Apoptosis

, Volume 22, Issue 11, pp 1380–1393 | Cite as

Glucotoxicity promotes aberrant activation and mislocalization of Ras-related C3 botulinum toxin substrate 1 [Rac1] and metabolic dysfunction in pancreatic islet β-cells: reversal of such metabolic defects by metformin

  • Sartaj Baidwan
  • Anil Chekuri
  • DiAnna L. Hynds
  • Anjaneyulu Kowluru
Original Paper

Abstract

Emerging evidence suggests that long-term exposure of insulin-secreting pancreatic β-cells to hyperglycemic (HG; glucotoxic) conditions promotes oxidative stress, which, in turn, leads to stress kinase activation, mitochondrial dysfunction, loss of nuclear structure and integrity and cell apoptosis. Original observations from our laboratory have proposed that Rac1 plays a key regulatory role in the generation of oxidative stress and downstream signaling events culminating in the onset of dysfunction of pancreatic β-cells under the duress of metabolic stress. However, precise molecular and cellular mechanisms underlying the metabolic roles of hyperactive Rac1 remain less understood. Using pharmacological and molecular biological approaches, we now report mistargetting of biologically-active Rac1 [GTP-bound conformation] to the nuclear compartment in clonal INS-1 cells, normal rat islets and human islets under HG conditions. Our findings also suggest that such a signaling step is independent of post-translational prenylation of Rac1. Evidence is also presented to highlight novel roles for sustained activation of Rac1 in HG-induced expression of Cluster of Differentiation 36 [CD36], a fatty acid transporter protein, which is implicated in cell apoptosis. Finally, our findings suggest that metformin, a biguanide anti-diabetic drug, at a clinically relevant concentration, prevents β-cell defects [Rac1 activation, nuclear association, CD36 expression, stress kinase and caspase-3 activation, and loss in metabolic viability] under the duress of glucotoxicity. Potential implications of these findings in the context of novel and direct regulation of islet β-cell function by metformin are discussed.

Keywords

Pancreatic β-cells Glucotoxicity Stress kinases Rac1 CD36 and metformin 

Abbreviations

AMPK

AMP-activated protein kinase

Arf6

ADP-ribosylation factor 6

CC3

Cleaved caspase-3

Cdc42

Cell division control protein 42

CD36

Cluster of differentiation36

ER stress

Endoplasmic reticulum stress

GEFs

Guanine nucleotide exchange factors

GGTase

Geranylgeranyl transferase

GGTI-2147

Geranylgeranyl transferase inhibitor

GSIS

Glucose-stimulated insulin secretion

HG

High glucose

LG

Low glucose

MTT

3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide

Nox2

NADPH oxidase 2

p38MAPK

p38 mitogen activated protein kinase

Rac1

Ras-related C3 botulinum toxin substrate 1

ROS

Reactive oxygen species

siRNA

Small (or short) interfering RNA

T2DM

Type 2 diabetes mellitus

Tiam1

T-lymphoma invasion and metastasis-inducing protein 1

Notes

Acknowledgements

This work is conducted for the partial fulfillment of an MS degree in Pharmaceutical Sciences [to SB] from Wayne State University. The authors thank Drs. Jairus Reddy, Anil Poudel, Naveen Mekala, Vino Cheriyan and Ms. Gurbani Bedi for technical assistance in certain aspects of these studies.

Author Contributions

SB and AC conducted studies and analyzed the experimental data. DLH developed specific experimental tools, reviewed and edited the manuscript. AK planned and supervised studies, and wrote and edited the manuscript.

Compliance with ethical standards

Conflict of interest

The authors declare no conflict of interest.

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Copyright information

© Springer Science+Business Media New York (outside the USA) 2017

Authors and Affiliations

  • Sartaj Baidwan
    • 1
    • 2
  • Anil Chekuri
    • 3
  • DiAnna L. Hynds
    • 4
  • Anjaneyulu Kowluru
    • 1
    • 2
    • 5
  1. 1.β-Cell Biochemistry LaboratoryJohn D. Dingell VA Medical CenterDetroitUSA
  2. 2.Department of Pharmaceutical SciencesWayne State UniversityDetroitUSA
  3. 3.Shiley Eye InstituteUniversity of CaliforniaLa JollaUSA
  4. 4.Department of BiologyTexas Woman’s UniversityDentonUSA
  5. 5.B-4237 Research ServiceJohn D. Dingell VA Medical CenterDetroitUSA

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