcine granulosa cell (GC) DNA synthesis and proliferation in vitro. The results showed RLX (1-1000 ng/ml), insulin (1-1000 ng/ml), and IGF-I (1-30 ng/ml) alone stimulated [3H]thymidine incorporation into DNA by GC from small, medium, and large porcine follicles (P < 0.05). The effect of RLX in combination with insulin or with IGF-I on DNA synthesis by GC from small and medium follicles was greater than the effect of either single factor (P < 0.05). In the large follicle, RLX increased GC DNA synthesis synergistically with insulin and IGF-I. FSH (100 ng/ml) or estradiol (10(-7) M) had no effect on basal or RLX-induced [3H]thymidine incorporation into GC DNA at any stage of follicular development. After 6 days incubation in vitro, RLX dose-dependently increased proliferation of GC from small follicles (P < 0.05). The increased cell number in response to RLX correlates with RLX-induced [3H]thymidine incorporation into DNA by these GC. Hydroxyurea, an inhibitor of DNA synthesis, blocked RLX-induced thymidine incorporation, indicating that the increase in thymidine incorporation in response to RLX was due to DNA synthesis and not repair. This is the first demonstration that RLX acts as a growth factor in the porcine follicle by increasing GC DNA synthesis and proliferation in vitro. RLX enhanced the growth-promoting effects of insulin and IGF-I on GC, which suggests an interaction of RLX with these proteins during follicular growth. These studies support the concept that locally produced thecal RLX alone or in combination with other growth effectors may play a key role in GC growth during porcine follicular development.

Insulin receptor : tyrosine kinase activity and insulin action

The first step in insulin action consists in binding of the hormone to specific cell surface receptors. This receptor displays two functional domains: an extracellular alpha-subunit containing the majority or the totality of the hormone binding site and an intracellular beta-subunit possessing insulin-stimulated tyrosine kinase activity. A general consensus has been reached in favour of the idea that this receptor enzymic function is essential for generation of the metabolic and growth-promoting effects of insulin. Concerning the mechanism of transmembrane signalling, we like to think that interaction of insulin with the receptor alpha-subunit triggers a conformational change, which is propagated to the beta-subunit and activates it. The active receptor kinase leads then to the phosphorylation of cellular protein substrates, which are likely to belong to two broad categories, those generating metabolic effects of insulin and those resulting in growth-promoting effects. The phosphorylated and active substrates then generate the final effects of insulin.

Effect of insulin on the growth of a human hepatoma cell line PLC/PRF/5: A possible role of insulin receptor.

Insulin produced increases in cell number, DNA synthesis and alpha-fetoprotein secretion of a human hepatoma cell line PLC/PRF/5. The insulin-treated cells showed a decrease in binding of 125I-insulin. The decreased binding was due to the decreased numbers as well as the low affinity constant of insulin receptor. These data suggest that insulin is a growth factor for the human hepatoma cells and that insulin receptor plays a role in the growth-promoting effect of insulin.

Coordinate roles of insulin and glucose on the growth of hepatoma cells in culture.

The growth-promoting effect of insulin on cells in culture is well established. In addition, most cells need glucose for growth, although hepatic cells, which have an active gluconeogenic pathway, can survive in glucose-free medium. Since insulin can inhibit gluconeogenesis, we have studied the effect of insulin on the growth of a well differentiated hepatoma cell line (Fao) in glucose-containing and glucose-free medium. In the presence of glucose, Fao cells had a doubling time of 26 h, and insulin stimulated cell growth in a concentration-dependent manner. Half-maximal stimulation occurred at physiological concentrations (3 × 10−11 m), and a maximal effect was observed at 10−8 m, with a 9-fold increase in total cell number after 6-14 days. In contrast, when glucose was omitted from the culture medium, the hepatoma cells grew slower (doubling time, 38 h), and insulin inhibited cell growth by up to 80%. The doseresponse curve for inhibition was similar to that for stimulation. Inhibition of growth correlat...

Evidence from monoclonal antibody studies that insulin stimulates deoxyribonucleic acid synthesis through the type I somatomedin receptor.

Somatomedin-C/insulin-like growth factor-I (Sm-C/IGF-I) and insulin stimulate DNA synthesis and cell replication in cultured human fibroblasts. It has been postulated that the growth-promoting actions of both peptides are mediated through the type I Sm-C/IGF-I receptor. This study tests this hypothesis using two recently developed monoclonal antibodies. The antibody designated sm 1.2 is directed to Sm-C, whereas the antibody designated alpha IR-3 is directed against the type I receptor for Sm-C/IGF-I. Radiolabeled monoclonal antibody alpha IR-3 was bound to human foreskin fibroblasts in a reversible time-dependent fashion, with 90% of the specific binding complete after 6 h of incubation at 15 C. Binding of [125I]alpha IR-3 was completely inhibited by excess unlabeled antibody, but not by 50 nM Sm-C or 1000 nM insulin. Specific binding of [125I]Sm-C fell to 27% of the control value in the presence of 50 nM alpha IR-3, and this concentration of antibody significantly reduced the mitogenic response to both Sm-C and insulin. Antibody sm 1.2 blocked the mitogenic response to exogenous Sm-C, but did not block the response to insulin; indeed, in some experiments, sm 1.2 enhanced the response to insulin. We postulate that this enhancement is the result of neutralizing endogenously produced Sm-like substances. This study provides further evidence that the growth-promoting effects of insulin in this cell type are the result of interaction with the Sm-C/IGF-I receptor.

Receptors and growth-promoting effects of insulin and insulinlike growth factors on cells from bovine retinal capillaries and aorta.

It has been suggested that elevated levels of insulin or insulin-like growth factors (IGFs) play a role in the development of diabetic vascular complications. Previously, we have shown a differential response to insulin between vascular cells from retinal capillaries and large arteries with the former being much more insulin responsive. In the present study, we have characterized the receptors and the growth-promoting effect of insulinlike growth factor I (IGF-I) and multiplication-stimulating activity (MSA, an IGF-II) on endothelial cells and pericytes from calf retinal capillaries and on endothelial and smooth muscle cells from calf aorta. We found single and separate populations of high affinity receptors for IGF-I and MSA with respective affinity constants of 1 X 10(-9) M-1 and 10(-8) M-1 in all four cell types studied. Specific binding of IGF-I was between 7.2 and 7.9% per milligram of protein in endothelial cells and 9.1 and 10.4% in the vascular supporting cells. For 125I-MSA, retinal endothelial cells bound only 1.7-2.5%, whereas the aortic endothelial cells and the vascular supporting cells bound between 5.6 and 8.5% per milligram of protein. The specificity of the receptors for IGF-I and MSA differed, as insulin and MSA was able to compete with 125I-IGF-I for binding to the IGF-I receptors with 0.01-0.1, the potency of unlabeled IGF-I, whereas even 1 X 10(-6) M, insulin did not significantly compete with 125I-MSA for binding to the receptors for MSA. For growth-promoting effects, as measured by the incorporation of [3H]thymidine into DNA, confluent retinal endothelial cells responded to IGF-I and MSA by up to threefold increase in the rate of DNA synthesis, whereas confluent aortic endothelial cells did not respond at all. A similar differential of response to insulin between micro- and macrovascular endothelial cells was reported by us previously. In the retinal endothelium, insulin was more potent than IGF-I and IGF-I was more potent that MSA. In the retinal and aortic supporting cells, no differential response to insulin or the IGFs was observed. In the retinal pericytes, IGF-I, which stimulated significant DNA synthesis beginning at 1 X 10(-9) M, and had a maximal effect at 5 X 10(-8) M, was 10-fold more potent than MSA and equally potent to insulin. In the aortic smooth muscle cells, IGF-I was 10-100 times more potent than insulin or MSA. In the retinal and aortic supporting cells, no differential response to insulin or the IGFs was observed. In the retinal pericytes, IGF-I, which stimulated significant DNA synthesis beginning at 1 X 10(-9) M, and had a maximal effect at 5 X 10(-8) M, was 10-fold more potent than MSA and equally potent to insulin. In the aortic smooth muscle cells, IGF-I was 10-100 times more potent than insulin or MSA. In addition, insulin and IGF-I at 1 X 10(-6) and 1 X 10(-8) M, respectively, stimulated these cells to grow by doubling the number of cells as well. In all responsive tissues, the combination of insulin and IGFs were added together, no further increase in effect was seen. These data showed that vascular cells have insulin and IGF receptors, but have a differential response to these hormones. These differences in biological response between cells from retinal capillaries and large arteries could provide clues to understanding the pathogenesis of diabetic micro- and macroangiopathy.

The Molecular Mechanism of Insulin Action

Insulin initiates its action by binding to a glycoprotein receptor on the surface of the cell. This receptor consists of an alpha-subunit, which binds the hormone, and a beta-subunit, which is an insulin-stimulated, tyrosine-specific protein kinase. Activation of this kinase is believed to generate a signal that eventually results in insulin's action on glucose, lipid, and protein metabolism. The growth-promoting effects of insulin appear to occur through activation of receptors for the family of related insulin-like growth factors. Both genetic and acquired abnormalities in the number of insulin receptors, the activity of the receptor kinase, and the various post-receptor steps in insulin action occur in disease states leading to tissue resistance to insulin action.

Growth-promoting effects of insulin and growth hormone in hypophysectomized and diethylstilboestrol-treated rats, and in rats fed 2-4-dinitrophenol.

Protamine zinc insulin has no growth-promoting effect in rats whose growth has been inhibited by diethylstilboestrol. Hypophyseal growth hormone stimulates growth in diethylstilboestrol-treated rats, and treatment with growth hormone and protamine zinc insulin results in an enhanced rate of growth. Protamine zinc insulin has growth-promoting properties in hypophysectomized rats, and cortisone acetate inhibits growth in insulin-treated hypophysectomized rats. 2-4-Dinitrophenol prevents growth and increases nitrogen excretion in young rats. Combined treatm ent with insulin, growth hormone and glucose induces growth and brings about positive nitrogen balance in dinitrophenol-poisoned rats. Adrenal, testis, liver and pancreas weights are normal in dinitrophenol-poisoned rats. The amount of extractable pancreatic insulin is also within normal limits. It is suggested that the functions of the hypophysis and pancreas are not seriously impaired, and that treatment with growth hormone and insulin promotes esterification of inorganic phosphate to high-energy phosphate compounds in the dinitrophenol-poisoned rats.