| Zinc is an essential trace element required for animals and human being, and plays an important role in physiological and nutritional functions in vivo. Several clinical symptoms of zinc deficiency are similar to those of essential fatty acid (EFA) deficiency; moreover, zinc deficiency has been shown to accentuate the symptoms of EFA deficiency, suggesting a role of zinc in fatty acid metabolism. However, the molecular mechanism has been still undefined until now. The present study was conducted to investigate the effect of dietary zinc deficiency and zinc overdose on the growth performance of SD rats and hepatic gene expression profiles, demonstrating that the expression levels of stearoyl-CoA desaturase-1 (SCD-1) and its transcription regulation factors were altered by dietary zinc status. And then, short-termed injection with zinc was carried out to investigate the effects of zinc on the levels of serum lipoproteins, triglyceride, total cholesterol, plasma hormones, fatty acid composition of liver and pasma, gene expression of SCD-1 and its regulation factors. Finally, primary hepatocyte culture was employed to display the regulation mechanism of zinc on SCD-1 expression. The main results were as follows:1. Effect of dietary zinc on growth performance and zinc levels in ratOne hundred male Sprague-Dawley rats, with a mean body weight of 90±5 g, were randomly divided into four groups after a 3-day period of acclimation. The first group was fed a zinc-deficient diet (ZD,3.20 mg/kg), the second group was pair-fed zinc-adequate diet with food intake of ZD group, the third group was fed zinc-adequate diet (ZA,46.39 mg/kg) and the last group was fed zinc-overdose diet (ZO.234.39 mg/kg). Twenty five rats in each group were fed for 5 weeks in barrier system. Food intake was recorded daily and body weight weekly. The results showed that:①ZD rats exhibited typical symptoms such as lethargy and lower activity. poor appetite, sparse and coarse hair, alopecia, dermal lesions and leanness; whereas ZO rats maintained good phenotype and ingestive behavior similar to ZA rats. Compared with PF group, body and tissues weights, zinc levels and activity of alkaline phosphatase (ALP) were significantly reduced (P<0.05) by zinc deficiency, however these parameters were markedly increased (P<0.05) in ZO group relative to ZA group. 2. Change in hepatic gene expression profiles in response to zinc statusTo monitor changes in gene expression of liver due to zinc deficiency and zinc overdose, the gene expression profiling was analyzed using cDNA arrays. Regulated genes were clustered according to known function. mRNA levels of 68 genes were found to be altered by zinc deficiency, and 76 genes were changed by zinc overdose. The proteins encoded by changed genes were involved in nutrient metabolism, ion transport, DNA transcription and translation, immunity, signaling and so on. Thereof, the mRNA level of SCD-1 was significantly reduced by zinc deficiency, but was unaffected by zinc overdose. While mRNA levels of PPARa and TRp were reduced in ZD groups, but increased in ZO groups.3. Regulation of zinc on fatty acid composition and the transcription levels of SCD-1 and its regulation factorsThe adaptive response of SD rats to intraperitoneal injection with Zn was conducted using lithium chloride (LiCl) as positive control, to determine the proper dose of injected zinc. After two zinc models were established by feeding SD rats with Zn-deficient (3.30 mg/kg) and Zn-adequate (38.89 mg/kg) diets, two factors experimental design was used to investigate the effect of short-termed zinc injection (A factor, with four doses of 0,0.5,1.0 and 2.0μg/g) and Zn models (B factor, Zn deficiency and Zn adequacy) on serum lipoproteins, total cholesterol, glucose, plasma hormones, hepatic and plasma fatty acid composition, gene expressions of SCD-1 and its regulating factors. Meanwhile, correlation analyses among the indices were performed in the current research. The main results were as follows:3.1 Adaptive response to intraperitoneal injection with zincThe rat injected with 150μg/g LiCl exhibited significantly less consumption of 0.1% saccharin solution than that of purified water (P<0.05). However, there is no difference in consumption of saccharin solution and purified water of rats injected with 1.0 and 2.0μg/g zinc, but rats injected with 3.0μg/g zinc showed seriously uncomfortable response, even death. The findings indicated that the dose of 2.0μg/g zinc and below did not cause malaise.3.2 Zinc-deficient and zinc-adequate modelsRats from ZD group exhibited typical symptoms of zinc deficiency, while ZA rats revealed normal behaviors. Compared with ZA rats, zinc concentrations of serum, liver, femur and skeletal muscle in ZD rats were reduced by 27.43%,15.39%,31.15% and 27.30%(P<0.05), respectively, suggesting that both Zn-deficient and Zn-adequate models were established successfully.3.3 Effect of short-term injected zinc [A (0) group as control group in both models](1) Serum biochemical indices:Levels of HDL and TC in ZA model and Glu in ZD model were significantly increased (P<0.05) by injected zinc, while hepatic FFA levels were markedly reduced (P<0.05) in both models; the levels of LDL in ZA model and TG in ZD model were elevated (P<0.05) by the zinc injected with 2.0μg/g.(2) Plasma hormones:In ZA model, concentrations of IRI, T3 and PG were increased (P<0.05), but these of T4 and leptin were reduced (P<0.05) by injected zinc; concentrations of IRI, T3, T4 and leptin were enhanced (P<0.05), while PG level was decreased (P<0.05) in ZD model.(3) Hepatic fatty acid composition:In ZA model, contents of C16:0 and C18:1 and ratio of C18:1 to C18:0 were reduced (P<0.05), but content of C16:1 and ratio of C 16:1 to C16:0 were elevated (P<0.05) by injection with 0.5μg/g zinc; contents of C16:0, C16:1 and C18:0 and ratio of C 16:1 to C16:0 were increased (P<0.05), while content of C18:1 and ratio of C18:1 to C18:0 were decreased (P<0.05) in both A(1) and A(2) groups. In ZD model, contents of C 16:0, C16:1 and C18:0 were increased (P<0.05), but content of C18:1 and ratio of C 18:1 to C18:0 were decreased (P<0.05) in both A(0.5) and A(1) groups; contents of C18:1 and C18:0 and their ratio were also reduced (P<0.05) in A(2) group.(4) Plasma fatty acid composition:In ZA model, contents of C 16:0 and C18:0 and C18:1 were decreased (P<0.05), but ratio of C16:1 to C16:0 was enhanced (P<0.05) in A(0.5) group; contents of C16:0 and C18:0 were reduced (P<0.05), while values of C16:1/C16:0 and C18:1/C18:0 were increased (P<0.05) in both A(1) and A(2) groups. Additionally, content of C16:1 was also increased (P<0.05) in A(2) group. In ZD model, all parameters, except value of C18:1/C18:0, were reduced (P<0.05) in A(0.5) group; the compositions of fatty acid were decreased (P<0.05) by the zinc injected with 1.0μg/g, Otherwise than unchanged in content of C18:0; contents of C16:1, C18:1 and the values of C16:1/C16:0 and C18:1/C18:0 were decreased (P<0.05), while content of C 18:0 was increased (P<0.05) by zinc injected with 2.0μg/g.(5) mRNA levels of SCD-1 and its transcription regulation factors:In ZA model, three injection doses of zinc increased the mRNA levels of SCD-1 and SREBP-lc (P<0.05); both 1.0 and 2.0μg/g of injected zinc elevated the mRNA level of PPARa (P<0.05); mRNA levels of TRp were also increased by administrated with 0.5 and 2.0μg/g zinc (P<0.05), respectively. In ZD model, SCD-1 mRNA level was increased by the injection dose of 2.0μg/g zinc (P<0.05), and both levels of PPARαand SREBP-lc mRNA were enhanced (P<0.05) in A(2) and A(1) groups, but TRβmRNA level was reduced (P<0.05) by injection dose of 0.5μg/g zinc.3.4 Effect of zinc models [ZA model as the control group]Levels of LDL, HDL, TG, TC, hepatic FFA, IRI, T3 and T4 were reduced (P<0.05) in ZD model, and there were no significant alternations (P>0.05) in PG and leptin levels. Meanwhile, the content of C16:1 and value of C16:1/C16:0 in liver and contents of C 18:0 and C 18:1 in plasma were lower (P<0.05) in ZD model, but the content of C 18:1 in liver was elevated (P<0.05). Relative gene expressions of SCD-1 and TRp were reduced (P<0.05) in ZD rats.3.5 Correlation analysis(1) Hepatic fatty acid composition and biochemical indices:a significant positive correlation was observed between C18:0 and T3 (P<0.05); there were significant positive correlations between content of C16:1 and biochemical parameters such as Glu, LDL, HDL, TC, PG, T3, IRI and leptin (P<0.05); obvious positive correlations were found between the value of C16:1/C16:0 and Glu, LDL, HDL, TC, TG, PG, T3, IRI and leptin (P<0.05), respectively; there was significant positive correlection between C18:0 and HDL (P<0.05); marked negative correlations were observed between C18:1 and LDL, HDL, TC, PG and leptin (P<0.05), respectively; in addition, there were also remarkable negative correlations between value of C18:1/C18:0 and Glu, LDL, HDL, TC and IRI (P<0.05), respectively.(2) Hepatic fatty acid composition and mRNA levels of SCD-1 and its transcription regulation factors:SCD-1 mRNA level were positively correlated (P<0.05) with content of C16:1 and the value of C16:1/C16:0, respectively, but negatively (P<0.05) with the content of C18:1 and value of C18:1/C18:0; a significant relationship was not found between PPARa mRNA level and fatty acid composition (P>0.05); there were positive correlations between TRp mRNA level and the contents of C16:0 and C18.1 (P<0.05), respectively, but negative (P<0.05) between TRp mRNA level and the value of C16:1/C16:0; SREBP-1c mRNA level was negatively correlated (P<0.05) with the content of C18:1 and value of C18:1/C18:0, respectively, but was positively (P<0.05) with the value of C16:1/C16:0.(3) Plasma fatty acid composition and biochemical indices:The positive relationship were TG vs value of C18:1/C 18:0, T3 vs C 18:0, T4 vs C18:0, leptin vs C18:0 (P<0.05); an obvious negative correlation was observed between TG and C18:1, T3 and C16:1/C16:0, leptin and C18:1/C18:0, (P<0.05); T4 level was negatively correlated with C16:1, C16:1/C16:0 and C18:1/C18:0 (P<0.05), respectively; there was a significant negative relationship between IRI and C16:0, C18:0 and C16:1/C16:0 (P<0.05), respectively.(4) Plasma fatty acid composition and the mRNA levels of SCD-1 and its transcription regulation factors:The content of C16:1 positvely corrected with mRNA levels of SCD-1 and SREBP-1c (P<0.05); C18:1 was observed to negatively correlate with TRp (P<0.05); there were significant negative relationships between C18:1/C18:0 and SCD-1 and SREBP-1c, respectively.(5) Hormone and the mRNA levels of SCD-1 and its transcription regulation factors:SCD-1 mRNA level was positively correlated (P<0.05) with PG, IRI, T3 and T4, respectively; both PG and T3 were positively correlated (P<0.05) with TRβmRNA level; SREBP-1c mRNA level was just positively correlated (P<0.05) with IRI level.4. Effects of zinc on the transcription levels of SCD-1 and its transcription regulation factors in primary hepatocyteWhen primary hepatocyte from zinc-deficient rats was maintained in the medium containing 400μM DTPA for 24 hours, SREBP-1c mRNA level was significantly reduced (P<0.05), while gene expression of PPARa was markedly up-regulated (P<0.05); transcription levels of SCD-1, TRp, SREBP-1c and PPARa were significantly down-regulated (P<0.05) when 600μM DTPA. Primary hepatocyte from zinc-adequate rats was cultured in the mediums including three zinc levels respectively. After 24 hours, SREBP-lc mRNA levels were remarkably increased (P<0.05) by zinc administrations. Both transcripts of SCD-1 and TRβwere up-regulated (P<0.05) by zinc administrations except adding 100μM zinc. Thereof, SCD-1 mRNA level of hepatocyte administrated with 50μM zinc was highest among the groups. Correlation analysis showed that transcription level of SCD-1 was positively correlated with the expression levels of TRp and SREBP-1c (P<0.05).
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