趙 娜,黃冬琴,詹曉梅,胡 敏
1980年Tatemoto等[1]從豬空腸粘膜萃取物中分離出一種36個(gè)氨基酸殘基縮合而成的多肽——酪酪肽[2],這種多肽與胰多肽(PP)和神經(jīng)肽Y(NPY)在結(jié)構(gòu)上具有高度一致性。目前,已經(jīng)在多種動(dòng)物以及人類體內(nèi)發(fā)現(xiàn)酪酪肽的存在。在循環(huán)系統(tǒng)中,酪酪肽以PYY1-36和PYY3-36兩種形式存在[3,4]。PYY1-36釋放入血后很快被二肽激肽酶酶切失去N端的兩個(gè)氨基水解后形成PYY3-36,PYY3-36是血液循環(huán)的主要形式[5]。
酪酪肽通過位于靶細(xì)胞的Y受體發(fā)揮生物學(xué)效應(yīng)。Y受體包含六種受體亞型(Y1、Y2、Y3、Y4、Y5、Y6),Y6受體在人體內(nèi)不發(fā)揮作用。各種受體亞型結(jié)構(gòu)、分布及其與酪酪肽的親和力都各有差異。除了Y3受體外,酪酪肽可以與其他幾種受體結(jié)合,對(duì)Y1,Y2,Y5受體親和力更強(qiáng),其中PYY3-36對(duì)Y2受體具有高親和力。Y2受體主要分布在下丘腦弓狀核、海馬、腸道、迷走神經(jīng)背側(cè)端神經(jīng)體內(nèi)[6],尤其在食欲調(diào)節(jié)中樞下丘腦弓狀核的NPY神經(jīng)元中高度表達(dá)[7]。
酪酪肽廣泛分布于胃腸道,主要由消化管的回腸、結(jié)腸和直腸上皮組織的L細(xì)胞以內(nèi)分泌和旁分泌兩種方式分泌[8],胰腺的內(nèi)分泌細(xì)胞、中樞神經(jīng)系統(tǒng)和末梢神經(jīng)系統(tǒng)也有表達(dá)[9]。酪酪肽的分泌受營養(yǎng)物質(zhì)成分對(duì)腸腔的刺激以及神經(jīng)體液因素的影響[4]。Renshaw等[10]研究表明,攝食15分鐘之內(nèi),在營養(yǎng)物質(zhì)到達(dá)L細(xì)胞之前,酪酪肽的分泌已經(jīng)增加,這說明酪酪肽釋放初期,神經(jīng)或激素可能參與了機(jī)制的調(diào)節(jié)。酪酪肽生物效應(yīng)的大小和持續(xù)時(shí)間還取決于機(jī)體攝入的食物熱量、營養(yǎng)成分和食物稠度[4,11,12]。機(jī)體攝入高蛋白質(zhì)食物后,血清酪酪肽濃度明顯高于攝入高脂肪或高碳水化合物食物[11]。Konturek等[13]證實(shí),外周循環(huán)血中酪酪肽峰值與食物熱量高度相關(guān),酪酪肽以營養(yǎng)依賴性的方式釋放,進(jìn)入血液循環(huán)后,其水平快速增加,飯后1小時(shí)~2小時(shí)達(dá)到峰值,并可持續(xù)升高數(shù)小時(shí)[4]。此外,膽汁鹽、胃酸以及調(diào)節(jié)因子如膽囊收縮素、血管活性腸肽等[14,15,16]都能刺激胃腸道細(xì)胞分泌酪酪肽。
1.4.1調(diào)節(jié)攝食PYY3-36具有促進(jìn)飽腹感和抑制攝食的作用。研究證實(shí),外周注射PYY3-36可抑制食欲,且攝食減少量與PYY3-36的注射量呈正比。在對(duì)動(dòng)物的實(shí)驗(yàn)研究中,Batterham等[17]對(duì)自由采食和先禁食24小時(shí)的大鼠腹腔注射PYY3-36(50μg·kg-1),每天注射兩次,連續(xù)七天,發(fā)現(xiàn)酪酪肽能抑制攝食和降低體重。Chelikani等[18]研究指出雄性SD大鼠靜脈注射PYY3-36后,出現(xiàn)食欲減退、攝食量下降。Papadimitriou等[19]對(duì)短尾猴側(cè)腦室灌注PYY3-36后,也發(fā)現(xiàn)短尾猴食欲明顯受到抑制,減少攝食量。Sileno等[20]對(duì)新西蘭兔每天一次較大劑量靜脈注射PYY3-36,連續(xù)注射14天后,體重明顯下降。
在對(duì)人類的研究中,Batterham等[17]對(duì)12名健康成人空腹注射正常餐后水平的PYY3-36,食欲明顯降低,24小時(shí)內(nèi)攝食量減少33%。2003年,該研究小組為評(píng)估酪酪肽對(duì)食欲和攝食量的影響,實(shí)驗(yàn)設(shè)計(jì)正常體重者和肥胖者(各12名)空腹注射PYY3-36兩小時(shí)后進(jìn)餐,發(fā)現(xiàn)肥胖個(gè)體能量攝入降低了30%,正常體重者降低了31%,且兩組24小時(shí)總能量攝入均顯著下降。這表明體重正常者和肥胖者對(duì)PYY3-36的厭食作用都比較敏感[21]。因此,酪酪肽可作為一種調(diào)節(jié)攝食的信號(hào)。
酪酪肽對(duì)攝食具有調(diào)節(jié)機(jī)制。攝食受下丘腦調(diào)節(jié),包括弓狀核黑素腎上腺皮質(zhì)激素和NPY系統(tǒng)。PYY3-36可經(jīng)血腦屏障進(jìn)入神經(jīng)中樞,通過下丘腦弓狀核的Y2受體抑制攝食。Y2受體是NPY的抑制突觸前受體,在弓狀核NPY神經(jīng)元上高度表達(dá),容易受周圍激素的影響。PYY3-36通過抑制NPY神經(jīng)末端的活性,從而激活鄰近的阿片-促黑素細(xì)胞皮質(zhì)素原(POMC)神經(jīng)元而發(fā)揮作用。酪酪肽受神經(jīng)內(nèi)分泌系統(tǒng)(腦-腸軸)調(diào)節(jié),由腸道內(nèi)分泌細(xì)胞分泌后作為控制能量攝入信號(hào),通過G蛋白偶聯(lián)作用于傳入神經(jīng)或通過血腦屏障直接作用于下丘腦弓狀核神經(jīng)元,引起飽腹感;還可能通過負(fù)反饋?zhàn)饔米钄囡嬍硵z入刺激所引起NPY和相關(guān)蛋白的表達(dá)和釋放,從而阻斷興奮下丘腦腹外側(cè)核引起的攝食[17]。
1.4.2增加能量消耗PYY3-36具有促進(jìn)能量消耗和脂肪氧化的作用。Adams等[22]對(duì)飲食誘導(dǎo)C57BL/6J雄性肥胖小鼠皮下注射PYY3-36(劑量為每天1mg·kg-1),連續(xù)處理七天。結(jié)果表明,PYY3-36處理組小鼠的采食量在第二至四天時(shí)顯著低于對(duì)照組,晝夜呼吸商也低于對(duì)照組,說明脂肪氧化作用增加。Van den Hoek等[23]對(duì)C57BL/6J雄性肥胖小鼠慢性注射PYY3-36的研究也表明呼吸商降低,氧化酶活性改變,脂肪氧化增加。Boey等[24]研究發(fā)現(xiàn),酪酪肽超表達(dá)的轉(zhuǎn)基因小鼠表現(xiàn)出基礎(chǔ)體溫升高及促甲狀腺釋放激素持續(xù)表達(dá),表明燃脂率和能量消耗增加降低血清甘油三酯水平及體重的增加。
Sloth等[25]對(duì)體重正常、超重及肥胖男性各12名采用盲目、隨機(jī)、交叉研究發(fā)現(xiàn),注射PYY3-36能夠促進(jìn)機(jī)體產(chǎn)熱、脂類分解。表明能量消耗和脂肪氧化率增加。Guo等[26]通過對(duì)29名健康受試者(其中13名是肥胖受試者)測(cè)量空腹和餐后30分鐘~180分鐘內(nèi)酪酪肽水平。研究發(fā)現(xiàn)空腹酪酪肽水平與肥胖各指標(biāo)及15小時(shí)的靜息代謝率呈負(fù)相關(guān),餐后酪酪肽峰值與24小時(shí)的呼吸商也呈負(fù)相關(guān),而餐后酪酪肽水平變化與餐后的飽感程度呈正相關(guān)。該研究表明內(nèi)源性酪酪肽可能通過對(duì)攝食、能量消耗和脂代謝的影響來參與體重的長期調(diào)節(jié)。同樣,Doucet等[27]對(duì)25名絕經(jīng)前期正常體重健康女性進(jìn)行空腹和早餐后三小時(shí)酪酪肽水平的測(cè)定發(fā)現(xiàn),餐后酪酪肽水平與餐后的能量消耗和食物熱效應(yīng)相關(guān)。以上研究均表明酪酪肽能降低食欲和增加能量消耗。
1.4.3抑制胰島素分泌
Nieuwenhuizen等[28]研究發(fā)現(xiàn),酪酪肽可以抑制胰島素的分泌。PYY3-36可能經(jīng)過下丘腦的Y2受體來影響胰島素的敏感性[29]。PYY3-36能抑制下丘腦弓狀核的神經(jīng)肽Y(NPY)神經(jīng)元的活性和激活鄰近的阿片-促黑素細(xì)胞皮質(zhì)素(POMC)神經(jīng)元,下丘腦中NPY的過度表達(dá)可引起嚙齒類的肥胖癥狀和胰島素抵抗;而α-黑色素細(xì)胞刺激素(POMC的一種產(chǎn)物)可以激活POMC神經(jīng)元從而提高胰島素敏感性[17]。
也有實(shí)驗(yàn)證明酪酪肽通過Y1受體影響其分泌。在胰島β細(xì)胞上存在Y1受體[30],酪酪肽通過降低環(huán)磷酸腺苷(cAMP)水平以及直接作用于胰島的Y1受體來抑制胰島素分泌[28,31]。酪酪肽還可以通過胰腺內(nèi)其它細(xì)胞來抑制β細(xì)胞分泌的胰島素,δ細(xì)胞分泌的生長素也能有效地抑制胰島素和胰高血糖素的分泌[32]。Karlsson等[33]還發(fā)現(xiàn)酪酪肽可通過間接減少胰高血糖素的作用來影響胰島素分泌,并認(rèn)為酪酪肽可能是以旁分泌方式來調(diào)節(jié)胰島素的分泌,但關(guān)于胰高血糖素和酪酪肽是否同時(shí)分泌尚不確定。
肥胖個(gè)體空腹血漿酪酪肽水平較低。Rahardjo等[34]對(duì)小鼠進(jìn)行22周飲食干預(yù),研究發(fā)現(xiàn)高脂飲食誘導(dǎo)的肥胖小鼠血漿酪酪肽循環(huán)水平比體重正常和低脂飼養(yǎng)小鼠更低。在人類,McGowan等[35]研究表明,空腹酪酪肽水平與體重指數(shù)(BMI)呈負(fù)相關(guān),超重者體內(nèi)PYY3-36水平較低,肥胖患者內(nèi)源性血漿酪酪肽水平在空腹和餐后均低于正常體重者[11,21]。Zwirska-Korczala 等[36]對(duì)患有代謝綜合癥的重度肥胖婦女測(cè)定空腹血漿PYY3-36水平,發(fā)現(xiàn)其濃度比正常體重組和患有代謝綜合癥的中度肥胖婦女低。Roth等[37]對(duì)73名肥胖兒童和45名正常體重兒童的研究也發(fā)現(xiàn),與正常體重兒童相比,肥胖兒童體內(nèi)內(nèi)源性酪酪肽水平明顯偏低,空腹酪酪肽水平不存在性別、青春期和青春前期間的差異,同時(shí)他還認(rèn)為較低的酪酪肽水平使受試者更易于發(fā)生肥胖。Li等[38]也證實(shí)肥胖者空腹酪酪肽低水平下,酪酪肽可能產(chǎn)生很弱的飽感信號(hào),從而加劇肥胖。Guo等[26]研究發(fā)現(xiàn)空腹酪酪肽水平與BMI和腰圍以及15小時(shí)的靜息代謝率都呈負(fù)相關(guān)。餐后肥胖組的酪酪肽水平比正常體重組低得多。酪酪肽循環(huán)水平與成人[39,40]、兒童[37]以及嬰兒[41]的脂代謝均呈負(fù)相關(guān)。因此,循環(huán)中低水平的酪酪肽可能參與肥胖的發(fā)病機(jī)理[42,43]。
研究發(fā)現(xiàn)減肥能使肥胖個(gè)體血液循環(huán)中低的酪酪肽濃度顯著升高。le Roux CW等[44]研究發(fā)現(xiàn),胃旁路術(shù)兩天后酪酪肽水平開始上升,食欲減退;給患者皮下注射生長抑素后,酪酪肽分泌受到抑制,食欲恢復(fù),攝食增加。Rosa等[45]研究也表明胃旁路術(shù)六周后,饑餓感降低,飽感增強(qiáng),血漿總酪酪肽水平升高。Alvarez_Bartolome等[46]研究發(fā)現(xiàn),重度肥胖患者酪酪肽水平比非肥胖者更低,而重度肥胖患者經(jīng)過垂直捆繃式胃成形術(shù)六個(gè)月及12個(gè)月后酪酪肽水平逐漸上升到正常水平。Valderas等[47]將肥胖者分為手術(shù)治療組和藥物治療組各八名與八名正常體重組隨訪兩個(gè)月,發(fā)現(xiàn)吻合胃繞道術(shù)、袖狀切除術(shù)組和藥物治療組減肥效果相似,手術(shù)組酪酪肽水平上升,食欲降低,而藥物治療組沒有發(fā)生顯著性變化;研究還提示酪酪肽水平升高及飽感增強(qiáng)可加強(qiáng)手術(shù)減肥的持續(xù)效果。肥胖者酪酪肽水平可隨著體重降低而逐漸上升,甚至能恢復(fù)到生理水平。然而,Paulina等[48]對(duì)30名肥胖成年人進(jìn)行八周飲食干預(yù)(分為低碳水化合物減肥組和低脂減肥組)研究發(fā)現(xiàn),減重后空腹和餐后血清PYY水平都下降了,這可能反映了一種維持能量平衡的代償反應(yīng)及會(huì)導(dǎo)致限制能量飲食期間減重困難。出現(xiàn)不同的研究結(jié)果可能是受試者沒有嚴(yán)格遵從飲食處方造成的。
中等強(qiáng)度的急性有氧運(yùn)動(dòng)可提高血漿酪酪肽水平。Broom等[49]對(duì)11名健康男學(xué)生進(jìn)行為期三周,每周一次八小時(shí)的實(shí)驗(yàn),受試者隨機(jī)交叉完成任何一組實(shí)驗(yàn)(有氧練習(xí)、力量練習(xí)和休息),力量練習(xí)組以80%RM的抗阻運(yùn)動(dòng)(舉重)持續(xù)90分鐘,休息六小時(shí);有氧練習(xí)以70%的最大攝氧量(VO2 max)跑臺(tái)持續(xù)一小時(shí),休息七小時(shí);另一組實(shí)驗(yàn)僅是安靜休息八小時(shí)。研究發(fā)現(xiàn)有氧運(yùn)動(dòng)的能量消耗高于抗阻運(yùn)動(dòng)和休息組,有氧運(yùn)動(dòng)期間和運(yùn)動(dòng)后一段時(shí)間饑餓感均受到抑制,且運(yùn)動(dòng)后酪酪肽水平在較長時(shí)間內(nèi)可持續(xù)一個(gè)較高水平。Ueda等[50]分別對(duì)肥胖和正常體重者(各7名)以50%VO2 max進(jìn)行持續(xù)一小時(shí)的騎自行車運(yùn)動(dòng),結(jié)果顯示,急性運(yùn)動(dòng)可以增加血漿中酪酪肽濃度,降低能量攝入和相對(duì)能量攝入;肥胖受試者運(yùn)動(dòng)抑制能量攝入的程度及能量消耗均顯著高于對(duì)照組,表明中等強(qiáng)度的急性運(yùn)動(dòng)可引起能量負(fù)平衡。Martins等[51]報(bào)道標(biāo)準(zhǔn)餐一小時(shí)后(飽腹?fàn)顟B(tài)下)對(duì)12名健康受試者以65%VO2 max進(jìn)行持續(xù)一小時(shí)的騎自行車運(yùn)動(dòng),發(fā)現(xiàn)運(yùn)動(dòng)期間血漿酪酪肽水平顯著增加,而且運(yùn)動(dòng)期間饑餓感顯著降低,但是這種效果很短暫,運(yùn)動(dòng)后即消失。與對(duì)照組相比,運(yùn)動(dòng)組機(jī)體相對(duì)能量攝入也顯著下降。研究認(rèn)為急性運(yùn)動(dòng)誘導(dǎo)的食欲降低可能會(huì)影響隨后的能量平衡,且可能與運(yùn)動(dòng)中酪酪肽升高有關(guān)系。此外,Cheng等[52]對(duì)12名有適量運(yùn)動(dòng)習(xí)慣的年輕男性(適量運(yùn)動(dòng)是指受試者在入選前至少進(jìn)行了三個(gè)月有氧運(yùn)動(dòng),每周二至四天,每次20分鐘~60分鐘)進(jìn)行三組隨機(jī)實(shí)驗(yàn)(飯后兩小時(shí)運(yùn)動(dòng)組、飯前一小時(shí)運(yùn)動(dòng)組、安靜進(jìn)餐能耗組)均以60%VO2 max進(jìn)行50分鐘的踏車運(yùn)動(dòng),研究發(fā)現(xiàn)飯后組50分鐘中等強(qiáng)度運(yùn)動(dòng)可延長食欲對(duì)攝食的抑制作用,而且血漿PYY3-36水平有上升的趨勢(shì);飯前組50分鐘中度強(qiáng)度運(yùn)動(dòng)可削弱運(yùn)動(dòng)后期的饑餓感,抑制飯后食欲的增加,這表明急性運(yùn)動(dòng)可降低他們的饑餓感,且饑餓程度受運(yùn)動(dòng)時(shí)間選擇的影響。
長期運(yùn)動(dòng)鍛煉降低體重后能夠增加肥胖青少年空腹酪酪肽總水平。Jones等[53]對(duì)肥胖青少年以60%~85%VO2 max的運(yùn)動(dòng)強(qiáng)度、每周三次持續(xù)鍛煉時(shí)間為一小時(shí)的有氧運(yùn)動(dòng)八個(gè)月后,體脂率降低2.2%,甘油三酯濃度降低23%,空腹酪酪肽循環(huán)總濃度增加23%,該研究表明,空腹酪酪肽濃度的增加暗示著可能會(huì)降低肥胖青少年的食欲。Roth等[37]通過對(duì)肥胖兒童運(yùn)動(dòng)鍛煉配合行為干預(yù)和飲食治療,每周一次持續(xù)一年的研究發(fā)現(xiàn),肥胖兒童酪酪肽基線水平較低,但減重后酪酪肽水平顯著增加,這暗示降低體重能使血漿酪酪肽水平恢復(fù)到生理水平。而Kelly等[54]將19名老年肥胖受試者分為適當(dāng)熱量飲食組和低熱量飲食兩組,然后以75% VO2 max每周五次,每次一小時(shí)、持續(xù)12周的中等強(qiáng)度有氧運(yùn)動(dòng)鍛煉(跑臺(tái)或踏車運(yùn)動(dòng)),研究發(fā)現(xiàn)干預(yù)后兩組受試者體重指數(shù)和脂肪量都下降,而空腹酪酪肽水平?jīng)]有變化。
酪酪肽具有抑制攝食、增加能量消耗和抑制胰島素分泌的作用。近年來,酪酪肽在肥胖領(lǐng)域的研究已經(jīng)取得了一些成果,但仍存在許多亟待解決的問題。酪酪肽抑制攝食作用時(shí)間的長短以及與其他食欲調(diào)節(jié)因子之間的相互作用還不明確,酪酪肽是如何結(jié)合和激活Y2受體發(fā)揮生物效用、酪酪肽進(jìn)入循環(huán)后對(duì)肥胖的作用及機(jī)制還需進(jìn)行進(jìn)一步的探討。目前更值得關(guān)注的是,急性運(yùn)動(dòng)以及長期運(yùn)動(dòng)鍛煉對(duì)肥胖個(gè)體酪酪肽水平的影響及其作用機(jī)制,不同強(qiáng)度運(yùn)動(dòng)、運(yùn)動(dòng)時(shí)間的選擇、運(yùn)動(dòng)方式以及運(yùn)動(dòng)時(shí)間對(duì)酪酪肽時(shí)效性的影響、運(yùn)動(dòng)誘導(dǎo)的酪酪肽變化對(duì)能量攝入是否有影響、酪酪肽對(duì)減肥效果和持續(xù)效果的時(shí)間等等都有待于更深入的研究。隨著對(duì)酪酪肽與肥胖及運(yùn)動(dòng)關(guān)系的不斷探究,酪酪肽將為肥胖癥的預(yù)防和治療開辟新的途徑。
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