Metabolic Stress Induces Caspase-3 Mediated Degradation and Inactivation of Farnesyl and Geranylgeranyl Transferase Activities in Pancreatic β-Cells.
OBJECTIVE
At least 300 prenylated proteins are identified in the human genome; the majority of which partake in a variety of cellular processes including growth, differentiation, cytoskeletal organization/dynamics and vesicle trafficking. Aberrant prenylation of proteins is implicated in human pathologies including cancer; neurodegenerative diseases, retinitis pigmentosa, and premature ageing syndromes. Original observations from our laboratory have demonstrated that prenylation of proteins [small G-proteins and γ-subunits of trimeric G-proteins] is requisite for physiological insulin secretion. Herein, we assessed the impact of metabolic stress [gluco-, lipotoxicity and ER-stress] on the functional status of protein prenylation pathway in pancreatic β-cells.


METHODS
Farnesyltransferase [FTase] and geranylgeranyltransferase [GGTase] activities were quantified by radioisotopic methods. Caspase-3 activation and FTase/GGTase-α subunit degradation were determined by Western blotting.


RESULTS
We observed that metabolic stress activates caspase-3 and induces degradation of the common α-subunit of FTase and GGTase-I in INS-1 832/13 cells, normal rodent islets and human islets leading to functional defects [inactivation] in FTase and GGTase activities. Caspase-3 activation and FTase/GGTase-α degradation were also seen in islets from the Zucker diabetic fatty [ZDF] rat, a model for Type 2 diabetes. Consequential to defects in FTase/GGTase-α signaling, we observed significant accumulation of unprenylated proteins [Rap1] in β-cells exposed to glucotoxic conditions. These findings were replicated in β-cells following pharmacological inhibition of generation of prenylpyrophosphate substrates [Simvastatin] or catalytic activity of prenylating enzymes [GGTI-2147].


CONCLUSIONS
Our findings provide the first evidence to suggest that metabolic stress induced dysfunction of the islet β-cell may, in part, be due to defective protein prenylation signaling pathway.
Department of Molecular and Cellular Endocrinology, Diabetes and Metabolic Research Institute, City of Hope/BRI, Duarte, USA.
Rajakrishnan
R
Veluthakal
Daleep K
DK
Arora
Marc L
ML
Goalstone
Renu A
RA
Kowluru
Anjaneyulu
A
Kowluru
D000328:Adult; D019883:Alkyl and Aryl Transferases / Q000378:metabolism*; D000818:Animals; D053898:Biosynthetic Pathways / Q000187:drug effects; D053148:Caspase 3 / Q000378:metabolism*; D002784:Cholesterol / Q000096:biosynthesis; D059865:Endoplasmic Reticulum Stress / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D005947:Glucose / Q000633:toxicity; D006801:Humans; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000201:enzymology*; D008055:Lipids / Q000633:toxicity; D008297:Male; D008954:Models, Biological; D017368:Protein Prenylation / Q000187:drug effects; D059748:Proteolysis / Q000187:drug effects; D051381:Rats; D019821:Simvastatin / Q000494:pharmacology; D013312:Stress, Physiological / Q000187:drug effects
D007328:Insulin; D008055:Lipids; D002784:Cholesterol; D019821:Simvastatin; D019883:Alkyl and Aryl Transferases; C083291:geranylgeranyltransferase type-I; C064911:p21(ras) farnesyl-protein transferase; D053148:Caspase 3; D005947:Glucose