Abstract
Whether dietary omega-3 (n-3) polyunsaturated fatty acid (PUFA) confers cardiac benefit in cardiometabolic disorders is unclear. We test whether dietary -linolenic acid (ALA) enhances myocardial resistance to ischemia-reperfusion (I-R) and responses to ischemic preconditioning (IPC) in type 2 diabetes (T2D); and involvement of conventional PUFA-dependent mechanisms (caveolins/cavins, kinase signaling, mitochondrial function, and inflammation). Eight-week male C57Bl/6 mice received streptozotocin (75 mg/kg) and 21 weeks high-fat/high-carbohydrate feeding. Half received ALA over six weeks. Responses to I-R/IPC were assessed in perfused hearts. Localization and expression of caveolins/cavins, protein kinase B (AKT), and glycogen synthase kinase-3 β (GSK3β); mitochondrial function; and inflammatory mediators were assessed. ALA reduced circulating leptin, without affecting body weight, glycemic dysfunction, or cholesterol. While I-R tolerance was unaltered, paradoxical injury with IPC was reversed to cardioprotection with ALA. However, post-ischemic apoptosis (nucleosome content) appeared unchanged. Benefit was not associated with shifts in localization or expression of caveolins/cavins, p-AKT, p-GSK3β, or mitochondrial function. Despite mixed inflammatory mediator changes, tumor necrosis factor-a (TNF-a) was markedly reduced. Data collectively reveal a novel impact of ALA on cardioprotective dysfunction in T2D mice, unrelated to caveolins/cavins, mitochondrial, or stress kinase modulation. Although evidence suggests inflammatory involvement, the basis of this “un-conventional” protection remains to be identified.
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Key Points
The hypothesis tested is that dietary ALA supplementation may be cardioprotective in T2D— including enhanced ischemic tolerance, survival signaling or responses to IPC —via shifts in conventional PUFA‐dependent mechanisms (caveolar makeup, kinase signaling, mitochondrial function, and inflammation) [46,60–62]. We employ a non‐genetic model of chronic T2D development (β‐cell stress + 21 weeks high‐fat/high carbohydrate (HFHC) feeding) in young adult mice, a model relevant to the pattern of T2D onset in humans. The focus here is on myocardial responses to I‐R and IPC, while assessing basal expression of caveolins 1 and 3 (known to influence ischemic tolerance and IPC together with cavins 1 and 4: more recently identified determinants of caveolar structure/function and stress‐resistance whose sensitivities to dietary lipids are unknown.
Six weeks ALA supplementation was of selective benefit in hearts of T2D mice, improving cardiac IPC without influencing intrinsic I‐R tolerance. This was associated with mixed changes in inflammatory mediators (including reduced Tnf). However, there were no changes in expression or sub‐cellular localization of caveolin and cavin proteins, despite reported effects of n‐3 PUFAs on raft microdomain formation, size, and structure. The benefit with ALA was not associated with changes in metabolic phenotype, baseline mitochondrial function, or baseline expression of p‐ AKT or p‐GSK3β.
The present study indicates that six weeks of moderate dietary ALA supplementation (10% of dietary fat calories) does not improve myocardial I‐R tolerance in chronic T2D, yet counters detrimental effects of IPC and restores functional protection. This selective benefit arises in the absence of changes in systemic metabolic phenotype, myocardial expression, and sub‐cellular localization of caveolins and cavins, and baseline mitochondrial function and expression/phosphorylation of AKT or GSK3b. Thus, while dietary ALA might present a useful strategy for improving or restoring cardioprotection in T2D, the mechanistic basis of this benefit requires further investigation. Shifts in inflammatory and VEGF signaling identified here await further study, while modification of caveolar structure and biophysical properties remains a possible determinant.