Objective

Investigations of autophagy in β-cells have usually focused on its homeostatic function. More dynamic roles in inhibiting glucose-stimulated insulin secretion (GSIS), potentially involving remodelling of cellular lipids, have been suggested from in vitro studies but not evaluated in vivo.

Methods

We employed temporally-regulated deletion of the essential autophagy gene, Atg7, in β-cells. Mice were fed chow or high-fat diets (HFD), in conjunction with deletion of Atg7 for the last 3 weeks (short-term model) or 9 weeks (long-term model). Standard in vivo metabolic phenotyping was undertaken, and 450 lipid species in islets quantified ex vivo using mass spectroscopy (MS). MIN6 cells were also employed for lipidomics and secretory interventions.

Results

β-cell function was impaired by inhibiting autophagy in the longer-term, but conversely improved by 3-week deletion of Atg7, specifically under HFD conditions. This was accompanied by augmented GSIS ex vivo. Surprisingly, the HFD had minimal effect on sphingolipid and neutral lipid species, but modulated >100 phospholipids and ether lipids, and markedly shifted the profile of polyunsaturated fatty acid (PUFA) sidechains from n3 to n6 forms. These changes were partially countered by Atg7 deletion, consistent with an accompanying upregulation of the PUFA elongase enzyme, Elovl5. Loss of Atg7 separately augmented plasmalogens and alkyl lipids, in association with increased expression of Lonp2, a peroxisomal chaperone/protease that facilitates maturation of ether lipid synthetic enzymes. Depletion of PUFAs and ether lipids was also observed in MIN6 cells chronically exposed to oleate (more so than palmitate). GSIS was inhibited by knocking down Dhrs7b, which encodes an enzyme of peroxisomal ether lipid synthesis. Conversely, impaired GSIS due to oleate pre-treatment was selectively reverted by Dhrs7b overexpression.

Conclusions

A detrimental increase in n6:n3 PUFA ratios in ether lipids and phospholipids is revealed as a major response of β-cells to high-fat feeding. This is partially reversed by short-term inhibition of autophagy, which results in compensatory changes in peroxisomal lipid metabolism. The short-term phenotype is linked to improved GSIS, in contrast to the impairment seen with the longer-term inhibition of autophagy. The balance between these positive and negative inputs could help determine whether β-cells adapt or fail in response to obesity.

中文翻译：

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自噬的短期抑制作用通过增加醚脂质和过氧化物酶体功能以及抵抗脂肪喂养后n-3多不饱和脂肪酸的消耗而有益于胰腺β细胞。

目的

β细胞自噬的研究通常集中在其稳态功能上。体外研究表明，在抑制葡萄糖刺激的胰岛素分泌（GSIS）中可能涉及细胞脂质的重塑中具有更多的动态作用，但尚未在体内进行评估。

方法

我们在β细胞中采用了必需自噬基因Atg7的时间调控缺失。在过去的3周（短期模型）或9周（长期模型）中，给小鼠喂以低脂饮食或高脂饮食（HFD），同时缺失Atg7。进行标准的体内代谢表型分析，并使用质谱（MS）对离体中的450种脂质进行定量。MIN6细胞也用于脂质组学和分泌干预。

结果

长期抑制自噬会损害β细胞功能，但相反，通过3周删除Atg7（特别是在HFD条件下）会改善β细胞的功能。这伴随着增强的GSIS离体。令人惊讶的是，HFD对鞘脂和中性脂质种类的影响最小，但调节了超过100种的磷脂和醚脂质，并将多不饱和脂肪酸（PUFA）侧链的轮廓从n3变为n6。这些变化被Atg7缺失部分抵消，这与随之而来的PUFA延长酶Elov15的上调一致。Atg7的丢失分别增加了血浆缩醛磷脂和烷基脂质，并与过氧化物酶体伴侣/蛋白酶Lonp2的表达增加有关，后者有助于醚脂质合成酶的成熟。在长期暴露于油酸盐（多于棕榈酸盐）的MIN6细胞中也观察到PUFA和醚脂质的消耗。通过敲除编码过氧化物酶体醚脂质合成酶的Dhrs7b抑制了GSIS。相反，由于油酸预处理而导致的GSIS受损可通过Dhrs7b过表达选择性地恢复。

结论

揭示了醚脂质和磷脂中n6：n3 PUFA比的有害增加是β细胞对高脂喂养的主要反应。短期抑制自噬可部分逆转这种情况，这会导致过氧化物酶体脂质代谢的代偿性变化。短期表型与GSIS的改善有关，而长期吞噬自噬会导致损害。这些积极和消极的投入之间的平衡可以帮助确定β细胞是适应肥胖还是失败。