Ketohexokinase (KHK) is the first and rate-limiting enzyme of fructose metabolism. Expression of the two alternatively spliced KHK isoforms, KHK-A and KHK-C, is tissue-specific and KHK-C is predominantly expressed in liver, kidney and intestine and responsible for the fructose-catabolizing function. While KHK isoform choice has been linked to the development of disorders such as obesity, diabetes, cardiovascular disease and cancer, little is known about the regulation of total KHK expression. In the present study, we investigated how hypoxic signaling influences fructose metabolism in the liver. Hypoxia or von Hippel-Lindau (VHL) tumor suppressor loss leads to the stabilization of hypoxia-inducible factors alpha (HIF-1α and HIF-2α) and the activation of their signaling to mediate adaptive responses. By studying liver-specific Vhl, Vhl/Hif1a, and Vhl/Epas1 knockout mice, we found that KHK expression is suppressed by HIF-2α (encoded by Epas1) but not by HIF-1α signaling on mRNA and protein levels. Reduced KHK levels were accompanied by downregulation of aldolase B (ALDOB) in the livers of Vhl and Vhl/Hif1a knockout mice, further indicating inhibited fructose metabolism. HIF-1α and HIF-2α have both overlapping and distinct target genes but are differentially regulated depending on the cell type and physiologic or pathologic conditions. HIF-2α activation augments peroxisome degradation in mammalian cells by pexophagy and thereby changes lipid composition reminiscent of peroxisomal disorders. We further demonstrated that fructose metabolism is negatively regulated by peroxisome-deficiency in a Pex2 knockout Zellweger mouse model, which lacks functional peroxisomes and is characterized by widespread metabolic dysfunction. Repression of fructolytic genes in Pex2 knockout mice appeared to be independent of PPARα signaling and nutritional status. Interestingly, our results demonstrate that both HIF-2α and peroxisome-deficiency result in downregulation of Khk independent of splicing as both isoforms, Khka as well as Khkc, are significantly downregulated. Hence, our study offers new and unexpected insights into the general regulation of KHK, and therefore fructolysis. We revealed a novel regulatory function of HIF-2α, suggesting that HIF-1α and HIF-2α have tissue-specific opposing roles in the regulation of Khk expression, isoform choice and fructolysis. In addition, we discovered a previously unknown function of peroxisomes in the regulation of fructose metabolism.

酮己糖激酶 (KHK) 是果糖代谢的第一个限速酶。两种选择性剪接的 KHK 亚型 KHK-A 和 KHK-C 的表达具有组织特异性，KHK-C 主要在肝脏、肾脏和肠道中表达，负责果糖分解代谢功能。虽然 KHK 异构体的选择与肥胖、糖尿病、心血管疾病和癌症等疾病的发生有关，但人们对总 KHK 表达的调节知之甚少。在本研究中，我们研究了缺氧信号如何影响肝脏中的果糖代谢。缺氧或 von Hippel-Lindau (VHL) 肿瘤抑制因子丧失会导致缺氧诱导因子 α（HIF-1α 和 HIF-2α）稳定并激活其信号传导以介导适应性反应。通过研究肝脏特异性 Vhl、Vhl/Hif1a 和 Vhl/Epas1 敲除小鼠，我们发现 KHK 表达被 HIF-2α（由 Epas1 编码）抑制，但在 mRNA 和蛋白质水平上不受 HIF-1α 信号传导抑制。 KHK 水平降低伴随着 Vhl 和 Vhl/Hif1a 敲除小鼠肝脏中醛缩酶 B (ALDOB) 的下调，进一步表明果糖代谢受到抑制。 HIF-1α 和 HIF-2α 具有重叠和不同的靶基因，但根据细胞类型和生理或病理条件而受到不同的调节。 HIF-2α 激活通过 pexophagy 增强哺乳动物细胞中的过氧化物酶体降解，从而改变脂质组成，让人联想到过氧化物酶体疾病。我们进一步证明，在 Pex2 敲除 Zellweger 小鼠模型中，果糖代谢受到过氧化物酶体缺陷的负向调节，该模型缺乏功能性过氧化物酶体，并具有广泛的代谢功能障碍。 Pex2 敲除小鼠中果糖分解基因的抑制似乎与 PPARα 信号传导和营养状况无关。有趣的是，我们的结果表明，HIF-2α 和过氧化物酶体缺陷都会导致 Khk 的下调，而与剪接无关，因为两种亚型 Khka 和 Khkc 均显着下调。因此，我们的研究为 KHK 的一般调节以及果糖分解提供了新的、意想不到的见解。我们揭示了 HIF-2α 的一种新的调节功能，表明 HIF-1α 和 HIF-2α 在 Khk 表达、异构体选择和果糖分解的调节中具有组织特异性的相反作用。此外，我们还发现了过氧化物酶体在果糖代谢调节中以前未知的功能。