A large number of physiological processes in the adult liver are regulated by nuclear receptors that require heterodimerization with retinoid X receptors (RXRs). In this study, we have used cre-mediated recombination to disrupt the mouse RXR␣ gene specifically in hepatocytes. Although such mice are viable, molecular and biochemical parameters indicate that every one of the examined metabolic pathways in the liver (mediated by RXR heterodimerization with PPAR␣, CAR␤, PXR, LXR, and FXR) is compromised in the absence of RXR␣. These data demonstrate the presence of a complex circuitry in which RXR␣ is integrated into a number of diverse physiological pathways as a common regulatory component of cholesterol, fatty acid, bile acid, steroid, and xenobiotic metabolism and homeostasis.Members of the nuclear receptor family regulate a broad range of developmental and physiological processes by binding to DNA response elements and regulating transcriptional activation (7). The retinoid X receptors (RXRs) are unique among the nuclear receptors in that they bind DNA as a homodimer and are required as a heterodimeric partner for a number of additional nuclear receptors to bind DNA (19). The latter receptors, termed the class II nuclear receptor subfamily, include many which are established or implicated as important regulators of gene expression in the liver (reviewed below). There are three RXR genes (18), coding for RXR␣, -␤, and -␥, all of which are able to heterodimerize with any of the class II receptors, although there appear to be preferences for distinct RXR subtypes by partner receptors in vivo (6).The physiological processes that are regulated by the class II receptors in the liver are diverse. LXR␣ is activated by oxycholesterol and promotes cholesterol metabolism (22). FXR (also known as RIP14) is part of an interrelated process, in that FXR is activated by bile acids (the end product of cholesterol metabolism) (17,21,34), which serve to inhibit cholesterol catabolism. CAR␤ is involved in phenobarbital induction of the cytochrome P450 2B10 (Cyp2B10) gene (9), which encodes a drug-and xenobiotic-metabolic enzyme. Interestingly, CAR␤ has constitutively activity, but becomes inactive in response to the steroid androstane (8). PXR is activated by a wide spectrum of steroids, and induces the Cyp3A gene (Cyp3A1 in mice; Cyp3A4 in humans) which encodes a broad-spectrum oxidase that is responsible for steroid degradation and for the catabolism of numerous pharmaceutical compounds (2, 11). PPAR␣ is activated by leukotriene B4 and by synthetic peroxisome proliferators, such as fibrates, and controls the expression of several genes which are related to fatty acid metabolism and processing (28). Although not discussed further in the context of this study, other class II nuclear receptors, including the retinoic acid and thyroid hormone receptors, are also likely to be important regulators of liver metabolism.In the adult liver, RXR␣ is the most abundant of the three RXRs (18), suggesting that it might have a prominent role in...

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Low fat dry fermented sausages were manufactured using controlled ripening conditions and a slow fermented process. The effect of fat content and ripening time on the chemical, colour, texture parameters and sensory acceptability was studied. The fat reduction in slow fermented sausages produced an increase in the pH decline during the first stage of the process that was favoured by the higher water content of the low fat sausages. Fat reduction did not affect the external appearance and there was an absence of defects but lower fat content resulted in lower sausage lightness. The sausage texture in low fat sausages caused an increase in chewiness and at longer ripening times, an increase in hardness. The sensory acceptability of the fermented sausages analyzed by internal preference mapping depended on the different preference patterns of consumers. A group of consumers preferred sausages with high and medium fat content and high ripening time. The second group of consumers preferred sausages with low ripening time regardless of fat content except for the appearance, for which these consumers preferred sausages of high ripening time. Finally, the limit to produce high acceptability low fat fermented sausages was 16% fat content in the raw mixture that is half the usual content of dry fermented sausages.

Dry fermented sausages with different fat contents were produced (10%, 20% and 30%). The effect of fat content and ripening time on sensory characteristics, lipolysis, lipid oxidation and volatile compounds generation was studied. Also, the key aroma components were identified using gas chromatography (GC) and olfactometry. High fat sausages showed the highest lipolysis and lipid oxidation, determined by free fatty acid content and thiobarbituric acid reactive substances (TBARS), respectively. A total of 95 volatile compounds were identified using SPME, GC and mass spectrometry (MS). Fat reduction decreased the generation of lipid derived volatile compounds during processing while those generated from bacterial metabolism increased, although only at the first stages of processing. The consumers preference in aroma and overall quality of high and medium fat sausages was related to the aroma compounds hexanal, 2-nonenal, 2,4-nonadienal, ethyl butanoate and 1-octen-3-ol which contributed green, medicinal, tallowy, fruity and mushroom notes.