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  • Thonzonium Bromide The observation that PNPLA in

    2019-07-08

    The observation that PNPLA3, in addition to its potential role as LPAAT and TAG lipase, may also act as RE hydrolase was made after the generation of the knockout, transgenic and knock in mouse models. Thus, in any of these mouse models the mobilization of RE stores has not been addressed. Yet, two studies [63,112] using primary human hepatic stellate Thonzonium Bromide and the human hepatic stellate cell line LX-2 demonstrated that expression of wild-type PNPLA3 but not of the I148M mutant PNPLA3 affects RE homeostasis in HSCs, concluding that the I148M variant represents a loss-of-function mutation. In line with such an assumption, humans carrying the I148M PNPLA3 mutation exhibit decreased plasma retinol levels and increased hepatic RE content [118,119].
    Targeting Ces3/TGH VLDLs are fundamental for the distribution of endogenous TAG from the liver to peripheral tissues. In liver, VLDLs are synthesized in hepatocytes and are stuffed with neutral lipids, a large portion being TAGs [120]. Storages sites of TAGs molecules are LDs, which may reside in the cytosol, in close proximity to the ER membranes or in the lumen of the ER [121]. Interestingly, expression of ATGL and in particular of HSL, the classical lipases of TAG catabolism, is very low in liver [45,55,122]. Expression of HSL is higher in the parenchymal than non-parenchymal cell fractions [55]. Furthermore, overexpression or ablation of HSL or ATGL in the liver does not affect hepatic VLDL secretion [69,123], suggesting that other lipases, which may reside in the lumen of the ER, are crucial for VLDL synthesis. One of the best-investigated luminal carboxylesterases in murine liver is Ces3/TGH (the human homologue is annotated as CES1 and is also termed neutral cholesteryl ester hydrolase, NCEH or CEH). Ces3 was initially isolated from porcine liver microsomes [124]. It was named TGH because of its in vitro lipase activity against medium- and long-chain TAG [124], although Ces3 also hydrolyzes CEs [125]. In the mouse, Ces3 is highly expressed in the liver and to a lesser extent in adipose tissue, heart, kidney, and small intestine [126]. Already initial studies [127] in the rat hepatoma cell line McA-RH7777 stably expressing rat Ces3 revealed that Ces3 promotes VLDL secretion Thonzonium Bromide while glycerol release was not changed. This was indicating that Ces3 rather plays a role in mobilizing hepatic TAG stores via VLDL secretion than via lipolysis, i.e. the catabolism of TAG from cytosolic LDs. This prominent role of Ces3 in VLDL secretion was further corroborated by inhibitor studies, where treatment of rat primary hepatocytes with a Ces3 specific inhibitor almost completely blunted apolipoprotein B-100 and TAG secretion into the supernatant [128]. Similar to cell experiments also the inducible transgenic expression of Ces3 in mice led to increased plasma TAG and apolipoprotein B-100 levels upon fasting [129]. Conversely, ablation of Ces3 in mice resulted in decreased plasma TAG and apolipoprotein B-100 and FFA levels in both fasted and fed state [130]. Authors argued that the decreased plasma FA levels in Ces3 knockout mice may be a result of decreased lipolysis in the adipose tissue, since Ces3 is also expressed in adipose tissue and participates in adipose tissue basal lipolysis [[130], [131], [132]]. However, it was unexpected that despite decreased VLDL secretion of Ces3 knockout mice, their livers were not steatotic but hepatic TAG content was even reduced [130]. Furthermore, Ces3 knockout mice exhibited increased respiratory quotient and increased glucose tolerance and insulin sensitivity, indicative for elevated glucose utilization because of decreased FA availability [130]. To address the specific role of Ces3 in hepatic TAG and CE homeostasis, two liver specific Ces3 knockout mouse models, one on wild-type background and the other on low-density lipoprotein receptor (Ldlr) knockout background, respectively, were generated [56,58]. Similarly, as observed in the global Ces3 knockout mouse model, also the liver specific Ces3 knockout mouse model on wild-type background exhibited decreased VLDL secretion [58]. Interestingly, in this same study it was found that male (but not female) mice exhibit increased hepatic TAG and CE contents, suggesting that TAG and CE may be the physiological substrates of Ces3. This increase in hepatic TAG and CE contents was accompanied by increased number of smaller cytosolic LDs [58], indicating that Ces3-deficiency counteracts FA supply for VLDL-TAG synthesis and thereby promotes the formation of nascent LDs in the cytosol. In contrast to global Ces3 knockout mice, liver specific Ces3 knockout mice showed unchanged plasma FA levels upon feeding or fasting as well as unaltered glucose tolerance and in females deteriorated insulin sensitivity [58]. Since decreased plasma FA levels as well as improved glucose tolerance and insulin sensitivity were solely observed in global Ces3 knockout mice [130], it is conceivable that reduced adipose TAG catabolism and consequently lower FA release provokes increased glucose utilization and not impaired VLDL secretion. The second mouse model, Ces3-deficiency on a Ldlr knockout background, showed unaltered hepatic TAG as well as CE contents, increased atherosclerotic lesions, and decreased fecal sterol content, indicating that hepatic Ces3 activity is required for efficient sterol elimination [56].