## 4 Discussion  
This study represents the first evidence of the biological activity of Sk-BvPP and Sd-BvPP extracts on the metabolism of human SM cells. All GPP extracts tested whatever the tissue and ripening stage had the ability to reduce lipid oxidative stress and to increase the oxidative capacity of palmitate-treated human SM cells. These data shed light on the impact of the tissue of origin and ripening stage of grapes against early muscle alterations favoring IR and the potential benefits of GPPs in muscle metabolism. Interestingly, while the TPC and TAC were higher in Sd compared to Sk and in the Bv stage compared to the M stage, it emerged that all extracts had comparable effects in boosting SM antioxidant response and oxidative capacity. The only exception was represented by the Sd-BvPP extract that, despite decreasing lipid oxidative stress and improving the oxidative capacity of palmitate-treated human SM cells, did not improve muscle insulin sensitivity. However, this observation implied that the efficacy of these extracts on insulin response was probably attributable to a more complex mechanism at play. Besides the direct antioxidant scavenging properties of GPP, they may synergistically/antagonistically interact with endogenous molecules to exert their biological activities [34]. As mentioned above, oxidative stress plays a pivotal role in the pathophysiology of SM IR and obesity, triggered by an excess of glucose and free fatty acids in the bloodstream, leading to heightened mitochondrial requirements, ineffectiveness of antioxidant systems and disease progression [35]. Therefore, mitigating oxidative stress becomes a crucial objective. This study demonstrated the ability of GPP extracts, regardless of the tissue type or ripening stage, to decrease palmitate-induced lipid peroxidation in human SM cells. These ROS  
Figure 5: Treatment of human SM cells with GPP extracts concomitantly to palmitate treatment improves insulin response except for Sd-BvPP extract. Representative Western blot of P-Akt and Akt following insulin stimulation of human SM cells treated or not with palmitate and GPP extracts. P-Akt/Akt levels are expressed as fold induction relative to baseline level of P-Akt/Akt fold induction without palmitate treatment that was set at one. n = 3. Data are expressed as means \(\pm\) SEM. Statistical significance was determined via one-way ANOVA. * \(p\leq\) 0.05, ** \(p<\) 0.01 and *** \(p<\) 0.0001.  
by-products attack is closely associated with insulin sensitivity. In the condition of IR, an increase in lipid peroxidation is observed at systemic level [35], but also at an SM level [36]. On the contrary, any improvement in insulin sensitivity is accompanied by a significant reduction in 4-HNE which is similarly observed following supplementation with antioxidants or implementation of exercise [37]. Since 4-HNE can form covalent adducts with insulin [38] leading to the functional alteration of the Akt pathway [39], lowering 4-HNE levels improves insulin sensitivity. Accordingly, in our study, a reduction in SM cells 4-HNE associated with an increase in insulin sensitivity has been also found, although not of the same amplitude for all extracts. Actually, Sd-BvPP did not improve SM cells' insulin sensitivity, whereas a lower 4-HNE level was observed compared to palmitate-treated cells, suggesting a nonlinear relation between these two parameters. Overall, the reduction in 4-HNE levels, compared to control palmitate-treated cells for Sk-BvPP, Sk-MPP and Sd-MPP, was sufficient to improve insulin sensitivity, although reaching a slightly higher level than the control cells. Metabolization of 4-HNE is achieved through GSH and glutathione-S-transferases; hence, an increase in metabolization could protect from aldehyde toxicity and muscle IR [11]. Furthermore, data displayed the enhanced activation of the Nrf2 antioxidant pathway. Treatment with palmitate led to a high phosphorylation of p62 in human SM cells, although not followed, as expected, by induction of the antioxidant HO-1.