固醇調(diào)控元件結(jié)合蛋白及其對(duì)乳脂合成的調(diào)節(jié)作用

王紅芳 楊維仁* 劉建新 楊在賓

（1.山東農(nóng)業(yè)大學(xué)動(dòng)物科技學(xué)院,泰安271018;2.浙江大學(xué)動(dòng)物科技學(xué)院奶業(yè)科學(xué)研究所,杭州 310029）

在動(dòng)物體內(nèi),脂類物質(zhì)有著多重復(fù)雜的功能,例如作為能源物質(zhì)、細(xì)胞膜的組分等。動(dòng)物產(chǎn)品中的脂類物質(zhì)與人類健康又有著密切的關(guān)系,例如乳脂成分和含量影響著乳制品的品質(zhì),進(jìn)而影響人類健康。近年來(lái)研究表明,動(dòng)物體內(nèi)的脂類物質(zhì)在動(dòng)物體內(nèi)的合成過(guò)程均受到固醇調(diào)控元件結(jié)合蛋白（sterol-regulation element binding proteins, SREBPs）的調(diào)控[1]。SREBPs能激活參與脂類（固醇類、脂肪酸、甘油三酯等）合成一連串酶的轉(zhuǎn)錄[2],是公認(rèn)的脂類物質(zhì)合成的關(guān)鍵調(diào)節(jié)因子。本文針對(duì)SREBPs的結(jié)構(gòu)功能、組織分布、合成調(diào)節(jié)以及其對(duì)乳脂合成的調(diào)節(jié)進(jìn)行全面綜述。

1 SREBPs的結(jié)構(gòu)、功能與組織分布

1.1 SREBPs的結(jié)構(gòu)

SREBPs是調(diào)控膽固醇和脂肪酸合成及體內(nèi)平衡的膜錨定轉(zhuǎn)錄因子[3-4],屬于螺旋-環(huán)螺旋亮氨酸拉鏈家族（basic helix-loop-helix-leucine zip-per fam ily,bHLH-LZ）[5]。SREBPs的無(wú)活性前體蛋白結(jié)合于內(nèi)質(zhì)網(wǎng)膜上,約由1 150個(gè)氨基酸組成。整個(gè)蛋白質(zhì)分為3個(gè)結(jié)構(gòu)域:1）N端480個(gè)氨基酸區(qū)域,內(nèi)包含轉(zhuǎn)錄激活區(qū)、富含絲氨酸和脯氨酸區(qū)、bHLH-LZ區(qū);2）2個(gè)伸入到內(nèi)質(zhì)網(wǎng)內(nèi)腔的疏水跨膜片段,這2個(gè)跨膜片段之間被一個(gè)約30個(gè)氨基酸大小的短環(huán)隔開;3）羧基端約590個(gè)氨基酸的區(qū)域,該區(qū)域是主要的調(diào)節(jié)區(qū)域,并且在羧基端形成了一個(gè)復(fù)雜的、能緊密結(jié)合于SREBPs裂解激活蛋白（SREBPs-cleacage-activating protein,SCAP）的區(qū)域,起到固醇感受器的作用[6-7]。目前發(fā)現(xiàn),SREBPs由2個(gè)基因編碼合成3個(gè)亞型SREBP-1a、SREBP-1c和SREBP-2,其中SREBP-1a和SREBP-1c來(lái)源于同一個(gè)基因SREBP-1,由于轉(zhuǎn)錄的起始位點(diǎn)不同使得SREBP-1a比SREBP-1c少了一段氨基酸序列,而SREBP-2亞型來(lái)源于單獨(dú)的基因SREBP-2基因[8],與SREBP-1基因擁有45%的同源序列[9]。

1.2 SREBPs的功能與組織分布

SREBPs的3個(gè)亞型具有不同轉(zhuǎn)錄激活作用。SREBP-1c主要調(diào)節(jié)脂肪酸合成和胰島素介導(dǎo)的葡萄糖代謝,尤其是脂肪形成;而SREBP-2主要與膽固醇代謝密切相關(guān);SREBP-1a兼?zhèn)淞松鲜龆叩淖饔肹4]。SREBP-1a的轉(zhuǎn)錄激活作用比SREBP-1c要強(qiáng),主要是因?yàn)樗腘端轉(zhuǎn)錄激活區(qū)域較長(zhǎng)[10]。上述三者除了作用不同外,在組織內(nèi)的分布也有明顯的區(qū)別,在人和小鼠等哺乳動(dòng)物體內(nèi),SREBP-1c是優(yōu)勢(shì)表達(dá)的亞型,尤其是在肝臟、白色脂肪組織、骨骼肌、腎上腺和大腦中含量比較高,而SREBP-1a主要在體外培養(yǎng)的細(xì)胞系或者是細(xì)胞增殖程度高的組織中表達(dá),例如脾臟和腸[11]。

2 SREBPs合成的調(diào)節(jié)

SREBPs合成的調(diào)節(jié)可以在不同的水平進(jìn)行,包括轉(zhuǎn)錄水平和轉(zhuǎn)錄后水平的調(diào)節(jié)。

2.1 SREBPs在轉(zhuǎn)錄水平的調(diào)節(jié)

影響SREBPs轉(zhuǎn)錄調(diào)節(jié)的因素很多,機(jī)制也相當(dāng)復(fù)雜,包括動(dòng)物的營(yíng)養(yǎng)水平、個(gè)別營(yíng)養(yǎng)因子、激素、核激素受體和某些酶類等,這些因子相互制約和影響,形成了一個(gè)龐大、復(fù)雜、靈活、多變的SREBPs轉(zhuǎn)錄調(diào)節(jié)網(wǎng)絡(luò)。

在嚙齒動(dòng)物的禁食后重新給飼的試驗(yàn)中,人們首次發(fā)現(xiàn)SREBP-1c的轉(zhuǎn)錄是可以調(diào)節(jié)的,給小鼠禁食后,小鼠的肝臟[12]、白色脂肪組織[13]和骨骼肌[14-15]中SREBP-1c的m RNA豐度明顯減少,說(shuō)明動(dòng)物營(yíng)養(yǎng)水平的高低可以調(diào)節(jié)SREBP-1c轉(zhuǎn)錄。但是對(duì)SREBPs的其他亞型沒有明顯影響。

有試驗(yàn)證明胰島素能誘導(dǎo)SREBP-1c的轉(zhuǎn)錄。例如,在離體的脂肪細(xì)胞[13]、肝細(xì)胞[16]中加入胰島素,發(fā)現(xiàn)SREBP-1c的轉(zhuǎn)錄上調(diào),在人體的脂肪組織和肌肉中也有類似發(fā)現(xiàn)[17-18]。將糖尿病小鼠與正常小鼠相比較,SREBP-1c的轉(zhuǎn)錄活性降低,注入胰島素后,SREBP-1c的轉(zhuǎn)錄會(huì)明顯上調(diào)[19]。由此可見,不論是離體細(xì)胞還是活體動(dòng)物中,胰島素都有誘導(dǎo)SREBP-1c轉(zhuǎn)錄的作用。其效應(yīng)途徑可能是通過(guò)3-磷脂酰肌醇激酶[phosphatidy l inositol 3 kinase,PI（3）-K]來(lái)實(shí)現(xiàn)的[20-21],并且有資料顯示其下游作用元件可能是蛋白激酶B（protein kinase B, PKB）和蛋白激酶C（protein kinase C, PKC）[21-24]。

肝臟X受體α（liver X receptorα,LXRα）是一種核激素受體,它可以被氧化固醇激活（膽固醇的衍生物）,激活的LXRα可以誘導(dǎo)SREBP-1c的轉(zhuǎn)錄。有研究證明,缺乏LXRα的動(dòng)物,SREBP-1c呈現(xiàn)出了較低的基因表達(dá)量,但是肝臟X受體β（liver X receptorβ,LXRβ）基因的敲除,卻對(duì)SREBP-1c的轉(zhuǎn)錄無(wú)影響[25-26]。飼喂動(dòng)物高膽固醇飼糧或者是LXRα促效藥都能激活LXRα,從而促進(jìn)SREBP-1c的轉(zhuǎn)錄[26-28],有人曾提出,LXRα誘導(dǎo)SREBP-1c轉(zhuǎn)錄是為了使其進(jìn)一步誘導(dǎo)脂肪酸的合成,能使得更多的脂肪酸和游離的膽固醇結(jié)合生成膽固醇酯,使過(guò)高的膽固醇濃度得到緩解[29]。LXRα也可以直接誘導(dǎo)個(gè)別脂肪酸生成基因的轉(zhuǎn)錄,例如脂肪酸合成酶（fat acid synthetase,FAS）[30]、乙酰輔酶A羧化酶（acetyl CoA carboxylase,ACC）[31]。但是飼喂SREBP-1c基因無(wú)效的小鼠（SREBP-1c null mice）LXRα促效藥卻不能有效促進(jìn)脂肪酸合成[32],這說(shuō)明SREBP-1c是LXRα誘導(dǎo)脂肪酸合成的必要中間元件。

也有資料顯示雄性激素和孕酮能誘導(dǎo)SREBP-1c的轉(zhuǎn)錄[33-35],但機(jī)理尚無(wú)報(bào)道,一般認(rèn)為這些激素有誘導(dǎo)脂肪合成的作用,而SREBP-1c被認(rèn)為是這一誘導(dǎo)過(guò)程中的中間調(diào)節(jié)物。

此外,一磷酸腺苷活化蛋白激酶（AM P-activated protein kinase,AMPK）對(duì)SREBP-1c的轉(zhuǎn)錄有抑制作用[36]。AMPK是一種催化AMP生成ATP的酶,通常在動(dòng)物體供能不足的情況下,AMPK活性增強(qiáng),此時(shí),往往伴隨著動(dòng)用體脂分解供能,因此AM PK活性增強(qiáng)會(huì)促進(jìn)脂肪酸分解,并通過(guò)抑制SREBP-1c的轉(zhuǎn)錄來(lái)抑制脂肪酸合成。

至于SREBP-1a和SREBP-2的轉(zhuǎn)錄,常在固醇缺乏的情況下被激活,SREBP-1a的轉(zhuǎn)錄激活通常在體外培養(yǎng)的細(xì)胞中實(shí)現(xiàn)[11]。而SREBP-2的轉(zhuǎn)錄主要是通過(guò)一個(gè)前饋機(jī)制,在SREBP-2基因中有固醇調(diào)節(jié)元件（stero l regulation elem ent,SRE）,細(xì)胞核中的SREBP-2（nSREBP-2）結(jié)合于SRE上,能激活SREBP-2自身的轉(zhuǎn)錄[37],有資料顯示SREBP-1c也有類似的前饋調(diào)節(jié)[38-39]。

2.2 SREBPs在轉(zhuǎn)錄后水平的調(diào)節(jié)

SREBPs轉(zhuǎn)錄后水平的調(diào)節(jié),概括起來(lái)可以分為3個(gè)途徑:固醇介導(dǎo)的SCAP/SREBPs復(fù)合物從內(nèi)質(zhì)網(wǎng)到高爾基體的轉(zhuǎn)運(yùn)和加工活化過(guò)程,活化后的SREBPs的修飾過(guò)程以及泛素-26S蛋白酶體對(duì)活性nSREBPs的降解作用。

SREBPs翻譯后的加工活化過(guò)程主要受細(xì)胞內(nèi)固醇水平的調(diào)節(jié)。SREBPs基因轉(zhuǎn)錄成m RNA后,繼而被翻譯成蛋白質(zhì),該蛋白質(zhì)無(wú)生物活性,被稱為SREBPs的前體蛋白。SREBPs前體蛋白與SCAP緊密結(jié)合并錨定在內(nèi)質(zhì)網(wǎng)膜上。當(dāng)細(xì)胞內(nèi)固醇含量較充足時(shí),SCAP與內(nèi)質(zhì)網(wǎng)上的insig蛋白（insulinsensitive protein,一種胰島素敏感蛋白）結(jié)合,使得SREBPs前體蛋白停留在內(nèi)質(zhì)網(wǎng)膜上,不參加后續(xù)的加工過(guò)程;當(dāng)細(xì)胞內(nèi)固醇含量低于生理要求時(shí), SCAP與insig蛋白斷裂,此時(shí)SCAP起到一個(gè)運(yùn)輸和伴侶蛋白的作用,以小泡的形式運(yùn)載著SREBPs前體蛋白到達(dá)高爾基體,在高爾基體的加工過(guò)程主要是由水解酶S1P（site 1 protease）和S2P（site 2 p rotease）起作用,經(jīng)2步水解反應(yīng),釋放出N端的轉(zhuǎn)錄激活區(qū)[40]。加工后的蛋白質(zhì)是一個(gè)二聚體,稱之為成熟的SREBPs,經(jīng)細(xì)胞內(nèi)核質(zhì)轉(zhuǎn)運(yùn)受體（importinβ,負(fù)責(zé)細(xì)胞內(nèi)大部分蛋白質(zhì)和核酸分子的跨核膜轉(zhuǎn)運(yùn)）運(yùn)輸進(jìn)入細(xì)胞核[41]內(nèi)發(fā)揮作用。

成熟的SREBPs進(jìn)入細(xì)胞核后,還要經(jīng)過(guò)一系列的修飾才能發(fā)揮作用。在細(xì)胞核內(nèi),成熟的SREBPs轉(zhuǎn)錄激活作用一般通過(guò)共價(jià)鍵修飾或者通過(guò)與其他蛋白質(zhì)的相互作用來(lái)調(diào)節(jié)[13]。進(jìn)一步研究發(fā)現(xiàn),胰島素也能調(diào)節(jié)細(xì)胞核內(nèi)成熟的SREBP-2和SREBP-1a的轉(zhuǎn)錄激活作用,這一調(diào)節(jié)主要通過(guò)活化的促細(xì)胞分裂蛋白激酶（m itogen-activated protein kinase,MAPK）途徑來(lái)實(shí)現(xiàn)。Ser-117是成熟的SREBP-1a的MAPK途徑的磷酸化位點(diǎn)[42]。體內(nèi)和體外的試驗(yàn)結(jié)果證明:成熟SREBP-2的磷酸化位點(diǎn)是Ser-432和Ser-455[43]。但是,成熟SREBPs通過(guò)MAPK途徑的磷酸化來(lái)調(diào)節(jié)轉(zhuǎn)錄激活的說(shuō)法是有爭(zhēng)議的,Ser-117也存在于SREBP-1c中,試驗(yàn)卻證明了,SREBP-1c并沒有通過(guò)MAPK的磷酸化途徑來(lái)調(diào)節(jié)轉(zhuǎn)錄激活[44-45]。

具有轉(zhuǎn)錄催化活性的SREBPs在細(xì)胞核內(nèi)還要受到泛素-26S蛋白酶體的快速降解。Botolin等[46]用試驗(yàn)證明加入26S的蛋白酶抑制劑后, SREBPs相對(duì)于未添加抑制劑組更加穩(wěn)定,目的基因的表達(dá)也相應(yīng)提高。

3 SREBPs對(duì)乳脂合成的調(diào)節(jié)

乳脂的成分很復(fù)雜,包括甘油三酯、甘油二酯、甘油一酯、甘油、游離脂肪酸和膽固醇等。其中甘油二酯、甘油一酯、甘油、長(zhǎng)鏈脂肪酸是極低密度脂蛋白（V LDL）在乳腺毛細(xì)血管中在脂蛋白酯酶（LPL）的作用下分解生成,后被乳腺上皮細(xì)胞吸收,在乳腺上皮細(xì)胞內(nèi)重新合成甘油三酯;中短鏈脂肪酸和少量膽固醇是乳腺上皮細(xì)胞內(nèi)源合成的。Peterson等[47]用試驗(yàn)證明了,SREBPs也廣泛分布于牛的乳腺組織中,即在乳腺組織中,SREBPs可以調(diào)節(jié)乳脂的生成。SREBPs的不同亞型作用的靶基因不同,它通過(guò)對(duì)不同靶基因的轉(zhuǎn)錄調(diào)節(jié)作用來(lái)實(shí)現(xiàn)對(duì)乳脂的內(nèi)源合成和外源攝取的調(diào)節(jié):SREBP-1c優(yōu)先活化脂肪酸合成的相關(guān)酶,如FAS、ACC等,進(jìn)而影響乳腺組織內(nèi)中短鏈脂肪酸的內(nèi)源合成;而SREBP-2則優(yōu)先活化膽固醇生物合成的相關(guān)基因,如羥甲基戊二酸單酰CoA合酶（hydroxymet hylglutaryl CoA synthase HMG CoA synthase）和羥甲基戊二酸單酰CoA還原酶（hydroxym et hylglutaryl CoA reductase,HMG CoA reductase）等酶的基因[48-49],這些基因參與乳脂中的膽固醇酯的合成;另外,SREBPs的目的基因還包括脂肪酸轉(zhuǎn)運(yùn)蛋白（FATP）基因,甘油酯合成相關(guān)酶基因,例如:甘油-1-磷酸脂酰基轉(zhuǎn)移酶（AGPAT）基因,長(zhǎng)鏈脂肪酸?；o酶A合成酶（ACSL）基因[50],這些基因分別與乳腺長(zhǎng)鏈脂肪酸的外源吸收和乳中甘油酯的合成有關(guān)。綜上所述,SREBPs對(duì)乳脂含量和成分的調(diào)節(jié)有3個(gè)途徑:1）可以通過(guò)影響甘油酯合成原料（中短鏈脂肪酸和膽固醇）的合成來(lái)間接影響乳脂的合成;2）可以通過(guò)調(diào)節(jié)脂肪酸轉(zhuǎn)運(yùn)蛋白來(lái)調(diào)節(jié)乳腺對(duì)脂類物質(zhì)的吸收,進(jìn)而影響乳脂含量;3）直接調(diào)節(jié)甘油酯合成相關(guān)酶來(lái)調(diào)節(jié)乳脂的合成。

4 小 結(jié)

SREBPs與脂肪酸合成有著密切的關(guān)系, SREBPs在奶牛的乳腺組織中廣泛分布,對(duì)奶牛乳脂的調(diào)控有著重要意義。從分子水平探索SREBPs調(diào)控乳脂的機(jī)理,可為生產(chǎn)中常見的奶牛乳脂合成降低提供理論依據(jù),進(jìn)而起到指導(dǎo)生產(chǎn)的作用。

[1] SH IMANO H.Stero l regulatory element-binding p roteins（SREBPs）:transcrip tional regulators of lipid regulators of lipid synthetic genes[J].Progress in Lipid Research,2001,40（6）:439-452.

[2] MCPHERSON R,GAUTHIER A.Molecular regulation of SREBP function:the Insig-SCAP connection and iso form-specific modu lation of lipid synthesis[J].Biochem istry and Cell Biology,2004,82 （1）:201-211.

[3] HORTON JD,GOLDSTEIN J L,BROWN M S. SREBPs:activators o f the comp lete p rogram of cho lesterol and fatty acid synthesis in the liver[J]. The Journal of Clinical Investigation,2002,109: 1125-1131.

[4] DELPH INE E,BRONWYN H,PASCALE B,et al.SREBP transcrip tion factors:master regulators o f lipid homeostasis[J].Biochim ie,2004,86: 839-848.

[5] HUA X,SAKA IJ,HO Y K,et al.Hairpin orientation o f sterol regulatory elemen t-binding p rotein-2 in cellmembranes as determ ined by protease protection[J].The Journal of Bio logical Chem istry, 1995,270:29422-29427.

[6] BROWN M S,GOLDSTEIN J L.The SREBP pathw ay:regulation o f cholestero l metabo lism by proteolysis o f amembrane-bound transcription factor[J].Cell,1997,89（3）:331-340.

[7] HORTON JD,SH IMOM URA I.Sterol regulatory elem ent-binding proteins:activators of cholestero l and fatty acid biosynthesis[J].Current Opinion in Lipidology,1999,10（2）:143-150.

[8] TONTONOZ P,K IM J B,GRAVES R A,et al. ADD 1:a novel helix-loop-helix transcrip tion factor associated with adipocy te determ ination and differentiation[J].Molecular and Cellular Biology,1993,13:4753-4759.

[9] HUA X,YOKOYAMA C,WU J,et al.SREBP-2,a second basic-helix-loop-helix-leuc ine zipper protein t hat stim ulates t ranscrip tion by binding to a sterol regulatory element[J].Proceedings of the National A cademy Sciences o f United States o f A-merica,1993,90（24）:11603-11607.

[10] SH IMANO H,HORTON J D,SH IMOMURA I, et al.Iso form 1c o f sterol regu latory element binding protein is less active than isoform 1a in livers of transgenic m ice and in cultured cells[J].The Journal o f Clinical Investigation,1997,99:846-854.

[11] SH IMOMURA I,SH IMANO H,HORTON J D, et al.D ifferential expression o f exons 1a and 1c in mRNAs for sterol regulatory element binding protein-1 in hum an andm ouse organs and cu ltured cells [J].The Journal o f Clinical Investigation,1997, 99:838-845.

[12] HORTON JD,BASHMAKOV Y,SH IMOMURA I,et al.Regulation of sterol regulatory element binding proteins in livers o f fasted and refed m ice [J].Proceedings of the National Academ y Sciences of United States of America,1998,95:5987-5992.

[13] KIM JB,SARRAF P,WRIGHT M,et al.Nutritional and insulin regu lation o f fatty acid synthetase and lep tin gene expression through ADD 1/SREBP-1[J].The Journal of C linical Investigation,1998, 101:1-9.

[14] BIZEAU M E,MACLEAN PS,JOHNSON G C, et al.Skeletal muscle stero l regulatory element binding p rotein-1c decreases with food dep rivation and increasesw ith feeding in rats[J].The Journal of Nutrition,2003,133:1787-1792.

[15] COMMERFORD S R,PENG L,DUBE J J,et al.In vivoregu lation of SREBP-1c in ske letalmuscle: effects of nutritional status,glucose,insulin,and lep tin[J].American Journal of Physiology Regulatory,Integrative and Comparative Physiology, 2004,287:R218-R227.

[16] FORETZ M,PACOT C,DUGA IL I,et al. ADD1/SREBP-1c is required in the ac tivation of hepatic lipogenic gene expression by glucose[J]. Mo lecular and Cellu lar Biology,1999,19:3760-3768.

[17] DUCLUZEAU P H,PERRETTI N,LAVILLE M,et al.Regulation by insulin of gene expression in human ske letalm uscle and adipose tissue.Evidence for specific de fects in type2 diabetes[J].D iabetes,2001,50:1134-1142.

[18] SEWTER C,BERGER D,CONSID INE R V,et al.H uman obesity and type2 diabetes are associated w ith alterations in SREBP1 isoform expression that are rep roducedex vivoby tumor necrosis factor-alpha[J].D iabetes,2002,51:1035-1041.

[19] SH IMOM URA I,BASHMAKOV Y,IKEM OTO S,et al.Insulin selectively increases SREBP-1c m RNA in the livers of rats with strep tozotocin-induced diabetes[J].Proceedings of the National A-cademy Sciences of United States o f Am erica, 1999,96:13656-13661.

[20] AZZO UT-M ARNICHE D,BECARD D,GUICHARD C,et al.Insulin ef fects on sterol regulatory-elem ent-binding p rotein-1c（SREBP-1c）transcrip tional activity in rathepatocy tes[J].The Biochem ical Journal,2000,350（Pt.2）:389-393.

[21] FLEISCHMANN M,IYNEDJIAN P B.Regulation o f stero l regu latorye lement binding p rotein 1 gene expression in liver:ro le of insulin and p rotein kinase B/cAkt[J].The Biochem ical Journal,2000, 349:13-17.

[22] RIBAUX P G,IYNEDIJIAN P B.Analysis of the role o f p rotein kinase B（cAKT）in insulin-dependent induction o f glucokinase and stero l regulatory element-binding protein 1（SREBP1）mRNAs in hepatocy tes[J].The Biochem ical Journal,2003, 376:697-705.

[23] MATSUMOTO M,OGAWA W,AK IMOTO K,et al.PKC lam bda in liver mediates insulin-induced SREBP-1c expression and determ ines both hepatic lipid content and overall insu lin sensitivity[J].The Journal of Clinical Investigation,2003,112:935-944.

[24] PORSTM ANN T,GRIFFITHS B,CHUNG Y L, et al.PKB/Ak t induces transcription of enzymes involved in cholesterol and fatty acid biosynthesis via activation o f SREBP[J].Oncegene,2005,24 （43）:6465-6481.

[25] PEET D J,TURLEY SD,MA W,et al.Cholesterol and bile acidmetabolism are impaired inm ice lacking the nuclear oxystero l recep tor LXR alpha [J].Cell,1998,93:693-704.

[26] REPA J J,LIANG G,OU J,et al.Regulation of mouse sterol regulatory element-binding protein-1c gene（SREBP-1c）by oxysterol recep tors,LXR alpha and LXR beta[J].Genes and Developm ent, 2000,14:2819-2830.

[27] SCHULTZ J R,TU H,LUK A,et al.Role of LXRs in control of lipogenesis[J].Genes and Development,2000,14:2831-2838.

[28] LAFFITTE B A,CHAO L C,LI J,et al.Activation o f liver X recep tor imp roves glucose tolerance through coordinate regulation o f g lucose metabolism in liver and adipose tissue[J].Proceedings of the National A cadem y Sciences o f United States of America,2003,100:5419-5424.

[29] REPA J J,LIANG G,OU J,et al.Regulation of mouse stero l regulatory elementbinding p rotein-1c gene（SREBP-1c）by oxysterol receptors,LXR alpha and LXR beta[J].Genes Developmen t,2000, 14:2819-2830.

[30] JOSEPH SB,LAFFITTE B A,PATEL PH,et al. Direct and indirect mechanisms for regulation of fatty acid synthase gene expression by liver X recep tors[J].The Journal o f Biological Chem istry, 2002,277:11019-11025.

[31] ZHANG Y,YIN L,HILLGARTNERF B.SREBP-1 integrates the ac tions of thyroid hormone,insu lin, cAMP,and medium-chain fatty acids on ACC alpha transcrip tion in hepatocy tes[J].Journal of Lipid Research,2003,44:356-368.

[32] LIANG G,YANG J,HORTON JD,et al.D im inished hepatic response to fasting/refeeding and liver X receptor agonists in m ice with selective deficiency o f sterol regu latory element-binding p rotein-1c[J].The Journal o f Bio logical Chem istry,2002, 277:9520-9528.

[33] HEEMERS H,MAES B,FOUFELLE F,et al. Androgens stimulate lipogenic gene expression in prostate cancer cells by ac tivation of the sterol regu latory element-binding p rotein cleavage activating p rotein/sterol regulatory element-binding p rotein pathw ay[J].Molecular Endocrinology,2001,15: 1817-1828.

[34] HEEM ERS H,VANDERHOYDONC F,ROSKAMS T,etal.Androgens stimulate coordinated lipogenic gene exp ression in norm al target tissuesin vivo[J].M olecular and Cellular Endocrinology, 2003,205:21-31.

[35] LACASA D,LE LIEPVRE X,FERRE P,et al. Progesterone stim ulates adipocyte determ ination and dif ferentiation 1/sterol regulatory e lem entbinding protein 1c gene exp ression.Potential mechanism for the lipogenic effect o f progesterone in adipose tissue[J].The Journal o f Bio logical Chem istry,2001,276:11512-11516.

[36] MCFADDEN JW,CORL B A.Activation of AMP-activated p rotein kinase（AMPK）inhibits fatty acid synthesis in bovine mamm ary epithelial cells[J]. Biochem istry and Biophysical Research Communication,2009,390（3）:388-393.

[37] SATO R,INOUE J,KAWABE Y,et al.Steroldependen t transcrip tional regulation of stero l regulatory element-binding p rotein-2[J].The Journal o f Bio logical Chem istry,1996,271:26461-26464. [38] AMEM IYA-KUDO M,SHIMANO H,YOSHIKAWA T,et al.Promoter analysis of themouse sterol regu latory elementbinding p rotein-1c gene[J].The Journal o f Biological Chem istry,2000,275: 31078-31085.

[39] HORTON JD,GOLDSTEIN J L,BROWN M S. SREBPs:activators o f the comp lete p rogram of cho lesterol and fatty acid synthesis in the liver[J]. The Journal of Clinical Investigation,2002,109: 1125-1131.

[40] RAWSON R B.The SREBP pathw ay-insights from Insigs and insec ts[J].Nature Reviews M olecular Cell Biolog,2003,4（8）:631-640.

[41] LEE S J,SEK IMOTO T,YAMASH ITA E,et al. The struc ture of im portin-beta bound to SREBP-2: nuclear import of a transcription factor[J].Science,2003,302:1571-1575.

[42] ROTH G,KOTZKA J,KREMER L,et al.MAP kinases Erk1/2 phosphory late sterol regulatory element-binding p rotein（SREBP）-1a at serine 117in vitro[J].The Journal of Biological Chem istry, 2000,275:33302-33307.

[43] KOTZKA J,LEHRS,ROTH G,etal.Insulin-ac-tivated Erk-m itogen-activated protein kinases phosphory late sterol regulatory element-binding protein-2 at serine residues 432 and 455in vivo[J]. The Journal of Biological Chem istry,2004,279: 22404-22411.

[44] WANG D,SUL H S.Insulin stimulation o f the fatty acid synthase p rom oter is mediated by the phosphatidylinositol 3-kinase pathw ay.Involvement of protein kinase B/Ak t[J].The Journal of Biological Chem istry,1998,273:25420-25426.

[45] IYNEDJIAN P B,ROTH R A,FLEISCHMANN M,et al.Ac tivation o f p rotein kinase B/cAk t in hepatocytes is suf ficient for the induction of exp ression of the gene encoding glucokinase[J].The Biochem ical Journal,2000,351（Pt.3）:621-627. [46] BOTOLIN D,WANG Y,CHRISTIAN B,et al. Docosahexaneoic acid（22∶6,n-3）regu lates rat hepatocyte SREBP-1 nuclear abundance by Erkand 26S proteasome-dependent pathw ays[J].Journal o f Lipid Research,2006,47:181-190.

[47] PETERSON D G,MATITASHV ILI E A,BAUMAN D E.The inhibitory effect o ftrans-10,cis-12 CLA on lipid synthesis in bovinem ammary epithelial cells invo lves reduced proteolytic activation o f the transcrip tion factor SREBP-1[J].The Journal o f Nutrition,2004,134:2523-2527.

[48] PAI J,GURYEV O,BROWN M S,et al.Differential stimu lation o f cholestero l and unsaturated fatty acid biosynt hesis in cells exp ressing individual nuclear sterol regulatory element-binding proteins [J].The Journal o f Biological Chem istry,1998, 273（40）:26138-26148.

[49] HORTON JD,SH IMOMURAL I,BROWN M S, et al.Activation o f cholesterol synthesis in p reference to fatty acid synt hesis in liver and adipose tissue of t ransgenic m ice overproducing sterol regulatory element-binding p rotein-2[J].The Journal of Clinical Investigation,1998,101（11）:2331-2339. [50] BIONAZ M,LOOR JJ.ACSL1,AGPAT 6,FABP3, LPIN1,and SLC27A 6 are the most abundant isoforms in bovine mammary tissue and their expression is af fected by stage of lactation[J].The Journalof Nutrition,2008,138:1019-1024.

*Correspond ing au thor,p rofessor,E-m ail:w ryang@sdau.edu.cn

（編輯 武海龍）