Intracellular depletion of insulin: a comparative study with palmitate, oleate and elaidate in INS-1 cells.
OBJECTIVE
Free fatty acids (FFAs) deplete the intracellular insulin stores of pancreatic beta-cells. It has been suggested that this results from a lipotoxic dysregulation of both insulin secretion and insulin synthesis. In the present study, this hypothesis was tested within a 12-h time-course by directly relating the FFA-induced loss of intracellular insulin to corresponding parameters of insulin secretion and de novo biosynthesis. Palmitate, cis-monoenic oleate and the trans-monoenic elaidate were employed as model FFAs to elucidate potentially different effects due to chain length and configuration.


METHODS
INS-1 cells were incubated for 1, 4 or 12 h with 11.2 mmol/l glucose with 200 micromol/l palmitate, oleate or elaidate and compared with non-FFA-exposed controls with respect to content and secretion of immunoreactive insulin (IRI). Biosynthesis of insulin was monitored by pulse-labeling experiments and by Northern blot analysis.


RESULTS
IRI content dropped by 50-60% after a short-term exposure with all FFAs employed (P< or =0.001). It tended to recover after 12 h of treatment with oleate and elaidate but not with palmitate. FFA treatment increased insulin secretion by 25% (P< or =0.05) which could not account quantitatively for the intracellular loss. FFA-induced changes in insulin biosynthesis did not correlate clearly with the FFA-induced intracellular loss.


CONCLUSIONS
The FFA-induced loss of IRI is an acute effect independent of the FFA employed. It cannot be sufficiently explained by FFA-induced perturbances of IRI secretion and biosynthesis. We therefore postulate an additional FFA-triggered mechanism, e.g. intracellular IRI degradation.
Bollheimer
L Cornelius
LC
Department of Internal Medicine I, University of Regensburg, 93042 Regensburg, Germany. cornelius.bollheimer@klinik.uni-regensburg.de
Kemptner
Daniela M
DM
Kagerbauer
Simone M
SM
Kestler
Thomas M
TM
Wrede
Christian E
CE
Buettner
Roland
R
D000818:Animals; D015152:Blotting, Northern; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000032:analysis / Q000096:biosynthesis* / Q000378:metabolism; D000078790:Insulin Secretion; D007340:Insulinoma; D007515:Islets of Langerhans / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism*; D019301:Oleic Acid / Q000494:pharmacology*; D009829:Oleic Acids; D019308:Palmitic Acid / Q000494:pharmacology*; D010190:Pancreatic Neoplasms; D011384:Proinsulin / Q000096:biosynthesis / Q000235:genetics; D011498:Protein Precursors / Q000096:biosynthesis / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D014407:Tumor Cells, Cultured
D007328:Insulin; D009829:Oleic Acids; D011498:Protein Precursors; D012333:RNA, Messenger; D019301:Oleic Acid; D019308:Palmitic Acid; C011459:elaidic acid; C013737:preproinsulin; D011384:Proinsulin; D005947:Glucose