Diabetes & The Endocannabinoid System: Prospects For Therapeutic Control

By:

Matthew Schnur

Quick Outline

      This will be a very detailed discussion, so lets put it in perspective

      First well discuss causes of diabetes

      Then move on to insulin receptor signaling and defects in this mechanism

      Next we will focus on the PPAR and cannabinoid CB1 & CB2 receptors

      Finally, it will all be tied together; how cannabinoid therapy treats the symptoms of Type 1 & Type 2 Diabetes

 

Diabetes Background

      Over 28 million Americans have diabetes (Type 1 or 2)

      80% of cases are diagnosed as Type 2

      The leading cause of blindness and amputations

      Diagnosed cases are rising exponentially-directly related to diet

      For every kg bodyweight over healthy BMI, a 7% increase in getting Type 2 is found

What is Diabetes?

      Type 1 (Diabetes Mellitus)

 

  An autoimmune disorder characterized by islet -cell destruction

 

  Plasma glucagon levels may be increased

 

  No detectable plasma insulin

What Is Diabetes?

      Type 2 (Diabetes Insipidus)

  Often environmentally induced in predisposed individuals

  Characterized by:

   Obesity

   Impaired IRS phosphorylation

   Impaired PI3K activity

   Impaired GLUT-4 translocation

   Increased FFA

 

Common Attributes To Both

      Both Type 1 and 2 patients have;

  Hypo/hyperglycemia

  Dyslipidemia

  Decreased immune function

  Poor wound healing

  Microangiopathies

   Neuropathy, retinopathy, nephropathy

  Depression & weight gain

   Both attributable to inflamm. TNF, IL-2, and IL-6

Causes of Diabetes

      Type 1:

  Only 30% identical twins will both have it

  MHC genes on chromosome 6

   Of 21 known DR alleles, DR3 & DR4 found in 95%

  -cell autoantibodies

   Directed against GAD (glutamic acid decarboxylase), unique to -cells

Causes of Diabetes

      Type 2

  A variety of theories, well focus on PPAR based

  Interruption of lipid homeostasis

-    Leads to increased FFA

-    FFAs normally decreased by PPAR activation

2. Activation of inflammatory cytokines normally suppressed by PPAR

Insulin Receptor Signaling

1. Insulin binds to the heterotetrameric IR (Insulin Receptor)

- Causes autophosphorylation of tyrosine residues

 

2. Tyrosine autophosphorylation causes dissociation of IRS-1 (Insulin Receptor Substrate-1)

- 4 IRS proteins;

* IRS-1 immediate activation of PI3K

* IRS-2 prolonged activation of PI3k

* IRS-3 & -4 inhibit PI3K activation

 

 

Insulin Receptor Signaling

3. Activation of PI3K

- Responsible for:

* Activ. of Akt/PKB (serine phosphorylation)

* GLUT-4 translocation

4. Activation of Ras/Raf

- Both PKB mediated or directly IRS activated

-Activates the MEK- ERK1/2 pathway

5. MEK & ERK1/2 Pathway

- Responsible for glycolysis & protein synthesis

- Activation of PPAR

Insulin Desensitization

       Besides tyrosine autophosphorylation, the IR has;

 

- Both serine & threonine residues capable of autophosphorylation

 

- Upon excess agonist activity, serine/threonine autophosp. causes a dissociation of IRS-1 without activation

 

- Results in loss of function IR, or only activation of IRS-2

* This is why we see IRS-2 activity in both Types

 

 

Insulin Desensitization

      Increased Fatty Acids

- Elevated FFAs lead to accumulation of

* DAG *fatty acyl-CoA

* ceramide

- These compounds are known to activate membrane bound PKC

- PKC causes serine phosphorylation of IRS-1 in lieu of IR mediated IRS-1 tyrosine phosphorylation

* Serine phosphorylation causes a dissociation between IRS-1 & PI3K

 

Insulin Resistance

3. TNF and inflammatory adipokines

- Chronic exposure to TNF to 3T3-L1 adipocytes resulted in 90% in GLUT-4 mRNA

- TNF has been found to:

* Repress expression of IRS-1 & GLUT-4

* Induce serine phosphorylation of IRS-1

* Increase FFA plasma levels

- TNF levels >2.5x higher in both Type 1 & 2 than in healthy patients

 

PPAR

      Peroxisome-proliferator activated gamma (PPAR)

      A nuclear receptor when activated dimerizes with retinoic X receptor

      A downstream mediator of IR MEK- ERK1/2 pathway

      Both PPAR & retinoic X receptor activation shown to enhance insulin sensitivity

      Ligands include mono- & poly-unsaturated fatty acids, PGs, the most commonly prescribed Type 2 diabetes medications thiazodolines (TZDs), and some NSAIDs (possible breakdown to AM404)

Functions of the PPAR

      Originally discovered to inhibit lipid peroxidation

      Agonist activity found to down regulate TNF gene

      Stimulates adipocyte differentiation & apoptosis

    Beneficial mostly for Type 2

      Represses gene expression of chemokines involved in insulin resistance:

    Leptin * Plasminogen activator-inhibitor-1

    Resistin * IL-6 & IL-11

      Induces gene expression of insulin sensitizing factors:

    Adiponectin * Fatty acid transport protein

    IRS-2

The Endocannabinoid System

      The CB1 & CB2 receptors are the most abundant G-protein coupled receptors in the human body

      Besides CB1 & CB2 endo- & phyto- cannabinoids also bind to the PPAR and TRPV1 vanilloid receptor

    The vanilloid receptor is expressed both in the islet -cells and smooth muscle cells

    Vanilloid receptor activation found to enhance insulin secretion and sensitivity

      Anandamide (arachidonylethanolamide) & 2-AG (arachidonylglycerol) are endocannabinoids

    These are under negative control of leptin

 

Endocann. Continued

    Leptin is a hormone secreted by adipose tissue and exerts its effects in the hypothalamus

 

    As previously mentioned, leptin increases insulin resistance

 

    Endocannabinoids are down-regulated by leptin

    Leptin causes an inhibition in the MAPK stimulated glycogen synthase activity of the CB1 receptor

 

 

The Cannabinoid Receptors

       The CB1 & CB2 receptors

    Both GPCR with Gi/o coupling

    CB1 also has Gs coupling ability under certain conditions

    Both coupled to activation of the PI3k-Akt/PKB pathway

    Both receptors shown to activate MAPKs via the Ras/Raf pathway

    P38 & p42/p44 MAPKs activated

    Shown to increase glycogen storage, glucose metabolism, c-fos expression

CB Receptors Continued

   Both receptors found to activate PLC

    PLC cleaves IP3

    IP3 releases Ca2+ from intracellular storage vesicles

   CB1 receptor also shown to inhibit K+ outflow & Ca2+ efflux

   CB2 not coupled to ion channels

CB & IR Interactions

CB Agonists

      Thus CB1 activation beneficial to insulin sensitivity and glucose metabolism

      CB2 is found predominantly in immune cells & adipocytes

      CB2 activation in B-cells, macrophages, T-cells, and monocytes is found to:

    Reduce TNF, IL-2, IL-6, and IL-11; all elevated in diabetics and correlated to insulin resistance

    Balance Th1/Th2 inflammatory cell profile

    Autoimmune Type 1 diabetes has activation of TH1/TH2

    IFN-, IL-12, and TNF associated with TH1, treatment with THC showed a marked decrease in mRNA levels of all

CB Receptors & -Cells

      Insulin secretion by -cells follows an oscillatory pattern

   Stimulated by & pattern of intracellular Ca2+

      Receptor localization:

   CB1 found mostly on -cells

   CB2 found on both - & -cells

   TRPV1 also found on -cells

      Cannabinoids found to/may:

    Reduce insulin secretion (metabolic syndrome X)

    CB1 may reduce cAMP dependent release of glucagon

    Enhance effects of insulin signaling

 

CB Receptors & -Cells

       The Evidence:

 

    Anandamide & 2-AG concentration in -cells under hyperglycemic conditions and decreases under hypoglycemic conditions

 

    Administration of insulin endocannabinoid levels

 

    Chronic activation of CB1 leads to up-regulation of PPAR (in adipocytes)

 

    Personal data:

    Smoking + insulin = ~18%> reduction in BGL

    Smoking alone = ~8% reduction

    No reduction when large quantities cannabis used + food

    Dangerous enhancement between exercise + cannabis + insulin combination can reduce insulin by 1/5

 

Non-CB Mediated Effects

      Both endo- & phyto- cannabinoids bind to the PPAR receptor

      Diabetics have a marked reduction in immune function & O2 transport

- IgA glycosylation 4x in both types of diabetics w/o complications, 33% more in Type 1

- IgM glycosylation even in healthy diabetics, 8% more in Type1

- Healthy individuals have 1-3% hemoglobin glycosylation, uncontrolled diabetics 20% (diagnostic tool HbA1c)

- Poor O2 transport by Hb leads to microangiopathies

- Other long lived proteins also get glycosylated; collagen, albumin, myelin

Non-CBR Mediated Effects

    Since protein glycosylation is an oxidative process, antioxidants have proven useful

    Preventative effects of Cannabis derived antioxidants on Hb glcosylation at [.5], [5], and [10]g

   Quercitan (flavanoid) 3%, 37%, 52%
   Kaempferol (terpenoid) 10%, 12%, 15%
   20 other flavanoids, also THC, CBD, CBC, and CBG all have antioxidant properties

    Hb glycosylation a Fenton Reaction

    NIH published paper on cyclic voltammetry & rat focal ischemia model: THC 20X potent the antioxidant than ascorbate

3. Cannabinoids (CBD) protect against myelin degradation, and excessive glutamatergic firing, a cause of one type of diabetic neuropathy (sensory)

- NMDA receptor induced intracellular Ca2+ accumulations cause neurotoxicity

 

Diabetic Retinopathy

       2 Phases:

 

- Nonproliferative

    Neovascularization resp. for

dev. of new blood vessels in
many tissues, especially the retina

    Growth mediated by VEGF

 

-      Proliferative phase

    Advanced stages of retinopathy

    Neovasc. Causes optic nerve damage & macular edema

    Leading cause of blindness

    all diabetics after 15 yrs

Retinopathy

       The VEGF Pathway

    Also actiavtes the PI3K-AKT/PKB pathway (like the CB receptors)

    Also activates the Ras/Raf dep. MAPK pathway just like the CB receptors

    Yet again, also activates the PLC-PKC pathway, and IP3 mediated intracellular Ca2+ release, like the CB receptors

    How then, can cannabinoids be beneficial?

 

Retinopathy & The CB Receptors

How Cannabinoids Benefit Retinopathy:

       Remember, 20 flavanoids + cannabinoid are antioxidants

-      The eye is rich with FFAs which are subject to oxidation (COX-2), typically elevated in diabetics

-      Cannabinoids prevent superoxide anion formation, and increase fatty acid metabolism

-       VEGF

- While VEGFR2 & CB receptors share nearly identical transduction mechanisms, cannabinoids inhibit VEGF gene transcription via other receptors, may not share similar phosphorylation patterns

- TNF increases VEGF mRNA, as does the Ils that are inhibited by CB activation

-       PEDF

- Pigment epithelial derived factor, a potent inhibitor of neovascukaarization via VEGF

- PEDF is inhibited by oxidative stress & TNF

 

Conclusions

      Diabetes is a simple disorder with complex pathways regulating insulin resistance/sensitivity and secondary pathology

      Nearly all complications to diabetes are the result of hyperglycemia

      After reviewing the IR, PPAR, CB1, CB2, and VEGF, we find that cannabinoid therapy for diabetes can:

     Reduce BGLs 2. Reduce HbA1c

     insulin sensitivity 4. glucose & lipid metabolism

     Prevent retinopathy 6. Inhibit inflammatory chemokines

     Neuroprotection 8. Improve O2 transport

References

       Asgary, S., et al. 1999. Anti-oxidant effect of flavanoids on hemoglobin glycosylation. Pharmaceutica Acta Helvetiae 73: 223-226.

       Blazquez, C., et al. 2004. Cannabinoids inhibit vascular endothelial growth factor pathway in gliomas. Cancer Research 64: 5617-5623.

       Caldwell, R.B., et al. 2005. Vascular endothelial growth factor and diabetic retinopathy: role of oxidative stress. Current Drug Targets 6: 511-524.

       Cussimanio, B.L., et al. 2003. Unusual susceptibility of heme proteins to damage by glucose during non-enzymatic glycation. Biophysical Chemistry 105: 743-755.

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       Gallily, R., et al. 2000. 2-arachidonylglycerol, an endogenous cannabinoid, inhibits tumor necrosis factor alpha production in murine macrophages, and in mice. European Journal of Pharmacology 406: R5-R7.

References

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References

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