Hepatic transcript profiling in early lactating dairy cows fed rumen-protected niacin during the transition from late pregnancy to lactation. Hepatic transcript profiling in early lactating dairy cows fed rumen-protected niacin during the transition from late pregnancy to lactation

In dairy cows, administration of high dosages of niacin (NA) was found to cause anti-lipolytic effects, which are mediated by the NA receptor hydroxyl-carboxylic acid receptor 2 (HCAR2) in white adipose tissue (WAT), and thereby to an altered hepatic lipid metabolism. However, almost no attention has been paid to possible direct effects of NA in cattle liver, despite showing that HCAR2 is expressed also in the liver of cattle and is even more abundant than in WAT. Due to this, we hypothesized that feeding of rumen-protected NA to dairy cows influences critical metabolic and/or signaling pathways in the liver through inducing changes in the hepatic transcriptome. In order to identify these pathways, we applied genome-wide transcript profiling in liver biopsies obtained at 1 wk postpartum (p.p.) from dairy cows of a recent study (Zeitz et al., 2018) which were fed a total mixed ration without (control group) or with rumen-protected NA from 21 d before calving until 3 wk p.p. Hepatic transcript profiling revealed that a total of 487 transcripts were differentially expressed [filter criteria fold change (FC) > 1.2 or FC < -1.2 and P < 0.05] in the liver at 1 wk p.p. between cows fed NA and control cows. Substantially more transcripts were down-regulated (n = 338), while only 149 transcripts were up-regulated by NA in the liver of cows. Gene set enrichment analysis (GSEA) for the up-regulated transcripts revealed that the most enriched gene ontology (GO) biological process terms were exclusively related to immune processes, such as leukocyte differentiation, immune system process, leukocyte differentiation, activation of immune response and acute inflammatory response. In line with this, the plasma concentration of the acute phase protein haptoglobin tended to be increased in dairy cows fed rumen-protected NA compared to control cows (P < 0.1). GSEA of the down-regulated transcripts showed that the most enriched biological process terms were related to metabolic processes, such as cellular metabolic process, small molecule metabolic process, lipid catabolic process, organic cyclic compound metabolic process, small molecule biosynthetic process and cellular lipid catabolic process. In conclusion, hepatic transcriptome analysis shows that rumen-protected NA induces genes which are involved mainly in immune processes including acute phase response and stress response in dairy cows at wk 1 p.p. These findings indicate that supplementation of rumen-protected NA to dairy cows in the periparturient period may induce or amplify the systemic inflammation-like condition which is typically observed in the liver of high-yielding dairy cows in the p.p. period. Overall design: For microarray analysis, n = 9 RNA samples each of the control group and the NA group were used for hybridization to the Affymetrix GeneChip Bovine Gene 1.0 ST array.

在奶牛中，高剂量烟酸（NA）给药已被证实可发挥抗脂解作用，该作用由白色脂肪组织（WAT）中的烟酸受体羟基羧酸受体2（HCAR2）所介导，进而改变肝脏脂质代谢。尽管已有研究证实奶牛肝脏中同样表达HCAR2，且其表达丰度甚至高于WAT，但目前几乎无人关注NA对牛肝脏可能存在的直接作用。
基于上述研究空白，本研究提出假设：给奶牛饲喂瘤胃保护性烟酸（rumen-protected NA），可通过改变肝脏转录组，调控肝脏内关键代谢及信号通路。为鉴定上述调控通路，本研究对一项近期研究（Zeitz等，2018）的奶牛肝脏活检样本开展全基因组转录组分析：该研究中的奶牛从产犊前21天至产后3周（p.p.），分别饲喂不含瘤胃保护性烟酸的全混合日粮（对照组）或含瘤胃保护性烟酸的全混合日粮，样本采集于产后1周。
肝脏转录组分析结果显示，产后1周时，饲喂烟酸组与对照组奶牛的肝脏中，共有487个转录本呈现差异表达[筛选标准：折叠变化（FC）>1.2或FC<-1.2且P<0.05]。其中下调转录本数量显著更多（n=338），而上调转录本仅149个。
对上调转录本的基因集富集分析（GSEA）显示，富集程度最高的基因本体（GO）生物学过程术语均与免疫过程相关，例如白细胞分化、免疫系统过程、白细胞分化、免疫应答激活及急性炎症反应。与此一致的是，与对照组相比，饲喂瘤胃保护性烟酸的奶牛血浆中急性期蛋白结合珠蛋白的浓度呈升高趋势（P<0.1）。
对下调转录本的基因集富集分析（GSEA）显示，富集程度最高的生物学过程术语与代谢过程相关，例如细胞代谢过程、小分子代谢过程、脂质分解代谢过程、有机环化合物代谢过程、小分子生物合成过程及细胞脂质分解代谢过程。
综上，肝脏转录组分析结果表明，产后1周时，瘤胃保护性烟酸可诱导奶牛肝脏中主要参与免疫过程（包括急性期应答与应激应答）的基因表达。上述研究结果提示，在围产期给奶牛补充瘤胃保护性烟酸，可能诱导或放大类似全身性炎症的状态，而该状态通常见于产后高产奶牛的肝脏中。
整体实验设计：本微阵列分析共使用对照组与烟酸组各9份RNA样本，均与Affymetrix GeneChip Bovine Gene 1.0 ST阵列进行杂交。