Tub Has a Key Role in Insulin and Leptin Signaling and Action In Vivo in Hypothalamic Nuclei

  1. Mario J.A. Saad
  1. Department of Internal Medicine, State University of Campinas, Campinas, Brazil
  1. Corresponding author: Mario J.A. Saad, msaad{at}fcm.unicamp.br.

Abstract

Mutation of tub gene in mice induces obesity, suggesting that tub could be an important regulator of energy balance. In the current study, we investigated whether insulin, leptin, and obesity can modulate Tub in vivo in hypothalamic nuclei, and we investigated possible consequences on energy balance, neuropeptide expression, and hepatic glucose metabolism. Food intake, metabolic characteristics, signaling proteins, and neuropeptide expression were measured in response to fasting and refeeding, intracerebroventricular insulin and leptin, and Tub antisense oligonucleotide (ASO). Tub tyrosine phosphorylation (Tub-p-tyr) is modulated by nutritional status. Tub is a substrate of insulin receptor tyrosine kinase (IRTK) and leptin receptor (LEPR)–Janus kinase 2 (JAK2) in hypothalamic nuclei. After leptin or insulin stimulation, Tub translocates to the nucleus. Inhibition of Tub expression in hypothalamus by ASO increased food intake, fasting blood glucose, and hepatic glucose output, decreased O2 consumption, and blunted the effect of insulin or leptin on proopiomelanocortin, thyroid-releasing hormone, melanin-concentrating hormone, and orexin expression. In hypothalamus of mice administered a high-fat diet, there is a reduction in leptin and insulin-induced Tub-p-tyr and nuclear translocation, which is reversed by reducing protein tyrosine phosphatase 1B expression. These results indicate that Tub has a key role in the control of insulin and leptin effects on food intake, and the modulation of Tub may contribute to insulin and leptin resistance in DIO mice.

Leptin and insulin signal adiposity to the hypothalamus and inhibit feeding, in part by reducing the expression of orexigenic neuropeptides and increasing the expression of anorexigenic neuropeptides (15). However, the network of signaling proteins induced by these hormones is not completely understood.

The protein Tub is primarily expressed in the central nervous system and has been implicated in energy balance homeostasis (614). Mice with a spontaneous mutation of the tub gene were called “tubby” mice and had development of late-onset obesity, insulin resistance, and sensory impairments (1520).

Inactivation of the tub gene by gene targeting is sufficient to induce the same phenotype as tubby mice; this suggests that the loss of Tub protein function is responsible for the tubby mice phenotype (21). Functions of Tub protein have been evaluated in only a few studies. Boggon et al. (22) suggested that the highly conserved COOH-terminal portion of the protein is a DNA-binding structure and that the NH2-terminal portion may be a regulator of transcription. They observed that on serotonin activation of 5-HT2c receptors, there was a translocation of Tub protein from the plasma membrane to the cellular nucleus of primary cultured neurons, suggesting that Tub may function as a transcription factor (22).

In addition to being a transcription factor, in vitro studies suggested that Tub might act as an adaptor protein downstream of the insulin receptor-signaling pathway (23). Moreover, it was also demonstrated that insulin induced Tub tyrosine phosphorylation in 3T3-L1 adipocytes (24). Together, these studies suggest that Tub is a substrate of the insulin receptor, at least in vitro. However, the role of Tub in insulin action and signaling in vivo has not yet been clarified.

In the hypothalamus, neuron populations of the arcuate nucleus (Arc), ventromedial hypothalamus, paraventricular nucleus (PVN), and lateral hypothalamus (LH) are responsive to insulin and leptin. Insulin acts through the insulin receptor (IR), which is a protein with endogenous tyrosine kinase activity, and in the hypothalamus insulin signals through IRS/PI3k/Akt/Foxo1 to control food intake (13,2530).

Leptin signals through the recruitment of the JAK2 to the leptin receptor (LEPR), where it phosphorylates several tyrosine residues on the LEPR. Signal transducer and activator of transcription (STAT) 3 proteins bind to the phosphorylated LEPR and are phosphorylated by JAK2. Phosphorylated STAT3 is translocated to the nucleus, where it is thought to bind to specific DNA sequences to regulate neuropeptide expression. Both leptin and insulin change the membrane potential of target neurons to control their firing rate, as well as neuropeptide and neurotransmitter release. In parallel, leptin and insulin signaling directly control transcription of neuropeptides in multiple hypothalamic nuclei. In the Arc, leptin and insulin increase the transcription of proopiomelanocortin (POMC), which is an anorexigenic neuropeptide (27,3133) and inhibit transcription of agouti-related peptide (AgRP) and neuropeptide Y (NPY), which are orexigenic neuropeptides (29). In the PVN, insulin and leptin induce an increase in thyroid-releasing hormone (TRH) expression, which is a positive regulator of energy expenditure (31). In the ventromedial hypothalamus, steroidogenic factor 1--expressing neurons respond to insulin and leptin to regulate food intake (34,35) and, in the LH, both hormones decrease the release of melanin-concentrating hormone (MCH) and orexin neuropeptides (36).

However, it has not yet been investigated whether, in the hypothalamus, insulin and leptin can modulate Tub phosphorylation and nuclear translocation, or whether this control might influence the expression of neuropeptides in physiologic conditions and also in obesity. In the current study, we investigated whether insulin, leptin, and obesity can modulate Tub in vivo in hypothalamic nuclei, and we investigated possible consequences on energy balance, neuropeptide expression, and hepatic glucose metabolism.

RESEARCH DESIGN AND METHODS

All experiments were approved by the Ethics Committee of the State University of Campinas. Eight-week-old male C57BL/6 mice obtained from the University of Campinas, São Paulo, Brazil, were assigned to receive a standard rodent chow or a high-fat diet (HFD) as previously described (37,38) and water ad libitum. For fasting and refeeding experiments, fasted (24 h) mice were allowed to refeed for 1 h or 2 h, and hypothalamic nuclei were dissected for protein studies, as described. All feeding tests were conducted between 8:00 a.m. and 10:00 a.m.

Intracerebroventricular cannulation.

Anesthetized mice were stereotaxically instrumented (Ultra Precise model 963; Kopf) to implant stainless steel cannulas (26-gauge; Plastics One) in the right lateral ventricle. The coordinates used from the bregma were: anterior/posterior, −0.5 mm; lateral, −1.3 mm; and dorso/ventral, −2.2 mm. Mice were single-housed after surgery and were allowed to recover for 5–7 days. The correct implantation of cannulas was checked by 10 ng angiotensin II intracerebroventricular (ICV) injection, which elicits an intake of water (39). Animals that did not reach this criterion were excluded from the experiments.

Hypothalamic nuclei dissection.

Arc, medial hypothalamus (MH; ventromedial and dorsomedial), PVN, and LH were quickly dissected in a stainless steel matrix with razor blades as described previously (40) and frozen in liquid nitrogen for further protein studies.

ICV injections.

To determine if insulin or leptin induces Tub tyrosine phosphorylation in vivo, overnight fasted mice fed chow or HFD received an ICV injection of insulin (human recombinant insulin; Eli Lilly, Indianapolis, IN) or recombinant leptin (Calbiochem, San Diego, CA), and hypothalamic nuclei were quickly dissected and frozen in liquid nitrogen for further protein studies. The time and dose responses to ICV insulin or leptin on Tub tyrosine phosphorylation were also investigated. To inhibit Tub (5′AGG AAC ACC TTC TTG CCA T 3′) or protein tyrosine phosphatase 1B (PTP1B) (5′CCC AGC AGC GGC TTC TGC AT 3′) expression in the hypothalamus, we developed antisense oligonucleotide (ASO) and administered them by ICV injections in mice fed chow or fed HFD, respectively. Both ICV ASO treatments were administered twice per day (8:00 a.m. and 5:00 p.m.) for 5 days. Body weight, epididymal fat mass, cumulative food intake, 4-h to 8-h food intake in response to ICV insulin and leptin, fasting blood glucose and blood glucose levels in response to pyruvate (pyruvate test) (41), and PEPCK expression in fasting livers were measured in Tub ASO-treated mice. We also determined O2 consumption and respiratory exchange rate (38). Akt, FoxO1, JAK2, and STAT3 phosphorylation in response to ICV insulin or leptin, respectively, were evaluated as described.

Immunoprecipitation and immunoblotting.

A pool of five to eight of each nucleus (Arc, MH, PVN, LH) was homogenized in extraction buffer. Immunoprecipitation (IP) and immunoblotting were performed as previously described (3739) using the following antibodies: antiphosphotyrosine, anti-IRβ (α-IR), anti-Tub (M-19), anti-β actin, anti-JAK2, anti-Akt, anti-STAT3, anti-FoxO1, anti-PTP1B, and anti-PEPCK (H-300) antibodies from Santa Cruz Technology (Santa Cruz, CA); and antipAkt Ser473, antipSTAT3, antipJAK2, and antipFoxO1 antibodies from Cell Signaling.

Coimmunoprecipitation and tyrosine kinase assay using Tub as a substrate.

The protocol described was adapted for ICV injections (42). Briefly, a low dose of insulin (0.2 μg) or leptin (0.1 ng) was injected into the lateral ventricle to trigger IR and JAK2 autophosphorylation. After 7–10 min, Arc, MH, PVN, and LH were dissected and homogenized in extraction buffer (38) for IR or JAK2 IP. In another group of animals, Arc, MH, PVN, and LH were dissected without insulin or leptin stimulation and samples underwent IP with anti-Tub antibody. Samples were collected on protein A-Sepharose and combined as followed: IP IR plus IP Tub and IP JAK2 plus IP Tub. Then, the kinase activity was performed as previously described (42).

Translocation of Tub from plasma membrane to the nucleus in hypothalamic nuclei.

Fasted animals received an ICV injection of insulin or leptin or acetylcholine (Ach) (100 µmol/L; Sigma-Aldrich, St. Louis, MO). The specific phospholipase Cβ (PLCβ) inhibitor U73122 (1 μmol/L; Sigma-Aldrich) was previously injected to block Tub nuclear translocation induced by the hormones. The U73343 (1 μmol/L; Sigma-Aldrich), which weakly inhibits PLCβ, was injected as a negative control. Hypothalamic nuclei or whole hypothalamus were quickly dissected and frozen in liquid nitrogen for further processing. A pool of 20 hypothalamic nuclei or a pool of four hypothalami were used for plasma membrane and nucleus separation as described previously (43,44). The procedure was performed at 4°C. Plasma membrane and nucleus fractions were analyzed by IP and immunoblotting with anti-Tub (M-19) antibody.

RNA extraction and real-time PCR.

Random fed or fasted (24 h) mice treated for 5 days with Tub ASO or sense received ICV insulin or leptin injection, and the hypothalami or hypothalamic nuclei were harvested after 6 h, quickly frozen in liquid nitrogen, and stored at −80°C for processing. Total RNA was obtained using RNeasy Mini Kit (Cat. 74106; Qiagen). Real-time PCR was performed using TaqMan RT-PCR Master Mix (Applied Biosystems) in an Mx3000P thermocycler (Stratagene). The Mx3000P software was used to calculate the cycle threshold for each reaction. Relative expression levels were determined using the comparative cycle threshold method with normalization of target gene expression levels to 18s. Primers and probes were purchased from Applied Biosystems and were described as TRH, Mm01182425_g1; NPY, Mm00445771_m1; AgRP, Mm00475829_g1; POMC, Mm00435874_m1; MCH, Mm01242886_g1; and Ore, Mm01964031_s1 for mouse. The polymerase chain reaction conditions were 2 min at 50°C, 10 min at 95°C, followed by 40 cycles at 95°C for 15 s and 60°C for 60 s. Real-time data were analyzed using the engine provided by Applied Biosystems.

Protein tyrosine phosphatase activity assay in immunoprecipitated Tub.

Protein tyrosine phosphatase (PTPase) activity was measured as previously described (45,46).

Statistical analysis.

Results are expressed as means ± SD. Significance was determined using two-tailed Student t test or one-way or two-way ANOVA with Bonferroni posttest, as appropriate, and differences were considered significant if P < 0.05. We used GraphPad Prism (GraphPad Software, San Diego, CA).

RESULTS

Tub protein is expressed in hypothalamic nuclei and is regulated by fasting and refeeding.

Previous studies have shown that Tub is expressed in the central nervous system (13). Here, we confirmed that Tub was expressed in multiple hypothalamic nuclei (Fig. 1A).

FIG. 1.

Expression of Tub, effect of nutritional states, and effect of insulin and leptin on Tub-p-tyr in hypothalamic nuclei. A: Representative blots of Tub protein expression in hypothalamic nuclei, including Arc, MH, PVN, LH, white adipose tissue (WAT), and liver. B: Effect of fasting (24 h) and refeeding on Tub-p-tyr in hypothalamic nuclei. Mice were killed at 10 min (C) or at 7 min (E) after hormone injections to determine the dose-response of ICV insulin or leptin on Tub-p-tyr. The different doses of insulin were: 0, 0.02, 0.2, and 2.0 μg; the different doses of leptin were: 0, 0.1, 1.0, 10, and 100 ng. To determine the time-course of Tub-p-tyr in response to insulin (D) or leptin (F), we used 2-µg (D) and 10-ng (F) doses, and mice were killed at different time points: 5, 10, and 15 min for insulin or 2, 5, 7, 10, 15, and 20 min for leptin. G: Tub-p-tyr is increased in response to ICV insulin (2 µg) or leptin (10 ng) in hypothalamic nuclei. Data are presented as means ± SD from 8 mice per nucleus or whole hypothalamus from 8 to 10 mice. One-way ANOVA with Bonferroni posttest was used. *P < 0.05 vs. other groups, †P < 0.05 vs. fast. α-PY, antiphosphotyrosine; ICV, ICV injection; IB, immunoblotting.

The Tub protein abundance was not different in hypothalamic nuclei after fasting and refeeding states. However, Tub-p-tyr was strongly induced after 1 h of refeeding, followed by a decrease after 2 h of refeeding in Arc, MH, PVN, and LH in control mice (Fig. 1B).

Insulin and leptin induce Tub tyrosine phosphorylation in hypothalamic nuclei in a time-dependent and dose-dependent manner.

Insulin-induced Tub-p-tyr was dose-dependent. The presence of phosphorylated Tub was detectable after ICV injection of as little as 0.02 μg insulin, half-maximal stimulation occurred with 0.2 μg of the hormone (Fig. 1C), and maximal stimulation was observed with 2.0 μg insulin (n = 3). Insulin-induced Tub-p-tyr occurred after 5 min of ICV insulin injection, and the maximal phosphorylation was observed at 10 min (Fig. 1D). The presence of phosphorylated Tub was detectable after ICV leptin injection of as little as 0.1 ng leptin, and the maximal stimulation was observed with 10.0 ng leptin (Fig. 1E). Leptin-induced Tub-p-tyr occurred after 2 min of ICV leptin injection, and the maximal phosphorylation was observed at 7 min (Fig. 1F).

Next, we compared the maximal effect of insulin and leptin in Arc, MH, PVN, and LH hypothalamic nuclei, and the results showed that in all these nuclei insulin or leptin induced similar increases in Tub-p-tyr (Fig. 1G).

Tyrosine kinase assay of IR and JAK2 using Tub as a substrate.

The IR kinase activity was increased significantly in Arc lysates after ICV insulin in vivo and there was a further increase after ATP addition in vitro in mice fed chow. Most importantly, Tub-p-tyr was increased in Arc of mice injected with a low dose of insulin, with a further increase after ATP addition in vitro, demonstrating enhanced IR kinase activity with Tub. These results were also demonstrated in MH, PVN, and LH (Fig. 2A).

FIG. 2.

Tyrosine kinase assay of IR and JAK2 using Tub as a substrate. A: 0.2 μg insulin or (B) 0.1 ng leptin was injected into lateral ventricle to increase IR or LEPR-associated JAK2 phosphorylation, respectively. Hypothalamic nuclei were dissected, and IP was performed with anti-IR antibody for insulin-stimulated mice (n = 7) or anti-JAK2 antibody for leptin-stimulated mice (n = 5). In another group of animals (n = 7), hypothalamic nuclei were dissected without insulin or leptin stimulation, and samples underwent IP with anti-Tub antibody. After combining IP samples (IR with Tub) for insulin-stimulated mice or (JAK2 with Tub) for leptin-stimulated mice, they were incubated with ATP and blotted with antiphosphotyrosine (α-PY). Representative blots show insulin-induced IR and Tub-p-tyr--induced and leptin-induced JAK2 and Tub-p-tyr in Arc, MH, PVN, and LH of mice fed chow. ICV, ICV injection.

Similarly to insulin, JAK2 kinase activity was increased significantly in Arc lysates in response to leptin injection, as demonstrated by an increase in JAK2 autophosphorylation. The addition of Tub to the reaction also demonstrated that after leptin injection, Tub was a substrate of JAK2 tyrosine kinase in this nucleus. These results were also demonstrated in MH, PVN, and LH (Fig. 2B). We reprobed the membranes and confirmed that the bands blotted with antiphosphotyrosine antibody were IR, JAK2, and Tub (Fig. 2A and B).

ICV insulin or leptin stimulation induces the nuclear translocation of Tub in hypothalamic nuclei.

Previous studies suggest that Tub acts as a transcription factor that can translocate from the plasma membrane to the nucleus after G-protein activation (47). Here, we observed that ICV insulin induced the nuclear translocation of Tub after 15 min in Arc, MH, and LH, and this translocation was maximal after 30 min in these nuclei. In PVN, insulin-induced Tub translocation was delayed compared with other nuclei and was evident at 90 min after the hormone injection. Leptin also was able to induce Tub nuclear translocation in Arc, MH, LH, and in PVN (Fig. 3AD). In Arc, MH, and LH, leptin-induced Tub nuclear translocation was observed at 7 min, but in PVN the maximal translocation was observed 20 min after leptin. We blotted 30 μg of protein from the plasma membrane and nucleus with anti-IR and anti-TATA-box antibodies, respectively, as controls for these preparations (data not shown).

FIG. 3.

Tub nuclear translocation in response to hormones in hypothalamic nuclei. A pool of 20 mice was used for each time point (AD). Arc, MH, PVN, and LH were dissected from the same mice, and the plasma membrane and nucleus fractions were obtained from the same samples. EI: Whole hypothalami of 14 mice were used. Plasma membranes and nuclear lysates underwent IP and were blotted (IB) with anti-Tub antibody. Tub nuclear translocation in response to ICV (2 μg) insulin (0, 15, 30, 90 min) or (10 ng) leptin (0, 7, 20 min) was investigated in Arc (A), MH (B), PVN (C), and LH (D) of mice on chow. Tub nuclear translocation in response to ICV (E) 100 μmol/L acetylcholine (Ach) (0, 30, 120 min), (F) 1 μmol/L specific PLCβ inhibitor (U73122) plus Ach (0, 10, 30, 120 min), (G) 1 μmol/L unspecific PLCβ inhibitor (U73343) plus Ach (0, 10, 30, 120 min), U73122 plus insulin (0, 30, 120 min) (H), and U73122 plus leptin (0, 120 min) (I) were investigated in the hypothalamus of mice fed chow diet. U73122 and U73343 inhibitors were administered 30 min before hormone injections. ICV, ICV injection.

Previous data showed that Ach stimulates translocation of Tub in Neuro-2A cells, and this involves PLCβ, which is blocked by a specific PLCβ inhibitor (U73122). We investigated whether Ach is able to induce Tub translocation in vivo in the hypothalamus of mice fed chow. We injected Ach (100 µmol/L) alone or U73122 before Ach ICV injection, or U73343, which weakly inhibits PLCβ, as a negative control. We observed that Ach induced Tub nuclear translocation, an effect that was blocked by U73122, but not by U73343 (Fig. 3EG). To investigate whether insulin- or leptin-induced Tub translocation also involved PLCβ, we injected insulin or leptin in mice previously treated with ICV U73122. The results showed that U73122 blocked insulin-induced and leptin-induced Tub translocation in vivo (Fig. 3H and I). It is interesting that ICV Ach altered TRH expression in the hypothalamus. Further, insulin or leptin induced an increase in POMC and TRH and a reduction in orexin or MCH expression, respectively. The ICV U73122 injection before Ach or insulin or leptin blunted these effects on neuropeptides (Supplementary Fig. 1AC).

Metabolic characteristics of mice treated with Tub ASO.

Control mice treated for 5 days with Tub ASO had a decrease of 80–90% in Tub expression in the hypothalamus compared with sense-treated mice (Fig. 4A). Mice treated with ASO had higher body weight and epididymal fat mass associated with an increase in food ingestion compared with sense-treated mice (Fig. 4BD). Additionally, there was a decrease in O2 consumption without changes in the respiratory exchange rate (Fig. 4E and F). There was also an increase in fasting blood glucose levels in ASO-treated mice (Fig. 4G). Furthermore, we observed higher blood glucose levels after pyruvate intraperitoneal injection at 15 and 30 min and enhanced PEPCK expression in fasting livers of mice treated with ASO (Fig. 4H and I). These differences persisted in pair-fed mice (Supplementary Fig. 2AD).

FIG. 4.

Metabolic characteristics of mice treated with Tub ASO. A: ICV injection of Tub ASO for 5 days decreases 80–90% of Tub expression in the hypothalamus compared with sense controls. Body weight (B), epididymal fat mass (C), and cumulative food intake (D) are enhanced in Tub ASO-treated mice. O2 consumption (E) is decreased in Tub ASO-treated mice without change in respiratory exchange rate (F) (RER). Fasting blood glucose (G), blood glucose after a pyruvate load (H), and PEPCK protein expression (I) are enhanced in mice treated with Tub ASO. For pyruvate tolerance test, we injected 2 g/kg sodium pyruvate (intraperitoneal) in fasted mice, and blood glucose levels were measured using a glucometer at 0, 15, 30, 60, 90, and 120 min. Data are presented as means ± SD from 8–10 mice. Two-tailed Student t test (A–C, G) or one-way ANOVA (D, F, I) or two-way ANOVA (E, H) with Bonferroni posttest were used. *P < 0.05 vs. sense.

Food intake and hypothalamic neuropeptide expression in response to insulin or leptin in mice treated with Tub ASO.

The anorexigenic effect of ICV insulin or leptin was measured by recording 4-h and 8-h food intake. Food intake was reduced in response to ICV insulin or leptin in mice treated with sense, but not in mice treated with ASO (Fig. 5A and B).

FIG. 5.

Food intake and hypothalamic neuropeptide expression in response to insulin or leptin in mice treated with Tub ASO. The 4-h (A) or 8-h (B) food intake in response to ICV insulin or leptin is impaired in mice treated for 5 days with Tub ASO. C–I: Neuropeptide expression (mRNA levels) in response to ICV insulin and leptin are altered in the hypothalamus of ASO-treated mice. J and K: Akt and Foxo1 phosphorylation are increased in response to ICV insulin in the hypothalamus of Tub ASO-treated mice. L and M: JAK2 and STAT3 phosphorylation in response to leptin are increased in the hypothalamus of ASO-treated mice. Data are presented as means ± SD from 8–10 mice. One-way ANOVA with Bonferroni posttest was used. Sense: ASO control. *P < 0.05 vs. sense; †P < 0.05 vs. saline sense; ‡P < 0.05 vs. saline and insulin sense; §P < 0.05 vs. saline, insulin, and leptin sense; ‖P < 0.05 vs. fed; ¶ vs. fast; # vs. fast with insulin (fast + I) and fast with leptin (fast + L) sense; ** vs. fast + L sense. AU, arbitrary units; ICV, ICV injection; Sal, saline; ins, insulin; lep, leptin; NPY, neuropeptide Y; AgRP, agouti-related peptide.

Next, we investigated whether ASO treatment for 5 days affects the influence of insulin or leptin on neuropeptide expression in the hypothalamus. As expected, fasting increased the expression of NPY, AgRP, MCH, and orexin, and insulin or leptin was able to markedly reduce expression of these neuropeptides in sense-treated mice (Fig. 5C, D, F, and G). However, in mice treated with ASO, leptin-induced reduction of MCH mRNA was blunted (Fig. 5F) and insulin-induced reduction of orexin was also blunted (Fig. 5G). In contrast, fasting decreased the expression of POMC and TRH, and insulin or leptin increased the expression of these neuropeptides in mice treated with sense; however, in mice treated with ASO, leptin-induced or insulin-induced increases were blunted for POMC and TRH (Fig. 5E and H).

In mice treated with ASO, insulin-induced Akt and Foxo1 phosphorylation and leptin-induced JAK2/STAT3 tyrosine phosphorylation were not altered, indicating that the blunted insulin and leptin effects on food intake in mice treated with ASO are independent from these pathways (Fig. 5I–L).

Insulin- and leptin-induced Tub-p-tyr in hypothalamic nuclei is blunted in DIO mice.

We next investigated the modulation of insulin-induced or leptin-induced Tub-p-tyr in obese mice. Acute treatment with ICV insulin increased Tub-p-tyr in Arc, MH, PVN, and LH from control mice, and this effect was reduced in mice fed a HFD (Fig. 6A). Similarly, ICV injection of leptin increased Tub-p-tyr in Arc, MH, PVN, and LH in control mice, and this effect was blunted in mice fed a HFD (Fig. 6B). Insulin-induced or leptin-induced Tub nuclear translocation was blunted in the hypothalamus of DIO mice (Fig. 6C).

FIG. 6.

Tub-p-tyr and translocation in response to insulin and leptin are impaired in mice fed HFD. Representative blots (n = 5 each nucleus) show insulin- (A) and leptin-induced (B) Tub-p-tyr in Arc, MH, PVN, and LH of mice fed chow or HFD. A pool of 20 mice was used for each time point. Plasma membranes and nuclear lysates underwent IP and were blotted (IB) with anti-Tub antibody. Tub nuclear translocation in response to ICV insulin (0, 15, 30, 90 min) or leptin (0, 7, 20 min) is investigated in the hypothalamus of mice fed a HFD. Data are presented as means ± SD. One-way ANOVA with Bonferroni posttest was used. α-PY, antiphosphotyrosine; *P < 0.05 vs. fast on chow diet.

PTP1B may play a role in the regulation of Tub-p-tyr in hypothalamic nuclei from DIO mice.

Because PTP1B dephosphorylates proteins that bind the IR and JAK2, we next investigated if insulin and leptin can induce an increase in Tub/PTP1B association in hypothalamic nuclei using coimmunoprecipitation. Our data showed that after these hormones were injected, there was an increase in Tub/PTP1B interaction in all hypothalamic nuclei investigated (Fig. 7AD). In addition, we investigated Tub/PTP1B association in mice fed a HFD, after leptin insulin infusion, and the results showed that in obese mice there was a marked increase in Tub/PTP1B association in response to insulin and leptin in Arc, MH, PVN, and LH nuclei (Fig. 7AD). Then, we treated mice with ICV PTP1B ASO for 5 days, which decreased PTP1B protein level in the hypothalamus (Fig. 7E), and we measured PTPase activity associated with PTP1B (Fig. 7F) and with Tub (Fig. 7G). We observed an increase in PTP1B and Tub associated with PTPase activity in the hypothalamus of mice fed a HFD, and this result was blunted by PTP1B-ASO treatment, suggesting that PTP1B may have phosphatase activity with Tub (Fig. 7F and G). Furthermore, we investigated the modulation of insulin-induced or leptin-induced Tub-p-tyr levels in mice fed a HFD. The results showed that the reduction in PTP1B protein level in hypothalamus was accompanied by an increase in Tub-p-tyr levels after leptin or insulin ICV treatment in DIO mice, suggesting that PTP1B may have a role in the dephosphorylation of Tub (Fig. 7H).

FIG. 7.

Tub/PTP1B association in response to insulin or leptin is higher in hypothalamic nuclei of mice fed a HFD. A–D: Representative blots show Tub/PTP1B association in response to ICV insulin (2 µg) or leptin (10 ng) in mice fed chow. This effect is greater in mice fed a HFD in Arc, MH, PVN, and LH of mice fed chow and HFD. E: PTP1B expression is reduced in mice fed a HFD treated with PTP1B ASO for 5 days. F: PTPase activity is increased by HFD and is reduced by PTP1B ASO treatment in mice fed a HFD. G: Tub associated with PTPase activity is greater in mice fed a HFD and is decreased by PTP1B ASO treatment. In this experiment, hypothalami were homogenized and centrifuged, and the supernatants were immunoprecipitated with anti-Tub antibody. Then, immune complexes were incubated with pp60c-src COOH-terminal phosphoregulatory peptide (TSTEPQpYQPGENL; Biomol) for 1 h at 30°C. To determine PTPase activity in a spectrophotometer, 40-μL aliquots with 100 μL Biomol Green reagent (Biomol) were used. H: Tub tyrosine phosphorylation in response to insulin or leptin is higher in mice fed a HFD treated with PTP1B ASO than in sense-treated mice. Data are presented as means ± SD from five mice per nucleus. One-way ANOVA with Bonferroni posttest was used. *P < 0.05 vs. sense, †P < 0.05 vs. mice fed chow treated with sense, ‡P < 0.05 PTP1B ASO groups. PTP1B, protein tyrosine phosphatase 1B; Sense, ASO control.

DISCUSSION

The results of the current study demonstrate that Tub is a direct substrate of insulin and leptin in vivo. Because the insulin receptor has tyrosine kinase activity, it induced Tub tyrosine phosphorylation directly; however, for leptin, similar to other substrates, it uses JAK2 as a kinase to induce Tub phosphorylation in Arc, MH, PVN, and LH. Our data also show that Tub-p-tyr is decreased during fasting and increased after refeeding, indicating that Tub is regulated by nutritional status. The combination of an increase in cumulative food intake, lower anorexigenic effect of insulin and leptin, and reduced O2 consumption may contribute to explain enhanced adiposity in mice treated with Tub ASO.

Previous data showed that Tub overexpression, restricted to the Arc by a herpes simplex viral system, had no effect on food intake in rats fed chow, suggesting that Tub expression beyond normal may not be functional (48). Here, we demonstrate that the inhibition of Tub expression by ASO in all hypothalamic nuclei enhanced food intake and adiposity. It is possible that the manipulation of the amount of Tub only in neurons from one nucleus induced such a discrepant result. The regulation of food intake and adiposity is made by an interaction of different kinds of neurons in multiple brain regions (15). The response is variable and depends on which neuron is targeted (15). The effect of Tub inhibition on the control of metabolism seen in our experiments with ASO may be a reflection of the circuit of leptin-responsive and insulin-responsive neurons in multiple hypothalamic nuclei. Further, although Tub acts as a transcription factor moving from the plasma membrane to the nucleus (47), it was not shown in this previous study (48) whether the elevated amount of Tub altered the translocation of Tub from the plasma membrane to the nucleus.

The results of the current study showed that after insulin and leptin stimulation, there was a clear translocation of Tub from the plasma membrane to the nucleus in the Arc, MH, PVN, and LH being slower in PVN. The delay in insulin- and leptin-induced Tub translocation in the PVN cannot be related to a lack of these hormones penetration into the area because PVN, as well as Arc, is periventricular. The IR expression in PVN is higher than in LH, but PVN has the lowest LEPR expression of all other nuclei, which may contribute to explaining the delay in leptin-induced Tub translocation in this nucleus. In addition, PVN receives neuronal projections from Arc; thus, we can speculate that modulation of neural activity in Arc induced by insulin or by leptin might delay the effect of these hormones on Tub translocation in PVN.

Previously, it was demonstrated that activation of G-protein αq by Ach releases Tub from the plasma membrane through the action of PLCβ, triggering translocation of Tub to the nucleus (47). In this regard, our data suggest that insulin and leptin also use PLCβ to induce Tub translocation, because an inhibitor of PLCβ was able to block the effects of insulin and leptin on Tub translocation. Here, we demonstrated that after reduction in Tub protein expression, insulin or leptin was unable to increase the mRNA of POMC in Arc or the mRNA of TRH in PVN. Furthermore, insulin and leptin were unable to decrease the mRNA of orexin and MCH, respectively, in LH, suggesting that Tub may have a role modulating neuropeptides expression.

The inhibition of Tub protein expression in mice treated with ASO induced an increase in fasting blood glucose, hepatic glucose production, and the PEPCK expression of the liver in mice fed a chow diet. This phenomenon seems to be independent of changes in body weight or food intake, because it persists in pair-fed mice. These data suggest that in parallel to control energy balance, Tub in the hypothalamus may play a role in the regulation of hepatic glucose metabolism.

Resistance to leptin and insulin signaling is likely to play a central role in obesity (39,49,50). Our data showing a reduction in Tub tyrosine phosphorylation and nuclear translocation in response to insulin or leptin may contribute to hypothalamic insulin and leptin resistance in mice fed a HFD.

In the current study, we observed that Tub associates with PTP1B, and this interaction is increased in mice fed a HFD. Further, we observed that there was an increase in Tub associated with PTPase activity in the hypothalamus of mice fed a HFD, and this result was blunted by PTP1B-ASO treatment, suggesting that Tub may be a PTP1B substrate. This event could be one of the molecular mechanisms by which leptin-induced or insulin-induced Tub tyrosine phosphorylation is blunted in mice fed a HFD, suggesting that in addition to dephosphorylating IR, IRS-1, and JAK2, PTP1B also may have phosphatase activity with Tub.

In summary, our data provide evidence that Tub is a substrate of IRTK and LEPR-associated JAK2 in vivo in hypothalamic nuclei. After leptin or insulin injection, Tub translocates to the nucleus (Fig. 8A), and this is associated with altered POMC, TRH, MCH, and orexin mRNA levels. The reduction in Tub protein expression in the hypothalamus by Tub ASO treatment increases food intake, adiposity, fasting blood glucose, and hepatic glucose production, and decreases O2 consumption, suggesting that Tub has a key role in the control of insulin and leptin effects on energy balance and glucose metabolism. In the hypothalamus of mice fed a HFD, there is a marked reduction in leptin- and insulin-induced Tub-p-tyr and translocation to the nucleus, which was accompanied by an increase in Tub/PTP1B interaction (Fig. 8B). These results demonstrate that Tub is part of the hypothalamic insulin and leptin signaling in vivo that acts independently of Akt/Foxo1 and STAT3 pathways, and its regulation has marked effects on metabolism (Fig. 8A and B).

FIG. 8.

Tub signaling in the context of other energy balance pathways. A: Insulin receptor has tyrosine kinase activity and is able to induce PI3K/Akt/Foxo1 pathway activation, altering transcription of neuropeptides involved in the regulation of energy balance. Because LEPR has no tyrosine kinase activity, it recruits JAK2. The LEPR/JAK2 complex binds and activates STAT3, which migrates to the nucleus, regulating feeding through neuropeptide expression. Here, we suggest that phosphorylated IR and JAK2 are also able to induce Tub-p-tyr and translocation to the nucleus, accompanied by changes in neuropeptide expression and regulation of energy balance. In addition, we demonstrate that acetylcholine stimulates translocation of Tub via PLCβ in vivo and insulin-induced and leptin-induced Tub translocation may occur via PLCβ. B: Furthermore, in obesity insulin-induced or leptin-induced Tub-p-tyr in Arc, MH, PVN, and LH are blunted in parallel to enhanced PTP1B activation.

ACKNOWLEDGMENTS

This work was supported by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo; 2008/55674-8), Sao Paulo, Brazil, CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) Universal (480131/2009-0), and by INCT (Instituto Nacional Ciência e Tecnologia de Obesidade e Diabetes; 573856/2008-7).

No potential conflicts of interest relevant to this article were reported.

P.O.P. performed experiments and wrote the manuscript. P.G.F.Q., A.M.C., A.C.S., D.G., J.M., and L.W. performed experiments. E.R.R., J.B.C.C., and L.A.V. contributed to discussion and reviewed the manuscript. M.J.A.S. also wrote the manuscript. M.J.A.S. and P.O.P. are the guarantors of this work and, as such, had full access to all of the data in the study and take responsibility for the integrity of the data and accuracy of the data analysis.

The authors acknowledge the technical assistance of Luis Janieri, Jósimo Pinheiro, Lígia Parreira Muniz, and Ramon Henrick Zorzeto dos Santos from the Department of Internal Medicine, State University of Campinas.

  • Received October 4, 2011.
  • Accepted May 7, 2012.

Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.

REFERENCES

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