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
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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.
KeywordsPancreatic β-cells Glucotoxicity Stress kinases Rac1 CD36 and metformin
AMP-activated protein kinase
ADP-ribosylation factor 6
Cell division control protein 42
Cluster of differentiation36
- ER stress
Endoplasmic reticulum stress
Guanine nucleotide exchange factors
Geranylgeranyl transferase inhibitor
Glucose-stimulated insulin secretion
NADPH oxidase 2
p38 mitogen activated protein kinase
Ras-related C3 botulinum toxin substrate 1
Reactive oxygen species
Small (or short) interfering RNA
Type 2 diabetes mellitus
T-lymphoma invasion and metastasis-inducing protein 1
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.
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.
This research is supported [to AK] by a MERIT Review Award from the US Department of Veterans Affairs [BX002801] and the National Institutes of Health [DK-74921 and EY-022230]. AK also received a Senior Research Career Scientist Award [13S-RCS-006] from the Department of Veterans Affairs.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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