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Original Research

MCPIP1 Deficiency Increases Insulin Sensitivity But Impairs Hepatic Insulin Signal Transduction in Mice

Brett B. Holdaway2, Joshua D. Moody2, Lauren F. Brooks2, Mingui Fu3, Pappachan Kolattukudy4, and Yingzi Chang1*
1Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri, USA
2Kirksville College of Osteopathic Medicine, Kirksville, Missouri, USA
3Department of Basic Medical Science, School of Medicine, University of Missouri Kansas City, Kansas City, Missouri, USA
4Burnett School of Biomedical Sciences College of Medicine, University of Central Florida, Orlando, Florida, USA

*Corresponding author: Dr. Yingzi Chang, M.D., Ph.D., Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, 800 West Jefferson Street, Kirksville, MO 63501, Tel: 1-660-626-2327; Fax: 1-660-627-2728; Email: ychang@atsu.edu

Submitted: 07-01-2015 Accepted: 09-08-2015  Published: 10-09-2015

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Article

 
Abstract

Background

Chronic inflammation is associated with pathogenesis of metabolic disorders, including insulin resistance. MCPIP1 (monocyte chemotactic protein [MCP]–induced protein 1; also known as ZC3H12A) is a newly discovered RNase that is essential in controlling inflammatory response and immune homeostasis. Our current study was designed to test if MCPIP1 deficiency affects glucose homeostasis by regulating the insulin signal transduction pathway in liver and/or adipose tissue, the two major organs that are involved in maintaining glucose homeostasis.

Methods

Both wild type C57/BL6 mice and MCPIP1 knockout (MCPIP1-/-) mice in C57/BL6 background were used. Glucose and insulin tolerance tests were conducted. Insulin-stimulated signal transduction was tested in both liver and adipose tissues.

Results

We found that MCPIP1-/- mice showed a significant increase in insulin sensitivity but delayed response to glucose challenge and prolonged recovery of plasma glucose levels following insulin challenge. We also found a substantial reduction of insulin-induced signal transduction and a significant decrease in IRS-1 expression and PI3 kinase activity in the liver. The morphological changes of the MCPIP1-/- mice livers mimicked hepatic cirrhosis. However, no changes were found in insulininduced AKT phosphorylation in adipose tissues.

Conclusion

MCPIP1 deficiency increased insulin sensitivity but induced hepatic remodeling and impaired hepatic insulin signal transduction. Our results suggest that MCPIP1 is involved in regulating glucose homeostasis but the mechanism is not understood.

Keywords: MCPIP1; Glucose Homeostasis; Hepatic Remodeling; Hepatic Insulin Signaling Transduction
 
Introduction

Excessive or prolonged inflammatory/immune events are associated with several chronic metabolic disorders, including insulin resistance. Studies found that cells treated with tumor necrosis factor-α (TNF-α) showed a decrease in insulin signal transduction, including insulin-induced tyrosine phosphorylation of IR and IRS-1 [1-3]. Blocking the function of TNF-α improved insulin sensitivity [4], suggesting that the enhanced inflammatory responses contribute to impaired insulin signaling and insulin sensitivity.

Liver and adipose are the two major organs that are involved in maintaining glucose homeostasis. Liver contains large populations of macrophages and natural killer cells that balance the production of pro- and anti-inflammatory cytokines, thus plays an important role in controlling initiation and termination of inflammatory responses. Studies have found that the liver from ob/ob mice show a significant increase in TNF-α expression and hepatic insulin resistance. Administration of TNF-α antibodies blocks hepatic inflammatory activities and increases insulin sensitivity [5-7], suggesting that increased hepatic inflammatory response contributes to insulin resistance and impairment glucose homeostasis. Adipocytes secrete monocyte chemoattractant protein 1 (MCP-1) leading to recruitment of macrophages and production of numerous inflammatory mediators [8,9]. Deficiency of MCP-1 and its receptor (CCR2) reduces macrophage infiltration in adipose tissue and improves systemic glucose homeostasis and insulin sensitivity [10]. Overexpression of MCP-1 in adipose tissue promotes macrophage infiltration and insulin resistance [11], implying that adipose tissue is a crucial site for metabolic and inflammation interactions.

MCPIP1 (monocyte chemotactic protein [MCP]–induced protein 1; also known as ZC3H12A) is a RNase induced in multiple cell lines by Lipopolysaccharide (LPS), interleukin 1-β (IL-1β), and monocyte chemotactic protein-1 (MCP-1) [12- 15]. Mice lacking MCPIP1 develop severe systemic inflammatory and autoimmune responses, an increase in inflammatory cell infiltration in peripheral tissues, anemia, severe splenomegaly, and lymphadenopathy [12]. Overexpression of MCPIP1 completely eradicates LPS- and IL-1β-induced JNK phosphorylation and NF-κB subunit p65 nuclear translocation [16], suggesting that MCPIP1 is an important inflammatory/ immune modulator. Our current study was designed to examine if MCPIP1 modulates systemic glucose homeostasis by regulating hepatic and/or adipose insulin sensitivity.

Materials and Methods

Materials

Polyclonal antibodies directed against phospho-AKT (ser473, Cat #9271), AKT (Cat #9272), insulin receptor substrate- 1(Cat #2832), PI3-kinase subunit p85 (Cat #4292), monoclonal anti-phospho-tyrosine (Cat #9411), anti-Rabbit IgG (Cat #7074), and anti-mouse IgG were purchased from Cell Signaling Technology (Danvers, MA). Polyclonal anti-insulin receptor β (C-19, sc-711) was obtained from Santa Cruz Biotechnology (Santa Cruz, CA). Regular human injection insulin (Novolin) was purchased from Med-Depot (Kirksville, MO). Protein G sepharose 4 fast flow was acquired from GE healthcare BioSciences (Cat #17-0618-01, Pittsburgh,PA). AccupPrimeTM SuperMix II was purchased from Invitrogen (Cat #12341-012, Grand Island, NY). MCPIP1 primers were ordered from IDT (Integrated DNA Technologies, Iowa). PI3-kinase activity ELISA: Pico was purchased from Echelon Biosciences Inc. (Cat # K-1000s, Salt Lake, UT). All other reagents were obtained from Sigma-Aldrich (St Louis, MO).

Experimental animals

Two pairs of MCPIP1 heterozygous mice in C57/BL6 background were cross-bred in our animal facility. MCPIP1 knockout (MCPIP1-/-) mice were verified by PCR. Animals were housed in a room with a controlled photoperiod of 12 hour light: 12 hour darkness and a temperature of 22° ± 2°C. Animals were given free access to a nutritionally balanced diet containing 11% fat and (5015, LabDiet, PMI Feeds, Inc., St Louis, MO, USA) and tap water. This study was carried out in compliance with the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. The animal protocol was approved by the Kirksville College of Osteopathic Medicine Animal Care and Use Committee. All experiments were conducted using mice between ages of 6-8 weeks old since MCPIP1-/- mice start dying at about 8-weeks old.

Measurement of glucose, insulin and protein concentrations

Fasting plasma glucose levels were measured with One- Touch UltraMini glucose meter and test strips (Wal-Mart, Kirksville, MO) by collecting blood from mouse tail veins. Serum insulin and protein concentrations were measured following the Manufacturers’ instruction. Briefly, after overnight fasting, blood was collected by cardiac puncture from anesthetized animals, allowed to clot at room temperature, and centrifuged at 1500 g at 4°C for 10 minutes. Serum was collected and frozen at -80°C until the assay was performed. Serum concentration of insulin was determined using a Rat/ mouse insulin ELISA kit (Cat # EZRMI-13K, Millipore, St. Charles, MO). Protein concentration in serum and tissue lysates was measured using BCA protein assay reagents (Cat #23228 and 23224, Pierce, Rockford, IL).

Glucose tolerance test

Glucose tolerance tests were conducted on animals after 16- 18 hour fast. Animals were injected with D-glucose (2 g/kg body weight) intraperitoneally. Blood glucose values in plasma samples (obtained by tail puncture) were determined at 0, 15, 30, 60, and 120 minutes after the injection [17].
 
Insulin tolerance test

Following 16-18 hour fasting, animals were subjected to an intraperitoneal injection of regular human injection insulin (0.75 IU/ kg body weight) [18]. Blood glucose levels were measured at 0, 15, 30, 60, and 120 minutes after the injection.

Liver and adipose tissue collection

To study the effects of MCPIP1-/- on insulin-stimulated insulin signal transduction pathway in liver and adipose tissues, 2 IU/kg of insulin or same amount of phosphate saline buffer was injected intraperitoneally after 16-18 hour food withdrawal by following modified protocols [17-19]. Mice were anesthetized with an intraperitoneal injection of sodium pentobarbital (50 mg/kg). Liver and adipose tissues (including inguinal, gonadal, abdominal and interscapular white fat pads) were harvested 15 minutes after insulin injection and weighed. The tissues were snap frozen in liquid nitrogen and kept at -80°C. Protein extraction was conducted following modified published protocol [19]. Briefly, liver and adipose tissues were homogenized in lysis buffer containing 25 mM Tris-HCI (pH 7.4), 100 mM NaF, 10 mM EDTA, 1% Igepal CA-630, and 2 mM sodium vanadate supplemented with 0.25 mM PMSF, 0.2 μM aprotinin, 5.25 μM leupeptin, 9 μM bestatin, 3.75 μM pepstatin A, and 3.5 μM E-64. Tissue lysate was centrifuged at 5,000 rpm for 15 minutes at 4°C. The supernatant was collected, further centrifuged at 32,000 rpm for 120 minutes at 4°C, and stored at -20°C for immunoprecipitation and direct Western blotting experiments respectively.

Immunoprecipitation

Immnunoprecipitation was performed by following modified Ozcan’s protocol [19]. Supernatants containing equal amounts of protein from liver tissue (3 mg) were incubated with antibodies (3 μg) directed against insulin receptor β (IRβ) or insulin receptor substrate-1(IRS-1) and protein G sepharose beads (25 μl) overnight at 4°C. The precipitates were washed with cold lysis buffer then boiled in 2 x Laemli sample buffer for 5 minutes.

Immunoblotting

Following immuniprecipitation, the precipitates were resolved with SDS-PAGE, transferred to a PVDF membrane (Millipore,Bedford, MA), blocked with 3% of nonfat dry milk, probed with anti-phospho-tyrosine (1:1000 dilution, to check tyrosine phosphorylation of IRβ and IRS-1) or anti-p85 (1:500 dilution, following immunoprecipitation with anti-IRS-1 to check IRS-1 and p85 association), and reprobed with either anti-IRβ (1:1000 dilution) or anti-IRS-1 (1:1000 dilution) to check for equal loading. Phosphorylation of AKT was determined by direct Western blotting using anti-phospho-AKT (1:1000 dilution) followed by re-probing with anti-AKT (1:1000 dilution). After incubation with the primary antibodies, membranes were incubated with HRPconjugated secondary antibodies (anti-mouse IgG, 1:15000 dilution; anti-rabbit IgG 1:5000 dilution), developed using Chemiluminescense Reagent Plus (PerkinElmer Life Science,  INC. Boston, MA), and detected by autoradiography using XRay film (Midsci, St. Louis, MO). Images were quantified by densitometric analysis using Image-J software. It should be noted that only insulin-induced phosphorylation of AKT and total AKT were assessed in adipose tissues since MCPIP1-/- mice have very limited amount of adipose tissue. We were unable to measure phosphorylation of IRβ, IRS-1, and neither the association of p85 and IRS-1.

PI3-kinase activity

PI3-kinase activity from liver samples was measured by following the manufacturer’s instruction. Briefly, 3 mg of liver lysate was incubated with 3 μg of anti-p85 (a subunit of PI3- kinase) for one hour at 4°C followed by the addition of 30 ul of 50% protein G-sepharose beads and rotation at 4°C overnight. The immunoprecipitate was collected by washing three times with ice-cold lysis buffer containing 2 mM sodium orthovanadate followed by three washes with 0.1 M Tris-HCl buffer (pH 7.4) containing 5 mM LiCl and 2 mM sodium orthovanadate, and three washes with 10 mM Tris- HCl (pH 7.4) containing 150 mM NaCl, 5 mM EDTA, and 0.1 mM sodium orthovanadate. The supernatant was aspirated after the last wash. 30 μl of kinase reaction buffer (provided in the kit) and 30 μl of 10 μM PI (4, 5) P2 (substrate) were added. The reaction proceeded for 3 hours at 37°C and was stopped by adding 90 μl of stop solution. The amount of PIP3 (end product of the kinase reaction) in the supernatant was detected using PI3-kinase activity ELISA kit.

Histology

Mice were anesthetized with an intraperitoneal injection of sodium pentobarbital (50 mg/kg) and fixed by perfusion with 3.7% of formaldehyde through the left ventricle. Liver tissues were dissected, further fixed in 3.7% of formaldehyde, processed using a Leica TP1020 semi-enclosed benchtop tissue processor (Leica Microsystems, Nussloch, Germany), and embedded in paraffin using Leico embedding station. Specimens were sectioned using a Leica RM2255 microtome. Cross-sections of 8 μm were stained with hematoxylin- eosin and trichrome. Images were captured with a Leica DFC400 digital microscope camera mounted on a Leica DM4000B microscope using Surveyor Software (objective imaging Ltd, Cambridge, United Kingdom).

Statistical analysis

Results are presented as means ± S.E.M. Data were analyzed with one-way ANOVA followed by the Newman-Keuls Multiple Comparison Test and Unpaired Student’s t-test using GraphPad Prism version 5 by GraphPad Software, Inc. (San Diego, CA, USA). The area under the curve (AUC) and inverse area under the curve were calculated using the AUC option in GraphPad Prizm and analyzed by Unpaired Student’s ttest. p<0.05 was considered as statistically significant.
 
Results

Basal blood insulin, glucose, and serum protein levels

It was previously reported that MCPIP1-/- mice exhibited growth retardation, systemic inflammation, autoimmune response, and premature death [12, 16], however, metabolic changes in these mice are unknown. As shown in the Table, comparing to wild-type littermates, fasting blood glucose and insulin concentrations from MCPIP1-/- mice were significantly lower (30% and 54% of reduction, respectively, p<0.05), suggesting that insulin sensitivity may be increased in MCPIP1-/- mice. In contrast to the basal levels of plasma glucose and insulin, the total serum protein concentration was considerably higher in MCPIP1-/- mice when compared to that in wild-type mice (about 53% of increase, p<0.05).

Table. Fasting serum glucose, insulin, and total protein concentrations (6 weeks old, Mean ± S.E.M).
 
Diabetes Table 8.1

Insulin sensitivity and glucose tolerance

Next, we conducted glucose and insulin tolerance tests using the same sets of animals. As shown in Figure 1a, in response to glucose challenge, blood glucose levels in MCPIP1-/- mice increased significantly at the beginning (15 and 30 minutes after glucose injection, p<0.05) but returned to baseline levels 120min after the glucose injection. No difference was observed in the area under the curve between wild type and their MCPIP1-/- littermates (Figure 1b. AUCglucose: 25180 ± 954.5 verses 27510 ± 2056, p>0.05). In contrast to the response to glucose challenge, the sensitivity of MCPIP1-/- mice to the insulin challenge was significantly increased, demonstrated as a rapid drop in fasting blood glucose levels (Figure 2a). This is reflected by a significant reduction of the area under the curve (Figure 2b. inverse AUCglucose: 12880 ± 794.8 verses 5949 ± 619.3, p<0.05). The glucose levels in MCPIP1-/- mice in response to insulin challenge stayed low and failed to return to baseline after 120 minutes (Figure 2a).
 
Diabetes Fig 8.1

Figure 1. Glucose tolerance test. After 16-18 hour fasting, 2 g/kg of glucose was administered to both wild-type and MCPIP1-/- mice via intraperitoneal injection. Blood glucose levels were measured by collecting blood from mouse tail vein using one-touch glucose meter. Results are expressed as the concentrations of glucose (1a.) and area under the curve (AUCglucose,1b.). The graph shows the mean ± S.E.M from 6 mice. *p<0.05 when compared to wild-type mice.
 
Diabetes Fig 8.2

Figure 2. Insulin tolerance test. 0.75 IU/kg of insulin was administered via intraperitoneal injection after 16-18 hour fasting. Blood glucose levels were measured using one-touch glucose meter by collecting blood from mouse tail vein. Results are expressed as the concentrations of glucose (2a.) and inverse area under the curve (inverse AUCglucose, 2b.). The graph shows the mean ± S.E.M from 6 mice. *p<0.05 when compared to wild-type mice.

Hepatic insulin signal transduction

Next, we examined the effect of MCPIP1 deficiency on hepatic insulin signal transduction. As shown in Figure 3, insulin- stimulated IRβ phosphorylation was reduced by 82% (p<0.01) in livers from MCPIP1-/- mice when compared to that from wild-type mice. No difference was observed in total IR expression levels. However, as shown in Figure 4, total IRS-1 expression levels in livers from MCPIP-/- mice were decreased to 39% of those from wild-type mice (Figure 4c). This decrease was consistent with a significant reduction in insulin-induced phosphorylation of IRS-1 (Figure 4b). In addition, insulin-induced serine phosphorylation of AKT in livers from MCPIP1-/- mice was markedly reduced compared to that from wild-type mice (about 59.8% of reduction, p<0.05, Figure 5). No difference was found in total AKT protein expression levels.

Figure 3. Insulin-induced IRβ phosphorylation in liver tissue.
 
Diabetes Fig 8.3

Following 16-18 hour fasting, mice received 2 IU/kg of insulin intraperitoneally.Liver tissues were removed 15 minutes after the injection. Tissues were homogenized and proteins were extracted in lysis buffer. Supernatants containing equal amounts of  protein (3 mg) were immunoprecipitated with anti-IR-β. Proteins were separated with SDS-PAGE, probed with anti-phosphotyrosine, stripped, and re-probed with anti-IR-β. The upper panel shows a representative blot and the lower panel shows a graph summarizing the results from 4 independent experiments. Results were calculated as the ratio of phosphorylated IRβ to total IRβ and expressed as mean ± S.E.M. **p<0.05 compared to wild-type control, ##p<0.05 compared to wild-type insulin.
Diabetes Fig 8.4

Figure 4. IRS-1 expression and insulin-induced IRS-1 phosphorylation in liver tissue. Following 16-18 hour fasting, mice received 2 IU/kg of insulin intraperitoneally. Livers were removed and homogenized and proteins were extracted in lysis buffer. Supernatants containing equal amounts of protein (3 mg) were immunoprecipitated with anti-IRS-1. Proteins were separated with SDS-PAGE, probed with anti-phosphotyrosine, stripped, and reprobed with anti-IRS-1. The image (Figure 4a) shows a representative blot of IRS-1 protein expression and insulin-induced IRS-1 phosphorylation. The graphs show the results of insulin-induced IRS-1 phosphorylation (4b, n=4) and total IRS-1 protein expression (4c, n=4). Results were calculated as arbitrary density of the bands and expressed as mean ± S.E.M. *p<0.05 compared to wild-type control, #p<0.05 compared to wild-type insulin.
 
Diabetes Fig 8.5
 
Figure 5. Insulin-induced AKT phosphorylation at serine 473 in liver tissue. Following 16-18 hour fasting, mice received 2 IU/kg of insulin intraperitoneally. Liver tissues were removed 15 minutes after the injection. Tissues were homogenized and proteins were extracted in lysis buffer. Supernatants containing equal amounts of protein (100 μg) were used to detect AKT phosphorylation levels. Proteins were separated with SDS-PAGE, probed with anti-phospho- AKT (ser473), stripped, and reprobed with anti-AKT. The upper panel shows a representative blot and the lower panel shows a graph summarizing the results from 4 independent experiments. Results were calculated as the ratio of phosphorylated AKT to total AKT and expressed as mean ± S.E.M. **p<0.05 compared to wildtype control, #p<0.05 compared to wild-type insulin.

Basal and insulin-stimulated hepatic PI3 kinase activity

As shown in Figure 6a, intraperitoneal injection of insulin in wild-type mice increased production of PIP3, an end product of PI3 kinase which serves as an indication of PI3 kinase activation. However, the same concentration of insulin failed to activate PI3 kinase in MCPIP1-/- mice. It should also be noted that the baseline activity of PI3 kinase in livers from MCPIP1-/- mice was also significantly lower than those from wild-type mice. No differences were observed in PI3 kinase protein expression between wild-type and MCPIP1-/- mice (Figure 6b).

Insulin-stimulated association of IRS-1 with p85 kinase in the liver

As shown in Figure 7, the basal level of association of IRS- 1 with p85 in the liver tissue was significantly lower in MCPIP1-/- mice when compared to wild-type mice. Administration of insulin failed to affect the association of IRS-1 with p85 in wild-type mice and MCPIP1-/- mice.
 
Diabetes Fig 8.6

Figure 6. PI3 kinase activity and p85 protein expression in liver tissue. Following 16-18 hour fasting, mice received 2 IU/kg of insulin intraperitoneally. 15 minutes after insulin injection, livers were removed and homogenized and proteins were extracted in lysis buffer. For PI 3-kinase activity assay (6a), 3 mg of liver lysate was immunoprecipitaed with 3 μg of anti-p85. The supernatant was incubated with PI3 kinase substrate, PIP2. The amount of PIP3 in the supernatant (end product of the kinase reaction) was detected using PI3-kinase activity ELISA kit. Results were calculated as pmol/mg of total protein and expressed as mean ± S.E.M from 4 independent experiments. For p85 protein expression (6b), supernatants containing equal amounts of protein (100 μg) were used. Proteins were separated with SDS-PAGE, probed with anti-p85, and re-probed with β-actin. The upper panel shows a representative blot and the lower panel shows a graph summarizing the results from 4 independent experiments. Results were calculated as the ratio of p85 to β-actin and expressed as mean ± S.E.M. *p<0.05 compared to wild-type control, #p<0.05 compared to wild-type insulin.

Figure 7. Association of IRS-1 with p85 kinase in liver tissue.
 
Diabetes Fig 8.7

Following 16-18 hour fasting, mice received 2 IU/kg of insulin intraperitoneally. 15 minutes after insulin injection, livers were removed and homogenized and proteins were extracted in lysis buffer. 3 mg of liver lysate was immunoprecipitaed with 3 μg of anti- IRS-1. The supernatant was collected and proteins were separated by SDS-PAGE and probed with anti-p85. The upper panel shows a representative blot and the lower panel shows a graph summarizing the results from 4 independent experiments. Results were calculated as arbitrary units of the band density and expressed as mean ± S.E.M. *p<0.05 compared to wild-type control.

Morphological changes of liver

As shown in figure 8, liver sections from MCPIP1-/- mice showed reduced hepatocytes and multiple foci containing collagen and bile ducts in the interlobular area in HE staining. Masson-Trichrome staining revealed matrix deposition mainly in the centrilobular area and portal tract.

Figure 8. Liver morphology in wild-type and MCPIP1-/- mice. Mice were anesthetized and fixed by in vivo perfusion with 3.7% of formaldehyde through the left ventricle. Livers were harvested, processed, embedded in paraffin, and sectioned. Cross-sections of 8 μm were stained with hematoxylin-eosin (HE) and Masson-Trichrome (MT). Images were captured. Red arrow indicates hepatocytes and green arrow indicates collagen.
 
Diabetes Fig 8.8

Insulin-induced AKT phosphorylation in adipose tissue

No differences in insulin-induced AKT phosphorylation were observed in adipose tissue as it occurred in liver tissue between wild type and MCPIP1-/- mice (Figure 9).

Figure 9. AKT phosphorylation at serine 473 in adipose tissue.
 
Diabetes Fig 8.9

Following 16-18 hour fasting, mice received 2 IU/kg of insulin intraperitoneally. Adipose tissues were removed 15 minutes after the injection. Tissues were homogenized and proteins were extracted in lysis buffer. Supernatants containing equal amounts of protein (100 μg) were used to detect AKT phosphorylation levels. Proteins were separated with SDS-PAGE, probed with anti-phospho-AKT (ser473), stripped, and re-probed with anti-AKT. The upper panel shows a representative blot and the lower panel shows a graph summarizing the results from 4 independent experiments. Results were calculated as the ratio of phosphorylated AKT to total AKT and expressed as mean ± S.E.M. *p<0.05 compared to wild-type control, #p<0.05 compared to wild-type insulin.

Discussion

Emerging evidence suggests that increased inflammatory responses in both liver and adipose tissues contribute to insulin resistance and impaired glucose homeostasis [5, 6, 8-11]. MCPIP1 is an important inflammatory/immune modulator. Overexpression of MCPIP1 suppresses inflammatory cytokine-induced signal transduction and MCPIP1 deficiency induces systemic inflammatory responses [15, 16, 20, 21], suggesting that MCPIP1 may be involved in regulating metabolic homeostasis by affecting hyperactive inflammatory responses and that MCPIP1 deficiency may lead to metabolic disturbances by down-regulating the body’s innate immune system, especially the function of liver and/or adipose tissue. To our surprise, our data revealed that MCPIP1 deficiency markedly increased insulin sensitivity but delayed the response to glucose challenge and postponed the recovery of glucose homeostasis following insulin challenge. To understand the underlying mechanism, we examined the insulin signal transduction pathway in both liver and adipose tissue. Our data showed that when compared to wild-type mice, insulin-induced hepatic insulin signal transduction in MCPIP1-/- mice was significantly impaired, manifested as reduced phosphorylation of IR, IRS-1, AKT, and association of IRS-1 with PI3Kinase. These changes are accompanied by noticeable reduction of hepatocytes, and increased bile duct growth, and matrix deposition in the liver sections from MCPIP1-/- mice. Grossly, we also observed enlarged and pale liver with an external nodular surface (data not shown), suggesting that a significant remodeling took place, which may have resulted from inflammatory response since recent studies found that MCPIP1-/- mice displayed increased macrophage infiltration in the bile duct of the liver [12, 22].

We speculated that the increased insulin sensitivity may be the result of an enhanced compensatory response from other tissues, such as adipose tissue. We used insulin-induced AKT phosphorylation as a surrogate of insulin sensitivity. However, no difference was observed between wild-type and MCPIP1-/- mice. The delayed recovery of plasma glucose levels in response to insulin challenge and the lower fasting basal plasma glucose concentrations in MCPIP1-/- mice may occur as the result of pathological changes observed in the liver, which not only lead to the impairment of insulin signal transduction but also glycogenolysis, an important function that is involved in maintaining glucose homeostasis.

In addition to the changes mentioned above, we also observed a lower baseline level of insulin in MCPIP1-/- mice than that in wild-type mice, which may be the result of increased insulin sensitivity. Several recent studies showed that MCPIP1-/- mice displayed an accumulation of immunoglobulins in multiple organs and upregulation of inflammatory cytokines [12, 16, 22]. Our data revealed a significant increase in total plasma protein concentration in MCPIP1-/- mice compared to wild-type mice. These results are consistent with the morphological changes of the liver that we observed in MCPIP1-/- mice, since one of the major functions of the liver is to maintain normal plasma concentrations of amino acids by promoting and suppressing ureagenesis. Hepatic remodeling caused by MCPIP1 deficiency impairs the ability of liver to metabolizing amino acid leading to hyperalbuminemia.

In conclusion, our current study demonstrates that MCPIP1 deficiency is associated with increased insulin sensitivity, impaired hepatic insulin signaling, and hepatic remodeling. Our results suggest that MCPIP1 plays an important role in regulating metabolic homeostasis and maintaining normal function of liver. The exact mechanism of increased insulin sensitivity in MCPIP1 deficiency mice is not understood. Further studies will be conducted to explore the possible mechanisms behind these findings, such as insulin signal transduction pathway and/or insulin receptor expression levels in other tissues including skeletal muscle.

Funding
 
This work was supported by Warner/Fermaturo grant awarded by A.T. Still University Board of Trustees and the grant awarded by A.T. Still University Biomedical Graduate Program.

Disclosures

The authors do not have anything to disclose.

Authorship Contributions

Participated in research design: Yingzi Chang

Conducted experiments: Yingzi Chang, Brett Holdaway, Joshua Moody, Lauren Brooks

Contributed new reagents or analytic tools: Yingzi Chang, Mingui Fu, Pappachan Kolattukudy

Performed data analysis: Yingzi Chang and Brett Holdaway Wrote or contributed to the writing of the manuscript: Yingzi Chang, Brett Holdaway, Joshua Moody, Mingui Fu, and Lauren Brooks.

Acknowledgement

Authors are grateful to the faculty and staff in Department of Pharmacology at A.T. Still University for their scientific and technical support. We would also like to acknowledge Dr. Maria Evans and Dr. Peter Kondrashov for lending their expertise to explaining the pathological results. We thank Michael Cramberg and Kelly Rogers for providing their valuable technical assistance.
 

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Cite this article: Chang Y. MCPIP1 Deficiency Increases Insulin Sensitivity But Impairs Hepatic Insulin Signal Transduction in Mice. J J Diab Endocrin. 2015, 1(1): 008.

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