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Published on: August 9, 2022
Adverse physicochemical properties of tripalmitin in beta cells lead to morphological changes and lipotoxicity in
J H Moffitt1, B A Fielding, R Evershed
1Oxford Centre for Diabetes, Endocrinology and Metabolism, Churchill Hospital, Oxford, OX3 7LJ, UK.
Aims/Hypothesis:
Long-term exposure of beta cells to lipids, particularly saturated fatty acids in vitro, results in cellular dysfunction and apoptosis (lipotoxicity); this could contribute to obesity-related diabetes. Our aims were to relate cell death to intracellular triglyceride concentration, composition and localisation following incubation of INS1 cells in saturated and unsaturated NEFA in high and low glucose concentrations.
Materials And Methods:
Insulin-producing INS1 cells were cultured (24 h; 3 and 20 mmol/l glucose) with palmitic, oleic or linoleic acids and the resulting intracellular lipids were analysed by gas chromatography and microscopy. Cell death was determined by quantitative microscopy and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and glucose-stimulated insulin secretion by ELISA.
Results:
All NEFA (0.5 mmol/l, 0.5% albumin) inhibited glucose-stimulated (20 mmol/l) insulin secretion. Cytotoxicity was evident only with palmitic acid (p<0.05), in which case intracellular triglyceride consisted largely of tripalmitin in angular-shaped dilated endoplasmic reticulum. Cytotoxicity and morphological disruption were reduced by addition of unsaturated NEFA. Triglyceride content (control cells; 14.5 ng/mug protein) increased up to 10-fold following incubation in NEFA (oleic acid 153.2 ng/mug protein; p<0.05) and triglyceride and phospholipid fractions were both enriched with the specific fatty acid added to the medium (p<0.05).
Conclusions/Interpretation:
In INS1 cells, palmitic acid is converted in the endoplasmic reticulum to solid tripalmitin (melting point >65 degrees C), which could induce endoplasmic reticulum stress proteins and signal apoptosis; lipid-induced apoptosis would therefore be a consequence of the physicochemical properties of these triglycerides. Since cellular triglycerides composed of single species of fatty acid are not likely to occur in vivo, destruction of beta cells by saturated fatty acids could be predominantly an in vitro scenario.
Insights
Saturated fatty acids like palmitic acid induce beta cell death in vitro by forming solid triglycerides in the endoplasmic reticulum. This lipotoxicity, driven by physicochemical properties, may be an artifact of in vitro conditions rather than a factor in obesity-related diabetes.
Area of Science:
- Cell biology
- Endocrinology
- Metabolic disease research
Background:
- Long-term exposure to lipids, especially saturated fatty acids, causes beta cell dysfunction and apoptosis (lipotoxicity).
- This lipotoxicity may contribute to obesity-related diabetes.
- Understanding the relationship between cell death and intracellular lipid accumulation is crucial.
Purpose of the Study:
- To investigate the link between cell death and intracellular triglyceride concentration, composition, and localization.
- To examine the effects of saturated and unsaturated non-esterified fatty acids (NEFA) on INS1 cells under varying glucose conditions.
Main Methods:
- INS1 cells were cultured with palmitic, oleic, or linoleic acids at different glucose concentrations.
- Intracellular lipids were analyzed using gas chromatography and microscopy.
- Cell death was quantified using microscopy and MTT assays; insulin secretion was measured by ELISA.
Main Results:
- Palmitic acid induced significant cytotoxicity and apoptosis, forming solid tripalmitin triglycerides within the endoplasmic reticulum.
- Unsaturated fatty acids reduced cytotoxicity and morphological disruption.
- All NEFA inhibited glucose-stimulated insulin secretion, with triglyceride content increasing up to 10-fold.
Conclusions:
- The physicochemical properties of tripalmitin, formed from palmitic acid in the endoplasmic reticulum, likely trigger apoptosis.
- Lipid-induced beta cell apoptosis may be primarily an in vitro phenomenon due to the formation of single-species triglycerides.
- These findings suggest that saturated fatty acid-induced beta cell destruction might be an artifact of in vitro experimental conditions.
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