摘 要: 胰島素作為機(jī)體所必需的激素,在動(dòng)物糖脂代謝、蛋白合成、生長(zhǎng)發(fā)育、性狀形成等生理過程中發(fā)揮重要作用。雞存在天然的胰島素抵抗,但其發(fā)生機(jī)制尚不清楚。本文綜述了雞肝臟、肌肉和脂肪組織胰島素通路的研究進(jìn)展,提出了雞胰島素信號(hào)通路和胰島素抵抗的未來研究方向,以期為深入探究人類胰島素抵抗以及雞肉品質(zhì)遺傳改良提供新思路。
關(guān)鍵詞: 雞;胰島素信號(hào)通路;胰島素抵抗;肝臟;肌肉;脂肪組織
中圖分類號(hào): S831.2
文獻(xiàn)標(biāo)志碼:A
文章編號(hào):0366-6964(2024)08-3288-09
收稿日期:2024-01-31
基金項(xiàng)目:國(guó)家自然科學(xué)基金項(xiàng)目(U23A20230)
作者簡(jiǎn)介:婁 明(1998-),女,黑龍江佳木斯人,博士生,主要從事動(dòng)物遺傳育種與繁殖的研究,E-mail:lou_ming@neau.edu.cn
通信作者:王 寧,主要從事動(dòng)物遺傳育種與繁殖的研究,E-mail:wangning@neau.edu.cn
Advances in the Study of Chicken Insulin Signaling Pathway
LOU" Ming1,2,3, LUO" Haoyu1,2,3, MU" Fang1,2,3, LI" Hui1,2,3, WANG" Ning1,2,3*
(1.Key Laboratory of Chicken Genetics and Breeding of Ministry of Agriculture and Rural Affairs, Harbin
150030," China; 2.Key Laboratory of Animal Genetics, Breeding and Reproduction, Education Department
of Heilongjiang Province, Harbin 150030," China;3.College of Animal Science and Technology, Northeast Agricultural University, Harbin 150030," China)
Abstract:" Insulin is an essential hormone and plays crucial roles in a variety of biological processes including glucose and lipid metabolism, protein synthesis, growth, development, and trait formation. Chickens have natural insulin resistance, but the cause of chicken insulin resistance is still unknown. This review summarizes the research progress on the insulin signaling pathway in chicken liver, muscle and adipose tissue, and then proposes future research directions in chicken insulin signaling pathway and insulin resistance, with a view to providing new ideas for in-depth exploration of human insulin resistance and genetic improvement of chicken meat quality.
Key words: chicken; insulin signaling pathway; insulin resistance; liver; muscle; adipose tissue
*Corresponding author:WANG Ning, E-mail: wangning@neau.edu.cn
胰島素是一個(gè)多功能激素,它能降低機(jī)體血糖水平,促進(jìn)糖原、脂肪和蛋白質(zhì)合成,此外,它還具有促進(jìn)細(xì)胞生長(zhǎng)、增殖以及抑制細(xì)胞凋亡的作用[1]。胰島素在調(diào)控動(dòng)物生長(zhǎng)發(fā)育、繁殖以及健康等方面發(fā)揮重要作用。缺乏胰島素或發(fā)生胰島素抵抗會(huì)導(dǎo)致機(jī)體代謝異常并誘發(fā)多種疾病[1]。目前人類對(duì)哺乳動(dòng)物胰島素的功能和作用機(jī)制已有了非常深入的了解,并建立了胰島素抵抗及其相關(guān)代謝疾病的多種診斷和治療方法。
胰島素能夠促進(jìn)哺乳動(dòng)物細(xì)胞攝取、存儲(chǔ)和利用葡萄糖,降低血糖水平[2]。雞與哺乳動(dòng)物的血液胰島素濃度相近,但雞的血糖水平卻遠(yuǎn)高于哺乳動(dòng)物和其他脊椎動(dòng)物[3],提示雞存在著天然的胰島素抵抗。多位學(xué)者通過體內(nèi)注射外源胰島素或胰島素抗血清,比較分析了雞與哺乳動(dòng)物之間[4]以及高、低體重雞之間[5]胰島素靶器官和靶組織(肝臟、肌肉、脂肪等)的胰島素信號(hào)通路活性差異,發(fā)現(xiàn)雞與哺乳動(dòng)物的胰島素信號(hào)轉(zhuǎn)導(dǎo)通路并不完全相同。
1 哺乳動(dòng)物胰島素信號(hào)通路
胰島素由胰臟的胰島β細(xì)胞合成。胰島β細(xì)胞首先合成前胰島素原(preproinsulin),經(jīng)過內(nèi)質(zhì)網(wǎng)和高爾基體的加工后,成熟的胰島素以分泌顆粒的形式儲(chǔ)存在β細(xì)胞的胞質(zhì)中。當(dāng)β細(xì)胞受到刺激,細(xì)胞通過胞吐作用將胰島素釋放到毛細(xì)血管,最終通過血液循環(huán)作用于全身靶組織和靶器官[6]。其中,肝臟、肌肉和脂肪組織是胰島素的主要靶器官。在肝臟中,胰島素促進(jìn)糖原合成,增加脂肪生成并降低糖異生[7];在骨骼肌中,胰島素增加葡萄糖轉(zhuǎn)運(yùn)和糖原合成,促進(jìn)骨骼肌對(duì)葡萄糖的利用和儲(chǔ)存,同時(shí)促進(jìn)肌肉組織的蛋白質(zhì)合成[8];在脂肪組織中,胰島素促進(jìn)脂肪組織葡萄糖吸收、脂肪生成,抑制脂肪的分解[9]。
胰島素通過與靶細(xì)胞的胰島素受體(INSR)結(jié)合發(fā)揮作用。胰島素受體是由2個(gè)α亞基和2個(gè)β亞基組成的跨膜蛋白復(fù)合體。胰島素結(jié)合α亞基的富含半胱氨酸的區(qū)域(CR)導(dǎo)致α亞基構(gòu)型變化[10],從而引起β亞基的酪氨酸殘基磷酸化和受體酪氨酸蛋白激酶的激活。激活的酪氨酸蛋白激酶招募并磷酸化胞質(zhì)內(nèi)的胰島素受體底物(IRSs)、SHC蛋白、生長(zhǎng)因子結(jié)合受體蛋白2(GRB2)等蛋白信號(hào)分子[11],進(jìn)而激活胰島素信號(hào)的兩個(gè)主要下游通路:磷酸肌醇3激酶(PI3K)/蛋白激酶B(PKB)通路和絲裂原活化蛋白激酶(MAPK)通路[12](圖1)。
PI3K通路主要參與葡萄糖代謝的調(diào)節(jié)。磷酸化的IRSs可以結(jié)合并激活PI3K,活化的PI3K則激活下游3-磷酸肌醇依賴性蛋白激酶(PDK)和mTOR復(fù)合物2(mTORC2),這兩者進(jìn)一步分別磷酸化Akt激酶的蘇氨酸(Thr 308)和絲氨酸(Ser 473),并激活A(yù)kt。激活的Akt磷酸化糖原合成酶激酶3(GSK-3)和FOXO1等底物,從而實(shí)現(xiàn)PI3K通路的代謝調(diào)節(jié)功能[13]。Akt抑制GSK-3的活性,從而激活糖原合成酶(GYS),導(dǎo)致葡萄糖利用和糖原合成增加[14]。Akt也可以抑制結(jié)節(jié)性硬化蛋白-2(TSC-2)活性,從而激活mTOR復(fù)合物1(mTORC1)及其下游靶蛋白核糖體蛋白S6激酶(S6K)、真核翻譯起始因子4E結(jié)合蛋白(4E-BP)和甾醇調(diào)節(jié)元件結(jié)合蛋白1c(SREBP1c),進(jìn)而促進(jìn)蛋白質(zhì)和脂質(zhì)合成。Akt同時(shí)抑制RabGAP TBC1結(jié)構(gòu)域家族成員4(TBC1D4)活性,調(diào)節(jié)葡萄糖轉(zhuǎn)運(yùn)。Akt也可以通過促進(jìn)FOXO降解以抑制FOXO介導(dǎo)的糖異生基因的轉(zhuǎn)錄,調(diào)節(jié)葡萄糖產(chǎn)生[15]。
MAPK通路主要通過調(diào)節(jié)細(xì)胞生長(zhǎng)相關(guān)基因的表達(dá)從而促進(jìn)細(xì)胞生長(zhǎng),也參與代謝調(diào)控[15]。磷酸化的Shc和/或IRSs募集GRB2和鳥苷釋放蛋白(SOS),并激活Ras蛋白和MAPKK激酶(Raf),而后引發(fā)MAPK激酶(MEK1/2)和MAPK(ERK1/2)被依次磷酸化和激活[14]。最后激活的ERK1/2易位到核內(nèi),磷酸化并激活E-26(ETS)轉(zhuǎn)錄因子家族成員、CREB以及C/EBPα等轉(zhuǎn)錄因子,從而調(diào)控細(xì)胞增殖、分化、凋亡、衰老和代謝等[16]。
胰島素信號(hào)通路異常會(huì)導(dǎo)致靶組織胰島素敏感性降低,從而引發(fā)胰島素抵抗(insulin resistance,IR)。雖然在胰島素信號(hào)轉(zhuǎn)導(dǎo)過程中任何一個(gè)環(huán)節(jié)發(fā)生胰島素反應(yīng)的受損都可以被定義為胰島素抵抗,但臨床醫(yī)學(xué)通常將胰島素對(duì)個(gè)體葡萄糖代謝作用降低的現(xiàn)象定義為胰島素抵抗[17]。
胰島素抵抗會(huì)導(dǎo)致人代謝紊亂,是糖尿病、高血壓、血脂紊亂及心腦血管病變等代謝性疾病的核心病理變化[18]。胰島素抵抗的根本原因尚未完全闡明,目前已知的胰島素抵抗發(fā)生機(jī)制有線粒體功能障礙、炎癥、氧化應(yīng)激、內(nèi)質(zhì)網(wǎng)應(yīng)激等。線粒體功能障礙和胰島素抵抗關(guān)系密切[19]。巨噬細(xì)胞以及促炎介質(zhì)(包括腫瘤壞死因子α和白介素1β等)的增加都會(huì)導(dǎo)致胰島素抵抗[20]。高活性分子如活性氧自由基(ROS)和活性氮自由基(RNS)在細(xì)胞內(nèi)過度積累,細(xì)胞就會(huì)發(fā)生氧化應(yīng)激,而氧化應(yīng)激會(huì)引起β細(xì)胞功能障礙、抑制GLUT4轉(zhuǎn)錄和運(yùn)輸、抑制胰島素信號(hào)分子的表達(dá)和激活,最終導(dǎo)致胰島素抵抗[21]。蛋白質(zhì)的過度合成、未折疊或錯(cuò)誤折疊的蛋白質(zhì)在內(nèi)質(zhì)網(wǎng)中積累的現(xiàn)象都被稱為內(nèi)質(zhì)網(wǎng)應(yīng)激。內(nèi)質(zhì)網(wǎng)應(yīng)激會(huì)促進(jìn)脂肪生成和糖異生、抑制IRS1激活,導(dǎo)致胰島素信號(hào)通路活性下降和胰島素抵抗[22]。
胰島素信號(hào)通路的信號(hào)分子突變也會(huì)導(dǎo)致胰島素抵抗。目前關(guān)于胰島素信號(hào)分子突變的研究大多集中在INSR突變,現(xiàn)已發(fā)現(xiàn)INSR基因的100多種突變,這些突變多位于INSRα亞基與胰島素的結(jié)合區(qū)域、β亞基的胞內(nèi)催化域以及前體蛋白的剪切位點(diǎn),這些突變會(huì)導(dǎo)致成熟INSR蛋白的合成減少、向質(zhì)膜的轉(zhuǎn)運(yùn)或再循環(huán)受損、與胰島素結(jié)合的親和力降低或酪氨酸激酶活性降低[23-25],從而導(dǎo)致胰島素抵抗。其他胰島素信號(hào)分子突變也與胰島素抵抗密切相關(guān),研究發(fā)現(xiàn)PI3K通路中編碼P85α的PIK3R1基因的Arg649Trp突變[26]以及Akt2的Arg274His突變[27]都對(duì)胰島素信號(hào)有顯著的抑制作用。全基因組關(guān)聯(lián)分析(GWAS)顯示,一些非胰島素信號(hào)分子的基因突變也與胰島素抵抗相關(guān),這些基因包括TCF7L2、PPARs、FTO、KCNJ11和PTEN等[28-29]。
表觀遺傳也在胰島素信號(hào)通路的調(diào)控和胰島素抵抗的發(fā)生中發(fā)揮重要作用。表觀全基因組關(guān)聯(lián)分析(EWAS)已經(jīng)篩選出大量與人胰島素抵抗相關(guān)的DNA差異甲基化區(qū)域(DMRS),這些DMRS包含PPARGC1A、ABcg1、SREBF1、ADIPOQ、TNFA、FKBP5、INSR和SLC2A4等胰島素信號(hào)通路的信號(hào)分子基因和胰島素反應(yīng)基因,這些基因的甲基化已被證實(shí)與二型糖尿病風(fēng)險(xiǎn)相關(guān)[30]。此外,大量研究已證實(shí),非編碼RNA(ncRNA)能通過調(diào)控胰島素信號(hào)分子的表達(dá)和活性等誘發(fā)胰島素抵抗的發(fā)生[31-32]。
2 雞胰島素信號(hào)通路
雞與人血液胰島素濃度相近,但雞的血糖濃度(2 g·L-1)約是正常人的1.7~2.5倍[33]。雞具有高血糖,但并沒有高血糖的臨床癥狀。有研究發(fā)現(xiàn),雞血清中存在一些重要代謝產(chǎn)物,如DL-精氨酸和?;撬岬?,這些代謝物能抑制雞體內(nèi)晚期糖基化終產(chǎn)物(AGEs)的水平,推測(cè)這些代謝物降低了雞高血糖的不良反應(yīng)[34]。此外,研究顯示,雞對(duì)哺乳動(dòng)物來源的胰島素具有一定的抵抗力[35]。目前普遍認(rèn)為雞存在天然的胰島素抵抗,推測(cè)胰島素抵抗可能會(huì)使雞避免高血糖所致的組織氧化損傷[36]。
雖然雞存在胰島素抵抗,但是注射胰島素抗血清可快速提升自由采食雞個(gè)體的血糖水平[37],這說明雞的血糖還是受到胰島素的調(diào)控。雞的胰島素敏感性似乎還存在發(fā)育階段性。有研究報(bào)道,肉雞從4周齡到12周齡血漿胰島素水平逐漸降低[38]。Chou和Scanes[39]采用葡萄糖鉗夾法檢測(cè)發(fā)現(xiàn),與幼齡(6~8周齡)白來航公雞相比,同品系成年公雞的胰島素敏感更高。另外,不同雞品系的胰島素敏感性也存在差異,研究發(fā)現(xiàn)21日齡絲羽烏骨雞和海蘭蛋雞的胰島素敏感性高于AA肉雞[40];5日齡重體型雞的胰島素敏感性高于輕體型雞[41]。不同羽色雞的胰島素水平也存在顯著差異,研究發(fā)現(xiàn)1周齡儋州黑羽公雞血清胰島素水平顯著高于麻羽公雞[42]。
雞和人胰島素受體具有相似酪氨酸激酶活性[43]。由于外顯子11的選擇性剪接,人INSR基因編碼INSR-A和INSR-B兩種INSR蛋白亞型,這兩個(gè)蛋白亞型的相對(duì)無序插入結(jié)構(gòu)域(αCT)長(zhǎng)度不同,兩者在胰島素信號(hào)轉(zhuǎn)導(dǎo)和功能上也存在差異[44]。本實(shí)驗(yàn)室及其他實(shí)驗(yàn)室的研究都顯示,與哺乳動(dòng)物INSR基因不同,雞INSR基因外顯子11不存在選擇性剪接[45-46],因此雞只有INSR-A蛋白亞型,不存在INSR-B亞型。INSR-B亞型的缺失可能是造成雞與哺乳動(dòng)物胰島素信號(hào)通路活性差異的原因之一。
胰島素通過介導(dǎo)葡萄糖轉(zhuǎn)運(yùn)蛋白(GLUT)易位調(diào)控葡萄糖的攝取[47],哺乳動(dòng)物擁有14個(gè)GLUT家族蛋白,其中GLUT4是哺乳動(dòng)物維持機(jī)體葡萄糖穩(wěn)態(tài)的一個(gè)重要轉(zhuǎn)運(yùn)蛋白,但雞缺失編碼GLUT4的SLC2A4基因[48],不過這不一定是雞高血糖的原因,因?yàn)殡u體內(nèi)依舊存在胰島素響應(yīng)葡萄糖轉(zhuǎn)運(yùn)機(jī)制。目前研究顯示,雞存在人SLC2A1、SLC2A2、SLC2A3、SLC2A5、SLC2A8、SLC2A9和SLC2A12的同源基因,并且這些基因的表達(dá)在雞體內(nèi)有明顯的組織特異性[49-50],其中SLC2A2和SLC2A9 mRNA在雞肝臟中表達(dá)量最高[5]。哺乳動(dòng)物GLUT2廣泛介導(dǎo)葡萄糖、半乳糖和果糖的轉(zhuǎn)運(yùn),并在維持肝細(xì)胞的葡萄糖攝取中發(fā)揮著重要作用[51-52],而GLUT9在哺乳動(dòng)物肝臟和腎臟中含量最高,主要負(fù)責(zé)尿酸的轉(zhuǎn)運(yùn)[53]。GLUT12是雞肌肉組織胰島素敏感型葡萄糖轉(zhuǎn)運(yùn)蛋白[49];GLUT12同樣存在于哺乳動(dòng)物的胰島素敏感組織(如骨骼肌和脂肪組織)中,GLUT12在維持骨骼肌葡萄糖攝取中發(fā)揮與GLUT4同等重要的作用[54]。相較于其他葡萄糖轉(zhuǎn)運(yùn)蛋白,GLUT1是雞脂肪組織胰島素反應(yīng)最敏感的葡萄糖轉(zhuǎn)運(yùn)蛋白[55],但GLUT1在哺乳動(dòng)物中一般負(fù)責(zé)中樞神經(jīng)系統(tǒng)的葡萄糖攝?。?6]。從這些研究結(jié)果可以看出,雞與哺乳動(dòng)物葡萄糖轉(zhuǎn)運(yùn)的胰島素調(diào)控機(jī)制存在差異。
2.1 雞肝臟的胰島素信號(hào)通路
胰島素信號(hào)通路的磷酸化檢測(cè)發(fā)現(xiàn),注射外源胰島素能激活雞肝臟的胰島素通路信號(hào)分子(INSR、IRS-1、Shc、PI3K)[4],且呈現(xiàn)劑量依賴性;而注射胰島素抗血清則會(huì)抑制雞肝臟胰島素信號(hào)通路(INSR、IRS-1、PI3K、Akt、GSK3、ERK2、核糖體蛋白S6)的活性,同時(shí)肝臟脂質(zhì)和葡萄糖代謝相關(guān)基因的表達(dá)也呈現(xiàn)明顯的胰島素依賴性,其中PPARγ、SREBP1、THRSPa、D2-脫碘酶、葡萄糖激酶(GK)和脂肪酸合成酶(FASN)表達(dá)降低,而D3-脫碘酶和IGFBP1表達(dá)增加[57]。這些結(jié)果與哺乳動(dòng)物研究結(jié)果相似,表明雞肝臟胰島素信號(hào)轉(zhuǎn)導(dǎo)通路是正常的。
2.2 雞肌肉的胰島素信號(hào)通路
Duchêne等[58]研究發(fā)現(xiàn),與肝臟組織不同,注射外源胰島素后,雞肌肉內(nèi)未見INSR、IRS-1和PI3K的激活(酪氨酸磷酸化),但卻發(fā)現(xiàn)Akt及其下游通路被激活。與此相一致,Dupont等[57]發(fā)現(xiàn)注射胰島素抗血清并不能引起雞肌肉組織INSR、IRS-1和PI3K磷酸化水平的顯著變化,但胰島素通路下游信號(hào)分子(Shc、Akt、GSK3、ERK2、S6K1、核糖體蛋白S6)的磷酸化水平降低。有研究在經(jīng)胰島素處理的雞成肌細(xì)胞中添加MAPK通路抑制劑U0126,結(jié)果發(fā)現(xiàn)U0126同時(shí)抑制雞成肌細(xì)胞中PI3K信號(hào)通路下游信號(hào)分子S6K1的活性,提示MAPK信號(hào)通路參與調(diào)控PI3K信號(hào)通路下游信號(hào)的激活[59]。比較研究發(fā)現(xiàn),在基礎(chǔ)狀態(tài)下雞肌肉組織的PI3K活性約為大鼠的30倍[4]。磷酸酶和張力蛋白同源物(PTEN)是PI3K信號(hào)通路的抑制劑,在雞肌肉生長(zhǎng)發(fā)育過程中PTEN蛋白的表達(dá)量及活性都顯著降低[60-61],由此可以推測(cè),PTEN的低表達(dá)可能是雞肌肉具有高PI3K活性的原因。哺乳動(dòng)物的PI3K等絲氨酸/蘇氨酸激酶可以磷酸化INSR和IRS-1的絲氨酸殘基,并同時(shí)抑制它們的活性及其下游信號(hào)通路的活性[62]。
綜合這些數(shù)據(jù)可以看出,與哺乳動(dòng)物不同,胰島素并不激活雞肌肉組織PI3K及其上游信號(hào)通路,但卻能激活其下游信號(hào)通路。注射胰島素可顯著提高雞肌肉組織中胰島素下游信號(hào)分子S6K1以及核糖體蛋白S6的活性[63-64],同時(shí)提高IRS-1絲氨酸殘基磷酸化水平[58]。雞肌肉組織的高PI3K活性可能負(fù)反饋抑制胰島素信號(hào)通路的上游信號(hào),使得肌肉胰島素信號(hào)通路上游信號(hào)分子處于失活狀態(tài),從而導(dǎo)致雞肌肉組織的胰島素抵抗。未來有必要驗(yàn)證這一推測(cè)。
2.3 雞脂肪組織的胰島素信號(hào)通路
目前胰島素在雞脂肪組織中的功能和作用機(jī)制尚不明確。研究發(fā)現(xiàn),注射胰島素抗血清并不影響雞脂肪組織胰島素通路的上游信號(hào)分子(INSR、IRS-1和Shc)和下游信號(hào)分子(Akt、S6K1、核糖體蛋白S6、AMPK、ERK2和P38)的活性,這些結(jié)果表明,與哺乳動(dòng)物脂肪組織不同,雞脂肪組織存在胰島素抵抗。雞脂肪組織的胰島素抵抗可能是由于雞脂肪組織INSR和IRS-1的蛋白表達(dá)量較低造成的[65]。此外,還可能是由于雞基因組缺失一些脂肪細(xì)胞因子基因造成的。脂肪組織能分泌多種脂肪細(xì)胞因子,其中一些脂肪細(xì)胞因子能夠調(diào)節(jié)胰島素信號(hào)通路的活性。基因組學(xué)研究顯示,與哺乳動(dòng)物相比,雞基因組缺失3個(gè)已知抑制胰島素敏感性的脂肪因子基因(抵抗素、腫瘤壞死因子α和纖溶酶原激活物抑制劑1)和一個(gè)已知增強(qiáng)胰島素敏感性的脂肪因子(網(wǎng)膜素)[66]。
雞脂肪組織的轉(zhuǎn)錄組學(xué)分析顯示,注射胰島素抗血清和禁食處理所影響的信號(hào)轉(zhuǎn)導(dǎo)、葡萄糖代謝和脂質(zhì)生成相關(guān)基因存在許多重疊,而且這些重疊基因的表達(dá)趨勢(shì)相似。然而脂肪組織代謝組學(xué)分析顯示,相較于禁食處理,注射胰島素抗血清后,雞脂肪組織丙氨酸、精氨酸、天冬氨酸等氨基酸水平顯著升高[67],這提示胰島素對(duì)雞脂肪組織氨基酸代謝有特殊調(diào)控。研究發(fā)現(xiàn),注射外源胰島素會(huì)增加雞脂肪組織中糖原合成關(guān)鍵基因PPP1R3C(蛋白磷酸酶1調(diào)節(jié)亞基3C)mRNA表達(dá),胰島素可能會(huì)促進(jìn)雞脂肪組織中葡萄糖轉(zhuǎn)運(yùn)[68]。但其他研究表明,注射胰島素并不能顯著增加雞脂肪組織葡萄糖吸收[69],因此,也有研究者認(rèn)為胰島素不是雞脂肪組織葡萄糖代謝的關(guān)鍵調(diào)節(jié)因子。與脂肪組織研究結(jié)果不同,在體外培養(yǎng)的雞原代脂肪細(xì)胞中,添加外源胰島素會(huì)增加Akt磷酸化、GLUT1蛋白水平[70]以及葡萄糖的攝取[71],表明體外培養(yǎng)的脂肪細(xì)胞具有胰島素敏感性。目前雞脂肪組織和脂肪細(xì)胞的研究結(jié)果不一致的原因還不清楚,這有待今后進(jìn)一步研究。
3 展 望
從目前的研究看,雞和哺乳動(dòng)物肝臟的胰島素信號(hào)通路的功能和作用機(jī)制相似,但雞脂肪和肌肉組織呈現(xiàn)明顯的胰島素抵抗。雞發(fā)生胰島素抵抗的原因還不清楚,為揭示雞胰島素抵抗發(fā)生的原因,未來有必要從如下幾個(gè)方面開展雞脂肪和肌肉組織的胰島素信號(hào)通路和胰島素抵抗研究。
INSR是胰島素信號(hào)傳導(dǎo)的關(guān)鍵信號(hào)分子。最新研究發(fā)現(xiàn),INSR除了定位于細(xì)胞膜作為胰島素受體激活胰島素信號(hào)通路外,還可進(jìn)入細(xì)胞核,作為轉(zhuǎn)錄因子直接調(diào)控胰島素相關(guān)功能靶基因的轉(zhuǎn)錄。研究發(fā)現(xiàn),與野生鼠相比,肥胖(ob/ob)小鼠肝臟染色質(zhì)結(jié)合的INSR水平顯著降低[72]。未來除了系統(tǒng)解析雞胰島素信號(hào)通路的信號(hào)分子,還需要利用ChIP-seq等高通量分析技術(shù),開展INSR靶基因的鑒定和功能分析。目前雞INSR的基因組結(jié)構(gòu)以及INSR表達(dá)調(diào)控機(jī)制尚不清楚,下一步需要開展雞INSR的基因組結(jié)構(gòu)、轉(zhuǎn)錄異構(gòu)體和蛋白質(zhì)異構(gòu)體分析,以及雞INSR的遺傳調(diào)控和表觀遺傳調(diào)控機(jī)制研究,確定INSR是否與雞胰島素抵抗有關(guān)。哺乳動(dòng)物INSR前體的切割[73]、INSR蛋白的內(nèi)吞作用[74]以及INSR蛋白的降解[75]都會(huì)影響胰島素敏感性,然而目前雞這方面的研究還是空白,未來有必要開展這方面的研究。除了INSR外,其他胰島素通路信號(hào)分子也可能與雞胰島素抵抗有關(guān),未來有必要可采用基因組重測(cè)序、轉(zhuǎn)錄組測(cè)序、蛋白質(zhì)組測(cè)序、蛋白翻譯后修飾以及表觀組測(cè)序等技術(shù),系統(tǒng)分析雞與哺乳動(dòng)物胰島素信號(hào)分子的遺傳和表觀遺傳差異,探索雞發(fā)生胰島素抵抗的原因。
目前轉(zhuǎn)錄組分析發(fā)現(xiàn),胰島素抗血清注射組和對(duì)照組(正常飼喂組)雞的肝臟和肌肉分別存在1 573和1 225個(gè)差異表達(dá)基因,其中只有42個(gè)差異表達(dá)基因與哺乳動(dòng)物的能量消耗、代謝調(diào)節(jié)以及胰島素抵抗有關(guān),而其余的大部分差異表達(dá)基因與目前已知的胰島素信號(hào)通路的關(guān)系還不清楚[76]。此外,哺乳動(dòng)物的最新研究顯示,胰島素抵抗的發(fā)生并不是單純的經(jīng)典胰島素信號(hào)通路信號(hào)轉(zhuǎn)導(dǎo)異常,更可能是產(chǎn)生了新的胰島素信號(hào)轉(zhuǎn)導(dǎo)通路[77]。未來可以開展多組學(xué)聯(lián)合分析來系統(tǒng)解析雞胰島素信號(hào)通路,鑒定雞胰島素信號(hào)通路分支路線、胰島素信號(hào)分子以及胰島素反應(yīng)基因。
雞作為一個(gè)重要的模式動(dòng)物和經(jīng)濟(jì)動(dòng)物,開展雞胰島素信號(hào)通路和胰島素抵抗研究將為人類胰島素抵抗的研究提供參考。胰島素在畜禽生長(zhǎng)、發(fā)育和繁殖中發(fā)揮重要作用[78],蛋雞和雄性肉雞的個(gè)體增重與血清胰島素水平存在顯著正相關(guān)[79]。胰島素在促進(jìn)雞原始生殖細(xì)胞(PGCs)增殖和抑制細(xì)胞凋亡中也發(fā)揮重要調(diào)控作用[80]。本實(shí)驗(yàn)室前期的肉雞血清生化指標(biāo)分析發(fā)現(xiàn),高脂系肉雞血清中胰島素含量顯著高于低脂系,高脂雞血清葡萄糖水平顯著低于低脂雞[81],提示胰島素信號(hào)通路在肉雞脂肪沉積中發(fā)揮著重要作用。深入研究雞胰島素信號(hào)通路和胰島素抵抗,將為揭示肉雞脂肪生長(zhǎng)發(fā)育和脂肪性狀形成的分子機(jī)制奠定基礎(chǔ),并將為控制雞脂肪過度沉積以及雞肉品質(zhì)的遺傳改良提供理論依據(jù)。
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(編輯 郭云雁)