María José García-Barrado

Ras-GRF1 signaling is required for normal β-cell development and glucose homeostasis

Development of diabetes generally reflects an inadequate mass of insulin‐producing β‐cells. β‐cell proliferation and differentiation are regulated by a variety of growth factors and hormones, including insulin‐like growth factor I (IGF‐I). GRF1 is a Ras‐guanine nucleotide exchange factor known previously for its restricted expression in brain and its role in learning and memory. Here we demonstrate that GRF1 is also expressed in pancreatic islets. Interest ingly, our GRF1‐deficient mice exhibit reduced body weight, hypoinsulinemia and glucose intolerance owing to a reduction of β‐cells. Whereas insulin resistance is not detected in peripheral tissues, GRF1 knockout mice are leaner due to increased lipid catabolism. The reduction in circulating insulin does not reflect defective glucose sensing or insulin production but results from impaired β‐cell proliferation and reduced neogenesis. IGF‐I treatment of isolated islets from GRF1 knockouts fails to activate critical downstream signals such as Akt and Erk. The observed phenotype is similar to manifestations of preclinical type 2 diabetes. Thus, our observations demonstrate a novel and specific role for Ras‐GRF1 pathways in the development and maintenance of normal β‐cell number and function.

Oral Insulin-Mimetic Compounds That Act Independently of Insulin

The hallmarks of insulin action are the stimulation and suppression of anabolic and catabolic responses, respectively. These responses are orchestrated by the insulin pathway and are initiated by the binding of insulin to the insulin receptor, which leads to activation of the receptor’s intrinsic tyrosine kinase. Severe defects in the insulin pathway, such as in types A and B and advanced type 1 and 2 diabetes lead to severe insulin resistance, resulting in a partial or complete absence of response to exogenous insulin and other known classes of antidiabetes therapies. We have characterized a novel class of arylalkylamine vanadium salts that exert potent insulin-mimetic effects downstream of the insulin receptor in adipocytes. These compounds trigger insulin signaling, which is characterized by rapid activation of insulin receptor substrate-1, Akt, and glycogen synthase kinase-3 independent of insulin receptor phosphorylation. Administration of these compounds to animal models of diabetes lowered glycemia and normalized the plasma lipid profile. Arylalkylamine vanadium compounds also showed antidiabetic effects in severely diabetic rats with undetectable circulating insulin. These results demonstrate the feasibility of insulin-like regulation in the complete absence of insulin and downstream of the insulin receptor. This represents a novel therapeutic approach for diabetic patients with severe insulin resistance.

Changes in plasma glucose and lactate evoked by α and β2‐adrenoceptor stimulation in conscious fasted rabbits

Summary— In conscious fasted rabbits, the iv infusion of salbutamol (3 μg/kg per min) and clonidine (2 μg/kg per min) induced a blood glucose increase amenable to blockade, respectively by ICI 118551 (1 μg/kg per min) and idazoxan (20 μg/kg per min). Amidephrine (10 μg/kg per min) and salbutamol mediated an increase in plasma lactate which was attenuated by prazosin (50 μg/kg, sc) and ICI 118551 respectively. Clonidine did not alter basal plasma lactate. The iv infusion of adrenaline (0.3 μg/kg per min) evoked an increase in plasma lactate more sensitive to blockade by ICI 118551 than by prazosin. ICI 118551 also shortened the hyperglycaemic response to adrenaline, 3‐Mercaptopicolinic acid (25 mg/kg) reduced salbutamol‐ and adrenaline‐mediated hyperglycaemia and increased at the same time the lactate/glucose ratio. Our data show that plasma lactate levels may be regulated by α1 ‐ and β2‐excitatory adrenoceptor stimulation. However, only the increase in blood lactate derived from β2‐adrenergic stimulation seems to contribute to the overall catecholamine‐mediated hyperglycaemia.

Amine oxidase substrates for impaired glucose tolerance correction

Amine oxidases are widely distributed from microorganisms to vertebrates and produce hydrogen peroxide plus aldehyde when catabolizing endogenous or xenobiotic amines. Novel roles have been attributed to several members of the amine oxidase families, which cannot be anymore considered as simple amine scavengers. Semicarbazide-sensitive amine oxidase (SSAO) is abundantly expressed in mammalian endothelial, smooth muscle, and fat cells, and plays a role in lymphocyte adhesion to vascular wall, arterial fiber elastic maturation, and glucose transport, respectively. This latter role was studied in detail and the perspectives of insulin-like actions of amine oxidase substrates are discussed in the present review. Independent studies have demonstrated that SSAO substrates and monoamine oxidase substrates mimic diverse insulin effects in adipocytes: glucose transport activation, lipogenesis stimulation and lipolysis inhibition. These substrates also stimulatein vitro adipogenesis. Acutein vivo administration of amine oxidase substrates improves glucose tolerance in rats, mice and rabbits, while chronic treatments with benzylamine plus vanadate exert an antihyperglycaemic effect in diabetic rats. Dietary supplementations with methylamine, benzylamine or tyramine have been proven to influence metabolic control in rodents by increasing glucose tolerance or decreasing lipid mobilisation, without noticeable changes in the plasma markers of lipid peroxidation or protein glycation, despite adverse effects on vasculature. Thus, the ingested amines are not totally metabolized at the intestinal level and can act on adipose and vascular tissues. In regard with this influence on metabolic control, more attention must be paid to the composition or supplementation in amines in foods and nutraceutics.ResumenLas amino-oxidasas (AO) están ampliamente distribuidas, desde microorganismos hasta vertebrados, y producen peróxido de hidrógeno y aldehído al catabolizar aminas biógenas o exógenas. Datos recientes ponen de manifiesto que las AO no pueden considerarse exclusivamente como depuradoras de aminas. La amino-oxidasa sensible a semicarbazida (SSAO) es muy abundante en ciertas células de mamíferos como las endoteliales, las células musculares lisas y los adipocitos, donde desempeña un papel importante en la adhesión de los linfocitos a las paredes vasculares, la maduración de las fibras elásticas arteriales, y el transporte de glucosa, respectivamente. Este último efecto es el que se presenta en esta revisión donde, además, se discuten las perspectivas abiertas como consecuencia de sus acciones insulino-miméticas. Los sustratos de SSAO y de monoamino-oxidasa mimetizan varios efectos de la insulina en los adipocitos: activación del transporte de glucosa y de la lipogénesis e inhibición de la lipólisis. También estimulanin vitro la adipogénesis. La administración agudain vivo de sustratos de AO mejora la tolerancia a la glucosa en ratas, ratones y conejos, y los tratamientos crónicos con benzilamina y vanadato tienen un efecto antihiperglucemiante en ratas diabéticas. Suplementos en la dieta con metilamina, benzilamina o tiramina han puesto de manifiesto una acción beneficiosa sobre el control metabólico en roedores a través de un aumento en la tolerancia a la glucosa o una disminución de la movilización lipídica, sin cambios notables en los marcadores plasmáticos de peroxidación lipídica o de glicación proteica, a pesar de unos efectos aterogénicos indeseables. Por tanto, las aminas ingeridas no se degradan totalmente a nivel de la barrera intestinal y pueden actuar sobre el tejido adiposo y vascular. En relación con esta influencia sobre el control metabólico, se concluye que es necesario prestar atención a la composición o adición como suplementos de estas aminas a los alimentos y medicamentos.

Role of μ-opioid receptors in insulin release in the presence of inhibitory and excitatory secretagogues

In mouse pancreatic islets incubated under static conditions, the inhibitory effects on glucose-evoked insulin release induced by adrenaline (1 microM), clonidine (2 microM) and UK 14,304 (brimonidine, 0.001-1 microM) were abolished by naloxone (30 nM). Only CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2), 0.1 microM), a very selective mu-opioid receptor antagonist, blocked the response to UK 14,304. Glucose-induced insulin secretion was attenuated by both beta-endorphin (0.01 microM) and endomorphin-1 (0.1 microM). Naloxone and CTOP prevented these inhibitory responses. The stimulatory effect of glibenclamide (1 microM) was also reduced by endomorphin-1. However, when islets were incubated in the presence of K(+) (30 mM), carbachol (100 microM) or forskolin (0.1 microM), neither the inhibitory effect induced by UK 14,304 was reversed by naloxone, nor endomorphin-1 altered the responses promoted by the excitatory agents. Thus, alpha(2)-adrenoceptor stimulation might inhibit glucose-induced insulin secretion by releasing endogenous opioids. Mu-Opioid receptor activation and opening of K(ATP) channels could be involved in the response.

Differential sensitivity to adrenergic stimulation underlies the sexual dimorphism in the development of diabetes caused by Irs-2 deficiency

The diabetic phenotype caused by the deletion of insulin receptor substrate-2 (Irs-2) in mice displays a sexual dimorphism. Whereas the majority of male Irs-2(-/-) mice are overtly diabetic by 12 weeks of age, female Irs-2(-/-) animals develop mild obesity and progress less rapidly to diabetes. Here we investigated β-cell function and lipolysis as potential explanations for the gender-related differences in this model. Glucose-stimulated insulin secretion was enhanced in islets from male null mice as compared to male WT whereas this response in female Irs-2(-/-) islets was identical to that of female controls. The ability of α(2)-adrenoceptor (α(2)-AR) agonists to inhibit insulin secretion was attenuated in male Irs2 null mice. Consistent with this, the expression of the α(2A)-AR was reduced in male Irs-2(-/-) islets. The response of male Irs-2(-/-) islets to forskolin was enhanced, owing to increased production of cAMP. Basal lipolysis was increased in male Irs-2(-/-) but decreased in female Irs-2(-/-) mice, concordant with the observation that adipose tissue is sparse in males whereas female Irs2 null mice are mildly obese. Adipocytes from both male and female Irs-2(-/-) were resistant to the anti-lipolytic effects of insulin but female Irs-2(-/-) fat cells were additionally resistant to the catabolic effects of beta-adrenergic agonists. This catecholamine resistance was associated with impaired generation of cAMP. Consequently, targets of cAMP-dependent protein kinase (PKA) which mediate lipolysis were not phosphorylated in adipose tissue of female Irs-2(-/-) mice. Our findings suggest that IRS-2 deficiency in mice alters the expression and/or sensitivity of components of adrenergic signaling.

Effects of verapamil and elgodipine on isoprenaline-induced metabolic responses in rabbits

Verapamil (0.17 microg kg(-1) min(-1) intravenous, i.v.) but not elgodipine (35 ng kg(-1) min(-1)) modestly enhanced the weak blood glucose increase induced by the i.v. infusion of isoprenaline (0.3 microg kg(-1) min(-1)) in conscious rabbits. However, elgodipine but not verapamil suppressed the increase in circulating insulin evoked by the agonist. Both drugs enhanced the rise in plasma lactate mediated by isoprenaline but only elgodipine potentiated the lipolytic effect of the agonist. In isolated islets elgodipine (10(-6) M) blocked forskolin (10(-6) M)-induced insulin release. However, in rabbit adipocytes elgodipine potentiated both glycerol release and cAMP accumulation induced by isoprenaline (10(-8)-10(-6) M). Excess K(+) (40-60 mM) did not alter basal lipolysis or the response to isoprenaline in either rabbit or mouse adipocytes. Therefore, Ca2+ influx through L-type Ca2+ channels does not seem to play a significant role in the lipolytic effect of isoprenaline. Metabolic alterations found with Ca2+ channel antagonists were of minor intensity and probably devoid of pathological implications.

Relevance of pituitary aromatase and estradiol on the maintenance of the population of prolactin-positive cells in male mice

In previous studies we demonstrated the expression of aromatase in pituitary cells. This expression is gender related, and is also associated with the presence of prolactinomas. To ascertain the relevance of aromatase in modulating the populations of prolactin-positive pituitary cells an immunocytochemical and morphometric study of prolactin-positive pituitary cells was carried out using the pituitary glands of adult male and female aromatase-knockout (ArKO) mice. Additionally has been determined if pituitary aromatase is involved in a gender-linked differentiated regulation of the prolactin-producing pituitary cells. Compared to wild-type mice, the knockout animals of both genders showed a significant decrease (p<0.01) in the cellular and nuclear areas of their prolactin cells, as well as in the percentages of the prolactin-positive cells and the proliferating prolactin cells. Our results suggest that estradiol is responsible for the maintenance of the population of prolactin cell in males and, so as not to disturb the endocrine reproductive environment, estradiol is synthesized inside the pituitary by circulating testosterone via means of aromatase P450, which acts in paracrine way. This new role for pituitary aromatase may well explain the previous findings establishing that the pituitary expression of aromatase is higher in males than in females, and the association between the development of prolactinomas and the increased expression of aromatase in tumours.

Is there an optimal dose for dietary linoleic acid?: Lessons from essential fatty acid deficiency supplementation and adipocyte functions in rats

Differential effects of n-3 and n-6 polyunsaturated fatty acids (PUFAs) have been demonstrated on adipose tissue physiology. Facing to the widely recognized beneficial effects of n-3 PUFAs, the n-6 PUFA effects remain controversial. Thus, we further analyzed the linoleic acid (LA) influence on adipocyte functions. To this aim, we treated by LA supplementation at three distinct doses (1, 2.5, or 5xa0% of energy intake) rats with essential fatty acids deficiency (EFAD). PUFA composition was determined in blood and white adipose tissue (WAT), while lipolytic and lipogenic activities were measured in isolated adipocytes. EFAD rats exhibited reduced WAT mass and increased EFAD biomarkers. WAT mass was completely recovered after supplementation, irrespective of LA dose. However, neither body mass nor EFAD biomarkers returned to control with 1xa0% LA, while LA abundance doubled in adipocytes from rats supplemented with 5xa0% LA. Regarding lipolysis, 2.5xa0% LA normalized the EFAD-induced alterations. A trend to decrease the maximal stimulation of lipolysis was observed with 1 and 5xa0% LA. Regarding lipogenesis, the lower and higher LA doses increased basal activity and hampered insulin to further stimulate glucose incorporation into lipids whereas 2.5xa0% LA normalized the basal or insulin-stimulated levels. Our results show that dietary linoleate at 2.5xa0% restored anatomical, biochemical, and functional disturbances induced by EFAD. At higher dose, LA tended to reduce triacylglycerol breakdown, to increase triacylglycerol assembly, and to provoke insulin resistance. Therefore, LA influence on adipocyte functions does not appear to follow a typical dose–response relationship, adding further complexity to the definition of its dietary requirement.

Adipocitocinas: implicaciones en el pronóstico y tratamiento farmacológico de patología cardiovascular

Las adipocitocinas, factores derivados del tejido graso con propiedades reguladoras, estan implicadas en la fisiopatologia de procesos ateromatosos y metabolicos tales como: cardiopatia isquemica, resistencia a la insulina, obesidad, dislipidemia y diabetes mellitus. El aumento de los depositos de tejido adiposo visceral determina una peor evolucion para esas afecciones. Farmacos como los inhibidores de la enzima convertidora de la angiotensina (IECA), las tiazolidindionas (glitazonas) o los antagonistas del receptor de angiotensina-II, generalmente asociados con la adecuada medicacion hipolipidemiante (estatinas, fibratos) o antiobesidad (orlistat, sibutramina, rimonabant), incrementarian las adipocitocinas con propiedades anti-inflamatorias e insulino-sensibilizadoras (es decir, adiponectina o visfatina), mientras que reducirian aquellas pro-inflamatorias y trombogenicas (como leptina, factor de necrosis tumoral [TNF]-a, inhibidor del activador del plasminogeno tipo 1 [PAI-1]). Por tanto, estas medidas terapeuticas farmacologicas podrian tener un efecto beneficioso al disminuir la morbimortalidad y mejorar el pronostico de los pacientes que padecen dichas enfermedades, todas ellas relacionadas con un elevado riesgo cardiovascular.