ABSTRACT
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.
Subject(s)
Insulin/biosynthesis , Islets of Langerhans/drug effects , Islets of Langerhans/metabolism , Oleic Acid/pharmacology , Palmitic Acid/pharmacology , Animals , Blotting, Northern , Glucose/pharmacology , Insulin/analysis , Insulin/metabolism , Insulin Secretion , Insulinoma , Islets of Langerhans/chemistry , Oleic Acids , Pancreatic Neoplasms , Proinsulin/biosynthesis , Proinsulin/genetics , Protein Precursors/biosynthesis , Protein Precursors/genetics , RNA, Messenger/analysis , Rats , Tumor Cells, CulturedABSTRACT
Glitazones are known to modulate fatty acid-induced effects on insulin secretion in the pancreatic beta-cell. The present study focused on combined effects of troglitazone and oleate on preproinsulin (PPI) biosynthesis. Insulin-producing INS-1 cells were incubated for 4 hr at 11.2mM glucose in the presence (O(+)) or absence (O(-)) of 200 microM oleate with (T(+)) or without (T(-)) 10 microM troglitazone. After cell lysis, cytoplasmic RNA was extracted and employed for Northern blotting and corresponding in vitro translation. Compared with untreated controls (CTRL=O(-)/T(-)), the cellular content of PPI-mRNA from cells which had been simultaneously treated by troglitazone and oleate (O(+)/T(+)) was significantly diminished (O(+)/T(+)=75+/-10% x CTRL; P=0.015). The PPI-mRNA content from those cells which had been exclusively exposed either to oleate (O(+)/T(-)) or troglitazone (O(-)/T(+)) did not significantly differ from that of the untreated controls. In spite of that decreased PPI-mRNA content, in vitro translation revealed the highest yield of newly synthesized PPI in RNA samples from those cells which had been simultaneously exposed to oleate and troglitazone before (O(+)/T(+)=1.6+/-0.3 x CTRL; P=0.01). It is concluded that troglitazone and oleate synergistically affect the translational rate at the level of the PPI-mRNA molecule.
