[摘" "要]" "目的:研究miR-221-3p對(duì)胰腺癌細(xì)胞凋亡的抑制作用及可能機(jī)制。方法:對(duì)人胰腺癌細(xì)胞株P(guān)ATU 8988T進(jìn)行miR-221-3p質(zhì)粒轉(zhuǎn)染,設(shè)為miR-221-3p mimics組、mimics NC組、miR-221-3p inhibitor組及inhibitor NC組,以未處理的PATU8988T細(xì)胞作為空白對(duì)照(NC)組。采用實(shí)時(shí)熒光定量 PCR(qRT-PCR)驗(yàn)證細(xì)胞轉(zhuǎn)染效率;采用流式細(xì)胞術(shù)觀察miR-221-3p對(duì)胰腺癌細(xì)胞凋亡的影響;采用Western blotting法測(cè)定各組細(xì)胞中p53、磷酸酶與張力蛋白同源物(phosphatase and tensin homolog,PTEN)蛋白表達(dá)。結(jié)果:qRT-PCR結(jié)果顯示,與NC組、mimics NC組及inhibitor NC組比較,miR-221-3p mimics組細(xì)胞miR-221-3p表達(dá)顯著增高,miR-221-3p inhibitor組表達(dá)明顯降低,差異均有統(tǒng)計(jì)學(xué)意義(P<0.05)。流式細(xì)胞術(shù)檢測(cè)結(jié)果顯示,與NC組、mimics NC組及inhibitor NC組比較,miR-221-3p inhibitor組細(xì)胞凋亡明顯增加,miR-221-3p mimics組細(xì)胞凋亡明顯減少,差異均有統(tǒng)計(jì)學(xué)意義(Plt;0.05)。Western blotting結(jié)果顯示,mimics NC組和inhibitor NC組細(xì)胞中p53、PTEN蛋白表達(dá)水平比較,差異無統(tǒng)計(jì)學(xué)意義(Pgt;0.05),而miR-221-3p mimics組細(xì)胞中p53、PTEN蛋白表達(dá)水平較mimics NC組明顯下降,差異均有統(tǒng)計(jì)學(xué)意義(P<0.05)。結(jié)論:PATU 8988T胰腺癌細(xì)胞中miR-221-3p表達(dá)上調(diào),細(xì)胞凋亡受到抑制。miR-221-3p可能通過下調(diào)p53、PTEN蛋白表達(dá)促進(jìn)胰腺癌的發(fā)生發(fā)展。
[關(guān)鍵詞]" "胰腺癌;miR-221-3p;p53;磷酸酶與張力蛋白同源物;細(xì)胞凋亡
[中圖分類號(hào)]" "R735.9 [文獻(xiàn)標(biāo)志碼]" "A [DOI]" "10.19767/j.cnki.32-1412.2024.03.003
Inhibitory effect of miR-221-3p on apoptosis of pancreatic cancer cells
and its mechanism study
QIAN Jing1, SHI Yu1, YAN Xiaodi1, BIAN Yinzhu2
(1Department of Radiotherapy, Affiliated Hospital of Nantong University, Jiangsu 226001; 2Department of Oncology, Yancheng First Hospital Affiliated to Medical College of Nanjing University/The First People’s Hospital of Yancheng City)
[Abstract]" "Objective: To study the inhibitory effect of miR-221-3p on apoptosis of pancreatic cancer cells and its possible mechanism. Methods: Human pancreatic cancer cell line PATU8988T was transfected with miR-221-3p plasmid and divided into the miR-221-3p mimics group, the mimics NC group, the miR-221-3p inhibitor group and the inhibitor NC group. Untreated PATU8988T cells were used as blank control (NC) group. Cell transfection efficiency was verified by" real-time fluorescence quantitative PCR (qRT-PCR); flow cytometry was used to observe the effect of miR-221-3p on apoptosis of pancreatic cancer cells; Western blotting was used to determine the expression of p53 and phosphatase and tensin homolog (PTEN) proteins in each group of cells. Results: qRT-PCR results showed that compared with the NC group, the mimics NC group, and the inhibitor NC group, the expression of miR-221-3p in the miR-221-3p mimics group was significantly increased, while the expression of miR-221-3p in the miR-221-3p inhibitor group was significantly reduced, with statistical difference (Plt;0.05). The results of flow cytometry showed that compared with the NC group, the mimics NC group, and the inhibitor NC group, the miR-221-3p inhibitor group showed a significant increase in cell apoptosis, while the miR-221-3p mimics group showed a significant decrease in cell apoptosis, with statistical difference (Plt;0.05). Western blotting results showed that there was no statistically significant difference in the expression levels of p53 and PTEN proteins between the mimics NC group and the inhibitor NC group (Pgt;0.05), while the expression levels of p53 and PTEN proteins in the miR-221-3p mimics group were significantly lower than those in the mimics NC group, and the differences were statistically significant (Plt;0.05). Conclusion: The expression of miR-221-3p is up-regulated in pancreatic cancer cells line PATU8988T, and cell apoptosis is inhibited. miR-221-3p may promote the occurrence and development of pancreatic cancer by down-regulating the expression of p53 and PTEN proteins.
[Key words]" "pancreatic cancer; miR-221-3p; p53; phosphatase and tensin homolog; cell apoptosis
胰腺癌是惡性程度極高的消化道腫瘤,病程短、進(jìn)展快、死亡率高?;颊咴缙诎Y狀不明顯,僅20%患者在確診時(shí)可手術(shù)切除,但遠(yuǎn)期效果也往往不理想,術(shù)后5年存活率低于20%[1-2]。因此,對(duì)胰腺癌進(jìn)行早檢測(cè)、早診斷,了解其發(fā)生發(fā)展的調(diào)控機(jī)制是提高生存率的關(guān)鍵。微小核糖核酸(microRNAs)是一種小分子非編碼核糖核酸,在基因轉(zhuǎn)錄后調(diào)控中發(fā)揮重要作用,參與腫瘤的發(fā)生發(fā)展。賈振飛等[3]研究顯示miR-151-3p表達(dá)水平對(duì)早期診斷胰腺癌具有一定價(jià)值。有研究發(fā)現(xiàn),miR-221-3p促進(jìn)胰腺癌細(xì)胞系的增殖[4-5],但miR-221-3p對(duì)胰腺癌細(xì)胞凋亡的影響尚不明確。任剛等[6]研究表明p53水平及遠(yuǎn)處轉(zhuǎn)移是影響胰腺癌患者預(yù)后的獨(dú)立因素。約78%胰腺癌細(xì)胞系中存在p53基因突變,主要為核苷酸轉(zhuǎn)換[7-8]。磷酸酶與張力蛋白同源物(phosphatase and tensin homolog,PTEN)通過去磷酸化調(diào)節(jié)細(xì)胞內(nèi)信號(hào)傳導(dǎo),大部分實(shí)體腫瘤如乳腺癌、前列腺癌、胰腺癌等存在PTEN基因突變、缺失或低表達(dá),導(dǎo)致PI3K/AKT信號(hào)通路活化,促進(jìn)細(xì)胞生長(zhǎng)和增殖[9-11]。本研究分析人胰腺癌細(xì)胞株P(guān)ATU 8988T中miR-221-3p不同轉(zhuǎn)染組細(xì)胞凋亡情況及p53、PTEN表達(dá)水平,探討miR-221-3p調(diào)控胰腺癌細(xì)胞凋亡的分子機(jī)制,為胰腺癌的診斷和治療提供新思路。
1" "材料與方法
1.1" "細(xì)胞及主要試劑" "人胰腺癌細(xì)胞株P(guān)ATU 8988T(福建醫(yī)科大學(xué)附屬腫瘤醫(yī)院腫瘤放射治療研究室提供),細(xì)胞培養(yǎng)基、胎牛血清(FBS)(美國Gibco公司),p53、PTEN抗體(美國Merck公司),Annexin V-FITC/PI 凋亡試劑盒(美國BD公司)。
1.2" "研究方法
1.2.1" "細(xì)胞培養(yǎng)及轉(zhuǎn)染:PATU 8988T細(xì)胞置于含10%FBS的DMEM培養(yǎng)液中,在36 ℃、5% CO2、飽和濕度培養(yǎng)箱中培養(yǎng),2~3天換液傳代一次。采用Lipofectamine 3000進(jìn)行轉(zhuǎn)染,按說明書制備相應(yīng)的轉(zhuǎn)染液,實(shí)驗(yàn)分為miR-221-3p mimics組、mimics NC組、miR-221-3p inhibitor組及inhibitor NC組,未作處理的PATU 8988T細(xì)胞為空白對(duì)照組(NC組)。
1.2.2" "qRT-PCR檢測(cè):取各組細(xì)胞提取RNA,逆轉(zhuǎn)錄cDNA,運(yùn)用qRT-PCR檢測(cè)miRNA的表達(dá),驗(yàn)證miRNA的轉(zhuǎn)染效率。miR-221-3p上游引物(Forward primer)與下游引物(Reverse primer)分別為5′-TAGCTACATCGGAACTGATTGTG-3′與5′-TGGTGTCGTGGAGTCG-3′。內(nèi)參U6的Forward primer與Reverse primer分別為5′-CTCGCTTCGGCAGCACA-3′與5′-AACGCTTCACGAATTTGCGT-3′。
1.2.3" "Western blotting檢測(cè):PATU 8988T細(xì)胞在6孔板中轉(zhuǎn)染24 h后,用4 ℃預(yù)冷PBS洗3次。加入裂解液200 μL,冰上放置30 min,渦流振蕩10 min。4 ℃低溫離心15 min,取上清。BCA法測(cè)定蛋白濃度,并計(jì)算蛋白變性所需相應(yīng)蛋白體積,變性冷卻后放置-20 ℃冰箱,再進(jìn)行Western blotting實(shí)驗(yàn):(1)蛋白電泳:準(zhǔn)備10%或12%的SDS-PAGE膠,每孔加入10 ~20 μL樣品。在120 V下電泳約1 h,直到染料前沿到達(dá)底部。(2)轉(zhuǎn)膜:甲醇潤濕PVDF膜,并用轉(zhuǎn)膜緩沖液平衡。將膠和膜夾在濾紙間,確保無氣泡。在100 V下轉(zhuǎn)膜1 h或按照設(shè)備說明操作。(3)封閉:使用5%脫脂奶粉封閉PVDF膜,在室溫?fù)u床上孵育1 h。(4)一抗孵育:將一抗按1 ∶ 1 000~1 ∶ 5 000稀釋在封閉液中,4 ℃孵育過夜,輕微搖動(dòng)。使用TBST緩沖液洗膜,每次5 min,共3次。(5)二抗孵育:將HRP標(biāo)記的二抗按1 ∶ 5 000~1 ∶ 10 000稀釋在封閉液中,在室溫下孵育1 h,輕微搖動(dòng)。洗膜方法同上。最后,使用化學(xué)發(fā)光試劑,按說明書操作,記錄蛋白條帶。
1.2.4" "細(xì)胞凋亡實(shí)驗(yàn):從細(xì)胞培養(yǎng)箱中取出轉(zhuǎn)染24 h~48 h后的細(xì)胞,棄去6孔板中的培養(yǎng)液,PBS漂洗一次,每孔加入不含EDTA的胰蛋白酶溶液,1~2 min后顯微鏡下觀察細(xì)胞狀態(tài),然后每孔添加2 mL新培養(yǎng)液,移液槍輕柔吹打,將細(xì)胞轉(zhuǎn)移至離心管中,室溫下1 000轉(zhuǎn)/min離心5 min。移去上清液,加入PBS緩沖液輕輕吹打,再次離心,重復(fù)洗滌3次。染色:離心管中加入1×Binding Buffer,混勻后在顯微鏡下進(jìn)行細(xì)胞計(jì)數(shù),調(diào)整細(xì)胞密度大于1×106個(gè)/mL。吸取100 μL細(xì)胞懸液至新的EP離心管中,分別加入5 μL熒光素異硫氰酸酯(FITC)-Annexin V和7.5 μL碘化丙啶(PI)。另取3支新的EP離心管(標(biāo)記為A、B、C管),各加入100 μL細(xì)胞懸液,A管不加染料,B管加5 μL FITC-Annexin V,C管加7.5 μL PI。輕輕震蕩混勻,室溫下避光孵育15 min。孵育后,將細(xì)胞懸液吸至流式用管中,每管加入400 μL 1×Binding Buffer,渦旋震蕩后進(jìn)行流式細(xì)胞檢測(cè)。
1.3" "統(tǒng)計(jì)學(xué)處理" "應(yīng)用SPSS 23.0軟件對(duì)數(shù)據(jù)進(jìn)行分析處理。計(jì)量資料以均數(shù)±標(biāo)準(zhǔn)差表示,多組間比較采用單因素方差分析,兩兩比較采用LSD-t檢驗(yàn)。計(jì)數(shù)資料以百分率表示,組間比較采用Fisher精確檢驗(yàn)。Plt;0.05為差異有統(tǒng)計(jì)學(xué)意義。
2" "結(jié)" " " 果
2.1" "各組細(xì)胞miR-221-3p相對(duì)表達(dá)量" "qRT-PCR檢測(cè)結(jié)果顯示,NC組、mimics NC組及inhibitor NC組細(xì)胞miR-221-3p表達(dá)比較,差異均無統(tǒng)計(jì)學(xué)意義(Pgt;0.05)。與NC組、mimics NC組及inhibitor NC組比較,miR-221-3p mimics組PATU 8988T細(xì)胞miR-221-3p表達(dá)顯著增高,miR-221-3p inhibitor組miR-221-3p表達(dá)顯著下調(diào)(Plt;0.05)。見圖1。
2.2" "miR-221-3p對(duì)細(xì)胞凋亡的影響" "與NC組、inhibitor NC組及mimics NC組比較,miR-221-3p inhibitor組細(xì)胞凋亡明顯增加,miR-221-3p mimics組細(xì)胞凋亡明顯減少(Plt;0.05),表明miR-221-3p對(duì)細(xì)胞凋亡具有抑制作用。見圖2。
2.3" "各組細(xì)胞p53、PTEN表達(dá)水平" "mimics NC組與inhibitor NC組細(xì)胞p53、PTEN蛋白表達(dá)比較,差異均無統(tǒng)計(jì)學(xué)意義(Pgt;0.05)。與inhibitor NC組比較,miR-221-3p inhibitor組細(xì)胞P53和PTEN蛋白表達(dá)明顯升高;與mimics NC組比較,miR-221-3p mimics組細(xì)胞p53和PTEN蛋白表達(dá)顯著降低,差異均有統(tǒng)計(jì)學(xué)意義(Plt;0.05)。見圖3。
3" "討" " " 論
p53是抑癌基因,缺失后腫瘤細(xì)胞快速增殖[16-18]。p53蛋白磷酸化、泛素化以及蛋白體降解均影響p53蛋白表達(dá)水平。KIM等[19-20]發(fā)現(xiàn),靶向突變型p53單抗可以降低胰腺癌轉(zhuǎn)移的發(fā)生率,環(huán)磷酸腺苷反應(yīng)元件結(jié)合蛋白1(cyclic adenosine monophosphate response element binding protein 1,CREB1)可以干擾突變型p53的協(xié)同作用,從而抑制胰腺癌細(xì)胞轉(zhuǎn)移。p53是調(diào)控miRNAs成熟的關(guān)鍵分子,轉(zhuǎn)化3T3細(xì)胞雙分2(transformed 3T3 cell double minute 2,MDM2)可與p53蛋白結(jié)合并抑制其募集,并且MDM2可作為E3泛素連接酶抑制p53蛋白的降解[16]。PTEN通過對(duì)磷脂酰肌醇3,4,5-三磷酸(PIP3)去磷酸化,調(diào)節(jié)細(xì)胞內(nèi)信號(hào)傳導(dǎo),參與多種腫瘤細(xì)胞的增殖、凋亡和轉(zhuǎn)移[21]。REN等[22]研究發(fā)現(xiàn)PTEN與ANKHD1相互作用抑制ANKHD1與p21啟動(dòng)子結(jié)合,從而誘導(dǎo)p21表達(dá)。PTEN失活導(dǎo)致Akt過度激活,進(jìn)而促使細(xì)胞增殖和抑制凋亡[23]。WU等[24]提出PTEN是miR-93-5p的直接靶點(diǎn),miR-93-5p可通過靶向PTEN/PI3K/Akt信號(hào)通路調(diào)節(jié)胰腺癌發(fā)生發(fā)展。
本研究結(jié)果顯示,NC組、mimics NC組及inhibitor NC組PATU 8988T細(xì)胞miR-221-3p表達(dá)水平比較,差異均無統(tǒng)計(jì)學(xué)意義(Pgt;0.05)。miR-221-3p mimics組細(xì)胞miR-221-3p表達(dá)明顯增高,miR-221-3p inhibitor組細(xì)胞miR-221-3p表達(dá)顯著下調(diào)(Plt;0.05)。流式細(xì)胞術(shù)結(jié)果顯示,miR-221-3p inhibitor組細(xì)胞凋亡明顯增加,miR-221-3p mimics組細(xì)胞凋亡明顯減少(Plt;0.05),說明miR-221-3p抑制細(xì)胞凋亡。Western blotting結(jié)果表明,miR-221-3p mimics組p53和PTEN蛋白表達(dá)較mimics NC組降低(Plt;0.05),表明miR-221-3p可能通過下調(diào)p53、PTEN蛋白表達(dá)而促進(jìn)胰腺癌發(fā)生發(fā)展。
綜上所述,胰腺癌細(xì)胞中miR-221-3p過表達(dá)可下調(diào)抑癌基因p53、PTEN表達(dá),進(jìn)而促進(jìn)胰腺癌的發(fā)生發(fā)展。本研究有望為胰腺癌的診斷和治療提供更為高效、精準(zhǔn)的手段,以滿足臨床需求,給患者帶來新的希望。
[參考文獻(xiàn)]
[1] 陳偉偉,劉承利. 循環(huán)腫瘤細(xì)胞在胰腺癌診療中的研究進(jìn)展[J]. 中國腫瘤臨床與康復(fù),2018,25(7):890-893.
[2] 蔣慧,鄭建明. 胰腺癌新輔助治療后病理評(píng)估的問題與挑戰(zhàn)[J]. 中華胰腺病雜志,2021,21(5):325-331.
[3] 賈振飛,王嘉毅,李少華. miR-151-3p在胰腺癌組織中表達(dá)及其臨床診斷價(jià)值[J]. 實(shí)用癌癥雜志,2023,38(10):1685-1690.
[4] 張堯,張鍇,康鵬東,等. 微小RNA-200c在胰腺癌中的研究進(jìn)展[J]. 臨床肝膽病雜志,2022,38(4):961-964.
[5] 黎文濤,伍小瓊,馮志鵬,等. 過表達(dá)miR-199-3p/5p誘導(dǎo)胰腺癌細(xì)胞BxPC-3、Panc-1鐵死亡的作用機(jī)制[J]. 中南醫(yī)學(xué)科學(xué)雜志,2023,51(3):344-348.
[6] 任剛,鄭吉春,張恒恒,等. 胰腺癌組織中Ki67、p53的表達(dá)及其與高劑量少分次放療生存時(shí)間的關(guān)系[J]. 現(xiàn)代腫瘤醫(yī)學(xué),2022,30(17):3156-3159.
[7] 鄧琳麗,王利. 姜黃素聯(lián)合西達(dá)本胺通過下調(diào)AKT磷酸化和上調(diào)p53表達(dá)調(diào)控SKM-1細(xì)胞增殖和凋亡[J]. 中國細(xì)胞生物學(xué)學(xué)報(bào),2021,43(8):1614-1621.
[8] 朱喜春,張永興,俞發(fā)榮. 不飽和脂肪酸對(duì)肝癌移植瘤小鼠P21、P53、P13K及PKB/Akt蛋白表達(dá)的影響[J]. 實(shí)驗(yàn)動(dòng)物科學(xué),2022,39(4):33-38.
[9] BURNETT M,RODOLICO V,SHEN F,et al. Path Visio Analysis:An Application Targeting the miRNA Network Associated with the p53 Signaling Pathway in Osteosarcoma[J].Biological cells,2021,45(1):17-26.
[10] 于江龍,阿不都拉·艾沙,楊巖,等. lncRNA-MALAT1基于p-PI3K/p-AKT通路對(duì)腦膠質(zhì)瘤細(xì)胞增殖、凋亡、侵襲的影響[J]. 實(shí)用癌癥雜志,2022,37(5):697-701.
[11] 周怡言,曾憲峰,劉鑫鑫. MiR-221靶向PTEN介導(dǎo)PI3K/AKT信號(hào)通路調(diào)節(jié)細(xì)胞自噬參與帕金森病的機(jī)制研究[J]. 腦與神經(jīng)疾病雜志,2021,29(7):411-417.
[12] 中國抗癌協(xié)會(huì)胰腺癌專業(yè)委員會(huì). 中國抗癌協(xié)會(huì)胰腺癌整合診治指南(精簡(jiǎn)版)[J]. 中國腫瘤臨床,2023,50(10):487-496.
[13] SOHRABI E,REZAIE E,HEIAT M,et al. An integrated data analysis of mRNA,miRNA and signaling pathways in pancreatic cancer[J]. Biochem Genet,2021,59(5):1326-1358.
[14] DITTMAR R L,LIU S Y,TAI M C,et al. Plasma miRNA biomarkers in limited volume samples for detection of early-stage pancreatic cancer[J]. Cancer Prev Res,2021,14(7):729-740.
[15] FU X F,ZHAO H C,YANG C L,et al. MicroRNA-203-3p inhibits the proliferation,invasion and migration of pancreatic cancer cells by downregulating fibroblast growth factor 2[J]. Oncol Lett,2021,22(2):626.
[16] 井佳瑜,董丹鳳,樊楊威,等. MDM2/MDMX-p53雙靶點(diǎn)蛋白增效厄洛替尼抗胰腺癌活性的研究[J]. 腫瘤預(yù)防與治療,2021,34(2):99-107.
[17] 郭斌,羅剛,李國灝. 腫瘤抑制因子第10號(hào)染色體上缺失與張力蛋白同源的磷酸酯酶基因通過活性氧/p38絲裂原活化蛋白激酶信號(hào)通路對(duì)前列腺癌細(xì)胞增殖和耐藥性的影響[J]. 中華實(shí)驗(yàn)外科雜志,2021,38(4):699-702.
[18] 唐春花,寧志豐,周雄飛,等. 異芒果苷通過調(diào)節(jié)PTEN-PI3K-AKt-mTOR通路抑制惡性膠質(zhì)瘤的惡性行為[J]. 云南中醫(yī)學(xué)院學(xué)報(bào),2022,45(1):65-71.
[19] KIM M P,LI X Q,DENG J,et al. Oncogenic KRAS recruits an expansive transcriptional network through mutant p53 to drive pancreatic cancer metastasis[J]. Cancer Discov,2021,
11(8):2094-2111.
[20] KIM M P,LI X,DENG J,et al. Mutant p53 and oncogenic KRAS converge on CREB1 to drive pancreatic cancer metastasis[J]. Cancer Research,2021,81(13):2417.
[21] 劉玉蘭,何鳳屏,郭偉強(qiáng),等. MicroRNA-10調(diào)控PTEN基因在促進(jìn)胰腺癌細(xì)胞增殖和遷移的作用[J]. 中國醫(yī)學(xué)創(chuàng)新,2022,19(25):29-34.
[22] REN D Y,SUN Y,LI D,et al. USP22-mediated deubiquitination of PTEN inhibits pancreatic cancer progression by inducing p21 expression[J]. Mol Oncol,2022,16(5):1200-1217.
[23] YANG Y,CHENG T,XIE P,et al. PMEPA1 interference activates PTEN/PI3K/AKT,thereby inhibiting the proliferation,invasion and migration of pancreatic cancer cells and enhancing the sensitivity to gemcitabine and cisplatin[J]. Drug Dev Res,2022,83(1):64-74.
[24] WU Y F,XU W X,YANG Y M,et al. miRNA-93-5p promotes gemcitabine resistance in pancreatic cancer cells by targeting the PTEN-mediated PI3K/akt signaling pathway[J]. Ann Clin Lab Sci,2021,51(3):310-320.
[收稿日期] 2023-12-26
(本文編輯" "趙喜)
[文章編號(hào)]1006-2440(2024)03-0229-05
[引文格式]錢靜,石宇,嚴(yán)曉娣,等. miR-221-3p對(duì)胰腺癌細(xì)胞凋亡的抑制作用及機(jī)制研究[J]. 交通醫(yī)學(xué),2024,38(3):229-233.
* [基金項(xiàng)目] 南通市科技計(jì)劃項(xiàng)目(JC22022003)。
** [作者簡(jiǎn)介] 錢靜,女,漢族,江蘇鹽城人,生于1982年2月,碩士,講師,副主任醫(yī)師,研究方向:腫瘤學(xué)。" "通信作者:卞銀珠,E-mail:bianyzyc@163.com