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 we’ll discuss causes of diabetes

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

•      Next we will focus on the PPARă 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 â-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. TNFá, 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%

–  â-cell autoantibodies

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

Causes of Diabetes

•      Type 2

–  A variety of theories, we’ll 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 PPARă

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 Ș 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 PKCè

- PKCè 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.  TNFá and inflammatory adipokines

    - Chronic exposure to TNFá to 3T3-L1 adipocytes         resulted in 90% « in GLUT-4 mRNA

    - TNFá has been found to:

          * Repress expression of IRS-1 & GLUT-4

          * Induce serine phosphorylation of IRS-1

          * Increase FFA plasma levels

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

 

PPARă

•      Peroxisome-proliferator activated gamma (PPARă)

•      A nuclear receptor when activated dimerizes with retinoic X receptor

•      A downstream mediator of IR – MEK- ERK1/2 pathway

•      Both PPARă & 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 PPARă

•      Originally discovered to inhibit lipid peroxidation

•      Agonist activity found to down regulate TNFá 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 PPARă and TRPV1 vanilloid receptor

–    The vanilloid receptor is expressed both in the islet â-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 Gái/o coupling

–    CB1 also has Gás 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 TNFá, 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 Ș activation of TH1/TH2

•    IFN-ă, IL-12, and TNFá associated with Ș TH1, treatment with THC showed a marked decrease in mRNA levels of all

CB Receptors & â-Cells

•      Insulin secretion by â-cells follows an oscillatory pattern

–   Stimulated by Ș &« pattern of intracellular Ca2+

•      Receptor localization:

–   CB1 found mostly on á-cells

–   CB2 found on both á- & â-cells

–   TRPV1 also found on â-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 & â-Cells

•       The Evidence:

 

–    Anandamide & 2-AG concentration in â-cells Ș under hyperglycemic conditions and decreases under hypoglycemic conditions

 

–    Administration of insulin « endocannabinoid levels

 

–    Chronic activation of CB1 leads to up-regulation of PPARă (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 PPARă receptor

•      Diabetics have a marked reduction in immune function & O2 transport

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

- IgM glycosylation Șeven 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]ìg

–   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

•    Ÿ 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 PLCă-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

        -        TNFá 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 & TNFá

 

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, PPARă, CB1, CB2, and VEGF, we find that cannabinoid therapy for diabetes can:

•     Reduce BGLs                  2.  Reduce HbA1c

•     Ș insulin sensitivity         4.  Ș 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.

•       Demuth, D.G. and Molleman, A.  2005.  “Cannabinoid Signaling”.  Life Sciences (Epub Ahead of Print).

•       El-Remessy, A.B., et al.  2006.  “Neuroprotective and blood-retinal barrier preserving effects of cannabidiol in experimental diabetes”. American Journal of Pathology 168(1): 235-244.

•       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

•       Guo, L. and Tabrizchi, R.  2005.  “Peroxisome proliferator activated receptor gamma as a drug target in the pathogenesis of insulin resistance”.  Pharmacology & Therapeutics (Epub Ahead of Print).

•       Hampson, A.J., et al.  1998.  “Neuroprotective antioxidants from marijuana”.  Annals New York Academy of Sciences 95: 8268-8273.

•       Juan-Pico, P., et al.  2006.  “Cannabinoid receptors regulate Ca2+ signals and insulin secretion in pancreatic â-cells”.  Cell Calcium 39: 155-162.

•       Kalia, K., et al.  2004.  “Non-enzymatic glycosylation of immunoglobulins in diabetic nephropathy”.  Clinica Chimica Acta 347: 169-176.

•       Li, X., et al.  2001.  “Examination of the immunosuppressive effect of delta-9-THC in streptozotocin-induced autoimmune diabetes”.  International Immunopharmacology 1: 699-712.

•       Marsicano, G., et al.  2002.  “Neuroprotective properties of cannabinoids against oxidative stress: role of the cannabinoid CB1 receptor”.  Journal of Neurochemistry 80: 448-456.

References

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•       http://www.biocarta.com/pathfiles/h_insulinPathway.asp

•       http://www.biocarta.com/pathfiles/h_vegfPathway.asp