Reduced endoplasmic reticulum (ER)-to-Golgi protein trafficking contributes to ER stress in lipotoxic mouse beta cells by promoting protein overload.
OBJECTIVE
Saturated fatty acids augment endoplasmic reticulum (ER) stress in pancreatic beta cells and this is implicated in the loss of beta cell mass that accompanies type 2 diabetes. However, the mechanisms underlying the induction of ER stress are unclear. Our aim was to establish whether saturated fatty acids cause defects in ER-to-Golgi protein trafficking, which may thereby contribute to ER stress via protein overload.


METHODS
Cells of the mouse insulinoma cell line MIN6 were transfected with temperature-sensitive vesicular stomatitis virus G protein (VSVG) tagged with green fluorescent protein to quantify the rate of ER-to-Golgi protein trafficking. I14 antibody, which detects only correctly folded VSVG, was employed to probe the folding environment of the ER. ER stress markers were monitored by western blotting.


RESULTS
Pretreatment with palmitate, but not oleate, significantly reduced the rate of ER-to-Golgi protein trafficking assessed using VSVG. This was not secondary to ER stress, since thapsigargin, which compromises chaperone function by depletion of ER calcium, markedly inhibited VSVG folding and promoted strong ER stress but only slightly reduced protein trafficking. Blockade of ER-to-Golgi protein trafficking with brefeldin A (BFA) was sufficient to trigger ER stress, but neither BFA nor palmitate compromised VSVG folding.


CONCLUSIONS
Reductions in ER-to-Golgi protein trafficking potentially contribute to ER stress during lipoapoptosis. In this case ER stress would be triggered by protein overload, rather than a disruption of the protein-folding capacity of the ER.
Preston
A M
AM
Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, NSW, 2010, Australia.|St Vincent's Clinical School, Faculty of Medicine, University of New South Wales, Sydney, NSW, Australia.|St Jude Children's Research Hospital, Memphis, TN, USA.
Gurisik
E
E
Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, NSW, 2010, Australia.|St Vincent's Clinical School, Faculty of Medicine, University of New South Wales, Sydney, NSW, Australia.
Bartley
C
C
Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, NSW, 2010, Australia.
Laybutt
D R
DR
Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, NSW, 2010, Australia.|St Vincent's Clinical School, Faculty of Medicine, University of New South Wales, Sydney, NSW, Australia.
Biden
T J
TJ
Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, NSW, 2010, Australia. t.biden@garvan.org.au.|St Vincent's Clinical School, Faculty of Medicine, University of New South Wales, Sydney, NSW, Australia. t.biden@garvan.org.au.
D000818:Animals; D045744:Cell Line, Tumor; D003513:Cycloheximide / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000502:physiology*; D017930:Genes, Reporter; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000502:physiology*; D007340:Insulinoma; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D019308:Palmitic Acid / Q000494:pharmacology; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology*; D011506:Proteins / Q000378:metabolism*; D013312:Stress, Physiological / Q000187:drug effects / Q000502:physiology*; D014162:Transfection; D014759:Viral Envelope Proteins / Q000378:metabolism*
C028391:G protein, vesicular stomatitis virus; D008562:Membrane Glycoproteins; D011506:Proteins; D014759:Viral Envelope Proteins; D019308:Palmitic Acid; D003513:Cycloheximide