Background

Obesity is a major risk factor for the development of type 2 diabetes mellitus (T2DM), with chronic high-fat intake promoting systemic insulin resistance and inflammation. The brain regulates key metabolic processes, and emerging evidence suggests that brain insulin resistance may influence eating behaviours and energy homeostasis, contributing to the pathophysiology of obesity and T2DM.

Aims

This study aims to explore the impact of palmitic acid exposure on insulin sensitivity and cellular stress responses in SH-SY5Y cells.  

Methodology

SH-SY5Y cells were treated with palmitate at physiologically relevant concentrations (100 -– 500 µM) to model high-fat dietary conditions. The impact of palmitate treatment on cell viability and metabolic activity was determined through Trypan Blue and MTT assays. Changes in insulin sensitivity were assessed by quantifying levels of phosphorylated Akt (p-Akt) via Western blot analysis. Markers of inflammation (IL-1α, IL-8), oxidative stress (GPX1, CAT) and palmitoylation enzymes (ZDHHC1 and ZDHHC12) were also evaluated using quantitative PCR.

Results

Cell viability using Trypan Blue assay revealed no increase in cell death following palmitate exposure after 24 and 48 hours (n = 2). Significant reductions in metabolic activity were observed after 24, 48, and 72 hours palmitate treatment via MTT assay. At 24 hours, there was a significant reduction in metabolic activity at 100 µM (48.5% ± 6.4%), 200 µM (49% ± 2.9%), 300 µM (48.5% ± 6.13%), 400 µM (47% ± 9.2%) and 500 µM (45.5% ± 11%) (n = 4, p < 0.0001 vs vehicle control for all concentrations). Similar trends were observed at 48- and 72 -hours palmitate treatment. Insulin-stimulated Akt phosphorylation was significantly reduced following palmitate treatment by 43% (±0.05%) at 200 µM and 45% (±0.07%) at 300 µM (n = 3, p < 0.001 vs. vehicle control). Additionally, 200 µM palmitic acid upregulated the pro-inflammatory cytokine IL-8 six-fold (645% ± 2.63%, n = 4, p < 0.01) and the ER stress marker DDIT3 four-fold (425% ± 2.63%, n = 4, p < 0.01), while also increasing the oxidative stress marker SOD1 two-fold (199% ± 0.74%, n = 4, p < 0.05). ZDHHC12, a palmitoylation enzyme involved in regulation of NLRP3, was reduced by 41% (± 0.2%, n = 4, p < 0.05), potentially leading to the disinhibition of NLRP3 inflammasome.

Conclusions

These findings demonstrate that palmitic acid impairs neuronal insulin signalling, reduces metabolic activity and induces cellular stress in SH-SY5Y cells. Our findings suggest that saturated fatty acids alone may disrupt key neuronal pathways involved in metabolic regulation. Ongoing research is focused on exploring the potential protective effects of unsaturated and polyunsaturated fatty acids, and their ability to counteract the palmitic acid induced impairment of insulin signalling and cellular stress markers in the brain.