NOMBRE Y FIRMA DEL REPRESENTANTE LEGAL
CONTRATO DE PRESTACIÓN DE SERVICIOS QUE CELEBRAN POR UNA PARTE LA UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO, A LA QUE EN LO SUCESIVO SE LE DENOMINARÁ “LA UNAM”,
III. DECLARAN AMBAS PARTES:
III. 1 Que leídas las anteriores declaraciones, reconocen mutuamente la personalidad que ostentan y acreditan estar de acuerdo con obligarse, conviniendo en celebrar el presente Contrato al tenor de
2. Que no podrá ser cancelada sin la conformidad previa y por escrito de “LA UNAM”, y
TXNIP was first shown to play a critical role in metabolic control when a mutant mouse strain, HcB-‐‑19/Dem (HcB-‐‑19) with a nonsense mutation in the TXNIP gene causing a lack of TXNIP expression, resulted in features of familial combined
hyperlipidemia (FCHL) including hypertriglyceridemia, hypercholesterolemia, elevated plasma apolipoprotein B and increased secretion of triglyceride-‐‑rich lipoproteins (Bodnar et al., 2002). It was later shown that no association was found between FCHL families and the TXNIP gene (Coon et al., 2004), but the impact of the deficiency of TXNIP on lipid metabolism was clear. In addition to hypertriglyceridemia, HcB-‐‑19 mice in the prolonged fasted state (≥18h) have elevated plasma free fatty acids, ketones and lactate with lower pyruvate levels in comparison to wild type controls (Bodnar et al., 2002; Donnelly et al., 2004; Hui et al., 2004; Sheth et al., 2005). Moreover, decreased TCA cycle flux as measured by CO2 production from isolated liver slices was significantly
decreased suggesting mitochondrial dysfunction with TXNIP deficiency (Bodnar et al., 2002).
Subsequently, mice generated with a targeted inactivation of TXNIP (TBP-‐‑2-‐‑/-‐‑)
displayed a similar phenotype with even more pronounced abnormalities, such as liver steatosis, severe gastrointestinal bleeding and hepatic-‐‑renal dysfunction after 24h fasting, and a predisposition to death under extreme fasting conditions ≥48h (Oka et al., 2006a). The phenotypic presentation in both of these mouse models of TXNIP deficiency points toward impairment in liver fatty acid flux through the TCA cycle, possibly sparing those fatty acids for incorporation into triglycerides and ketones (Bodnar et al., 2002; Donnelly et al., 2004; Oka et al., 2006a; Sheth et al., 2005).
In addition to its function in lipid metabolism, TXNIP is a key regulator in glucose homeostasis. TXNIP is transcriptionally upregulated by glucose (Stoltzman et al., 2008) and its mRNA expression is elevated in muscle of insulin resistant and diabetic humans (Parikh et al., 2007). Moreover, silencing TXNIP expression in human adipocytes and skeletal muscle myocytes enhances glucose uptake, whereas TXNIP overexpression inhibits glucose uptake. Under prolonged fasted conditions, both the HcB-‐‑19 and TBP-‐‑2-‐‑/-‐‑ mouse models of TXNIP deficiency display higher insulin with
lower blood glucose levels in comparison to wild type controls (Bodnar et al., 2002; Donnelly et al., 2004; Hui et al., 2004; Sheth et al., 2005), which was shown to be due to a defect in hepatocyte glucose production (Chutkow et al., 2008) and increased insulin secretion and sensitivity (Hui et al., 2004).
A third and fourth model of TXNIP ablation generated through a Cre-‐‑loxP-‐‑ mediated gene recombination (TKO) and targeted gene deletion (TXNIP-‐‑null), respectively, share similar phenotypes to the previous two models (HcB-‐‑19 and TBP-‐‑2-‐‑/-‐‑)
with the exception of normal insulin levels (Chutkow et al., 2010; Chutkow et al., 2008; Hui et al., 2008). The reason for the varied fasting insulin levels between the models is unknown, but Chutkow et al. (2008) suggest that it could be caused by experimental conditions for fasting and susceptibility or modifier gene effects resulting from strain differences. Nonetheless, in all models in which TXNIP is disrupted, there is consistent enhancement of glucose tolerance, insulin sensitivity and augmented glucose transport
in some peripheral tissues (Chutkow et al., 2010; Chutkow et al., 2008; Hui et al., 2008; Oka et al., 2006a). For instance, TXNIP null mice challenged on a 4-‐‑week high fat diet were insulin sensitive and demonstrated augmented glucose uptake into skeletal muscle and white adipose tissue by 30 and 40%, respectively, relative to WT controls (Chutkow et al., 2010). These conditions occured in spite of a 2-‐‑fold increase in adiposity due to hyperphagia. TXNIP deficiency similarly improved glucose intolerance and insulin resistance in the skeletal muscle of diabetic ob/ob mice crossed with TBP-‐‑2-‐‑/-‐‑ mice
(ob/ob•TBP-‐‑2-‐‑/-‐‑), without amelioration of obesity or obesity-‐‑induced adipocytokines
(Yoshihara et al., 2010).
Interestingly, the peripheral tissue phenotypes as a result of TXNIP ablation vary. TKO (total body knock-‐‑out) mice created by Hui et al. (2008) exhibited increased insulin signaling, insulin sensitivity and glycolysis in oxidative tissues (skeletal muDeBalsi.Thesis.9.5.13scle and hearts), but not in lipogenic tissues (adipose and liver). This is in contrast to the enhanced glucose uptake reported by Chutkow et al. (2010) in both skeletal muscle and liver of TXNIP null mice, which may be due to strain differences or specific aspects of the gene-‐‑targeting approaches. Also in the TKO mice, oxidative tissues displayed impaired mitochondrial glucose and fatty acid oxidation and were predicted to have disrupted mitochondrial respiration, while no comparable studies were conducted with the TXNIP null mice of Chutkow et al (2010). Conversely, in other studies mitochondrial fatty acid oxidation was not impaired in the hearts of
HcB-‐‑19 mice, although the TXNIP-‐‑deficient hearts preferentially used fatty acids as a substrate over glucose as measured by percentage of acetyl-‐‑CoA originated from fats or glucose for the TCA cycle (Sheth et al., 2005). More interestingly, in this same study fatty acid oxidation in skeletal muscle was dramatically increased by 35% in the TXNIP deficient mice as compared to controls, but only in a prolonged fasted state.
In summary, although TXNIP is ubiquitously expressed in all tissues (Junn et al., 2000), existing data suggests that the precise metabolic function of TXNIP varies among tissues and possibly under different physiological conditions (i.e. extreme fasting/starvation). Further, genetic ablation of TXNIP appears to compromise mitochondrial oxidative function across multiple catabolic pathways and impair respiratory/ETC function in some peripheral tissues but systematic comparisons of mitochondrial function in disparate tissues under various physiological states have not been addressed. These studies will be described in chapter 3.