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Page 1: Diabetes Mellitus Type II

Diabetes Mellitus Type II

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Beta Cell Failure in DM T2

signaling pathways implicated in β-cell failure

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Wnt Signaling Pathway

Controls organismal growth and differentiation

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Wnt signalling Pathway and DM T2

1. Homozygous mutation of LRP5 in mice leads to defective glucose-stimulated insulin secretion from isolated islets in vitro.

2. Components of the Wnt pathway are present in the adult pancreas, and in particular multiple members of the frizzled family of Wnt receptors have been identified in the islet.

3. TCF7/L2 gene polymorphism affects B-cell function

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While most studies have found little evidence that Wnt signaling is involved

in endocrine differentiation or in the adult islet,

there is some evidence for an

effect of Wnt signaling on β-cell replication.

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Main ObjectiveTo study the role of Wnt signaling

in human diabetes

Systematically examinedcomponents of the

pathway in the pancreasof normal & DMT2

Mouse Studies

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METHODS TISSUE PREPARATION1. Paraffin-embedded from human pancreases: 5 non-diabetic & 9 DM T2

2. Freshly isolated & cultured pancreases: 3 non-diabetic (Isolated human islets)

3. Human Fetal Pancreases in 18-24 gestational weeks4. Murine Pancreaes (Balb/c or C57/bl mice)5. NEPCs

WESTERN BLOT1. Whole-cell extracts

IMMUNO-HISTOCHEMISTRY1. Paraffin samples2. Frozen samples

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I. Objective: To determine if TCF7L2 is upregulated in islets

of type II diabetic patients

•TCF7L2 is a protein acting as a transcription factor.•Several SNPs of the gene are associated with Type 2 DM

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TCF/L2 TCF3

TCF7L2 is upregulated in islets of type II diabetic

patients

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II. Objective: To determine if Wnt2b

is upregulated in

type II diabetes

To test the hypothesis that the induction of TCF factors, being a both effector as well as a downstream target of

Wnt Signaling, is a result of a more global activation of Wnt signaling

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B Catenin

Wnt2bIn islets

Wnt 2b is upregulated in Type 2 Diabetes

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III. Objective:To determine if

B-catenin

is upregulated in

type II diabetes

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B-CateninIn islets

Beta-catenin in human islets of all 5 nondiabetics was markedly lowerThan in the surrounding exocrine

Tissue where it was stronglyExpressed (figs. 2f-2h)

Beta-catenin is strongly expres in the islets of all DM T2 to a

Level approximately half that of the Surrounding exocrine tissue

(figs 2h-2j)

insu

lin

insu

lin

insu

lin

islet

islet

insulin

B-catenin B-catenin

insulin

B-catenin

B-catenin

DM T2 DM T2

Normal Normal

Human B-cells lack Detectable B-catenin

expression but it is highly upregulated in

DM T2

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IV. Objective: To determine if the terminal effectors

(c-myc & cyclin D) are upregulated in DM T2

TCF/LEF factors activate a number of terminal effectors Of Wnt signaling

(c-myc and cyclinD1)

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cNORMAL

DM T2

CyclinD1In islets

C-mycIn islets

Terminal effectors of Wnt signaling

(c-myc & cyclinD1)are upregulated in

human type 2 Diabetes

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MOUSE MODELV. Objective:

To determine whether Wnt activation was an early or late

event in diabetes pathogenesis

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high-fat dietnormal chow

Harvested pancreases examined for c-myc expression

OBESE BUT WITH NORMAL FBS

12 weeks

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co

ntro

l

High-fat

cont

rol

High-fat

cont

rol

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Figure 6: Wnt signaling in high-fat fed mice. •In normal mouse pancreas, β-catenin (green in (a) and (b)) is expressed in islets as identified by somatostatin (red in (a)) and colocalizes with insulin (red in (c)). •C-myc (red in (d) and (e)) was not expressed in islets of normal mice (marked by dotted lines and glucagon in green in (d)) • but was induced in islets and some ducts of high-fat fed mice (islets marked by glucagon in green). •C-myc expression is quantitated in (f).

B -catenin

B -catenin

somatostatin insulin

Islets

C-myc

C-myc

High-Fat

Normal

Normal

Normal

Normal

Glucagon

Expression of the Wnt target gene c-myc

is an early response to high-fat diet

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Summary1. B-catenin in human islets of all 5

nondiabetics was markedly lower than in the surrounding exocrine tissue, where it was strongly expressed.

2. Expression of the Wnt2b, B-catenin, TCF7/L2, & terminal effectors (c-myc & cyclinD1) are all upregulated in the islets in type II diabetes

3. The mouse model suggested that obesity alone may be sufficient to induce Wnt activation, which would mark it as an early event in the pathogenesis of type II diabetes.

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ConclusionType II Diabetes activates the Wnt signaling pathway

specifically in the Beta cells of the islets of Langerhans

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Thank You!!!

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• Immunohistochemical analysis of insulin (green) and anti-sFRP (red in (f) and (g)) detects sFRP on islet cells only when sFRP has been added to the culture media.

• sFRP exposure led to inhibition of β-catenin ((h) versus (i), quantitated in (j)) and c-myc ((k) versus (l), quantitated in (m)).

• Error bars = mean +/−s.e.m. ∗P < .05. Scale bars in (f)–(m): 25 μm.

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• To pursue the role of Wnt activation in the islet, it would be desirable to have an in vitro model. Thus, we examined Wnt activation in isolated islets.

• Surprisingly, when cultured human islets were examined by Western blotting, β-catenin, which is low or absent in the islet compared with surrounding tissue in situ, was expressed at a higher level than in the nonendocrine pancreatic cells (NEPCs) [23] (Figure 4(a)).

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• To pursue the finding that β-cells with low-insulin expression had a pattern of catenin expression resembling that in the exocrine pancreas, pancreas sections were immunostained for the acinar marker amylase as well as insulin, revealing that low-insulin β-cells coexpressed amylase (Figures 5(f), 5(g), 5(h)).

• The insulin/amylase doublepositive cells expressed PDX-1 (Figure 5(i)), which in the adult pancreas is restricted to β-cells and is never expressed in mature acinar cells, indicating that the weak insulin expression was not artifactual.

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• To further explore whether the areas containing the insulin/amylase double-positive cells arose by alteration of preexisting β-cells or by induction of insulin expression in preexisting exocrine cells, as has been described in some β- cell regeneration models [37–39], we examined those areas for glucagon expression.

• Consistently, high levels of glucagon and a lack of amylase were observed in all α-cells, whether the islets exhibited high or low insulin expression (Figure 5(l)).

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• Figure 1. Schematic representation of WNT/TCF signalling. Secreted WNTs bind to FZD and LRP receptors, which in turn inactivate the degradation complex comprising AXIN, DVL and GSK3B.

• This results in non-phosphorylated b-catenin entering into the nucleus and binding to TCF7L2, thus activating a wide variety of genes.

• TCF7L2 could regulate several genes – tissue specifically influencing both insulin secretion and insulin sensitivity. For example, TCF7L2 regulates b-cell growth and differentiation. TCF7L2 also activates the expression of proglucagon gene, which encodes the GLP-1 (glucagons like peptide-1) and thus promotes insulin secretion. Alterations in this pathway (in TCF7L2 risk variants) could lead to reduced secretion of GLP-1 and hence defective insulin secretion.

• In addition, altered WNT signalling (in TCF7L2 risk variants) could be expected to influence adipose tissue growth and development and thus BMI. Increased pro-inflammatory signals (IL-6, TNF-a) and altered adiponectin from the adipocytes (through their endocrine function) might result in skeletal muscle insulin resistance. With regard to microvascular complications of diabetes, TCF7L2 may also influence mesangial cell expansion and retinal neovascularization.