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. 2016 Oct 5;6:34684. doi: 10.1038/srep34684

LC-MS Based Sphingolipidomic Study on A2780 Human Ovarian Cancer Cell Line and its Taxol-resistant Strain

Hao Huang 1,2, Tian-Tian Tong 1, Lee-Fong Yau 1, Cheng-Yu Chen 1, Jia-Ning Mi 1, Jing-Rong Wang 1,a, Zhi-Hong Jiang 1,b
PMCID: PMC5050431  PMID: 27703266

Abstract

Drug resistance elicited by cancer cells continue to cause huge problems world-wide, for example, tens of thousands of patients are suffering from taxol-resistant human ovarian cancer. However, its biochemical mechanisms remain unclear. Sphingolipid metabolic dysregulation has been increasingly regarded as one of the drug-resistant mechanisms for various cancers, which in turn provides potential targets for overcoming the resistance. In the current study, a well-established LC-MS based sphingolipidomic approach was applied to investigate the sphingolipid metabolism of A2780 and taxol-resistant A2780 (A2780T) human ovarian cancer cell lines. 102 sphingolipids (SPLs) were identified based on accurate mass and characteristic fragment ions, among which 12 species have not been reported previously. 89 were further quantitatively analyzed by using multiple reaction monitoring technique. Multivariate analysis revealed that the levels of 52 sphingolipids significantly altered in A2780T cells comparing to those of A2780 cells. These alterations revealed an overall increase of sphingomyelin levels and significant decrease of ceramides, hexosylceramides and lactosylceramides, which concomitantly indicated a deviated SPL metabolism in A2780T. This is the most comprehensive sphingolipidomic analysis of A2780 and A2780T, which investigated significantly changed sphingolipid profile in taxol-resistant cancer cells. The aberrant sphingolipid metabolism in A2780T could be one of the mechanisms of taxol-resistance.

Ovarian cancer is the most aggressive gynecologic cancer and thus a leading cause of cancer-related mortality in women worldwide1. At present, the most effective strategy for ovarian cancer is combination therapy based on cytoreductive surgery and chemotherapy with taxanes (e.g. taxol), but intrinsic or acquired tumor chemoresistance remains the most important clinical problem and a major obstacle to a successful therapy2. According to a systematic literature review, 69 of the total 137 acquired drug-resistant cell lines were resistant to taxol3. Seventy-five percent of ovarian cancer patients initially respond to platinum or taxane based chemotherapy; however, most of them eventually develop chemotherapy resistance4. Many factors can lead to drug resistance, including increased drug efflux, drug inactivation, alterations in drug target, processing of drug-induced damage, and evasion of apoptosis5. Mechanisms including overexpression of drug resistant associated proteins6 and activation of some signaling pathways7 have been implicated in resistance to taxol, but the overall molecular mechanisms of taxol resistance still need further elucidation.

Sphingolipids (SPLs) are a kind of membrane and intracellular lipids that typically play structural roles and act as signaling molecules and/or modulators of signaling pathways associated with cell survival8. Besides the most widely studied bioactive SPL - ceramide, the relationship between cancer and other SPL has been extensively studied, including sphingosine 1-phosphate (S1P)9, glucosylceramide (GluCer)10, sphingosine and C1P11. Growing evidence showed that sphingolipids are deeply involved in the regulation of apoptosis as well as the apoptosis resistance that is displayed by cancer cells12. Qualitative and quantitative assessment of SPLs could reveal novel biomarkers for early diagnosis of cancer13.

There are several studies focused on the sphingolipidomics of A2780 Human Ovarian Cancer cell line14,15, as well as its fenretinide-resistant16 and multidrug-resistant strains17,18. Valsecchi M et al. have characterized the sphingolipidomes in N-(4-hydroxyphenyl)retinamide (4-HPR) and 4-oxo-N-(4-hydroxyphenyl)retinamide (4-oxo-4-HPR) treated A2780 cells by ESI-MS, revealed that the two drugs differentially affect the early steps of SPL synthesis19. In 4-HPR resistant A2780 cells (A2780/HPR), a remarkable alteration of sphingolipid metabolism with respect to both of the parental sensitive A2780 cells and 2780AD cells has been revealed20. Increasing evidence suggests the change of SPL metabolism can be (one of) the crucial mechanism of drug resistance in A2780 cells. However, till now, there is no sphingolipidomic study on taxol resistant A2780 cells (named as A2780T, TA2780, A2780/Taxol, or A2780/PTX in literature). Therefore, a comprehensive sphingolipidomic study is required for elucidating the mechanisms underlying the resistance of A2780T cells to taxol treatment.

In the current study, SPLs in A2780 and A2780T were comprehensively profiled and quantitatively determined by using a well-established LC-MS approach developed in our lab21. It appears to be a promising tool for viewing overall sphingolipidomic difference between taxol-sensitive and -resistant strain of A2780.

Results

Comprehensive identification of sphingolipids in A2780 cells

Duplicate analyses of pooled samples of A2780 and A2780T cells (QC samples) were carried out to achieve comprehensive profiling of SPLs in these two cell lines. Ultra-high performance liquid chromatography coupled with Q-TOF mass spectrometry (UHPLC-Q-TOF MS) is an effective and sensitive analytical tool to separate and identify SPLs in a complex mixture. By integrating the high efficient separation offered by UHPLC, high-resolution mass spectrum obtained by MS and MS/MS on Q-TOF, as well as comparing the data with those of reference standards and searching against our personal database, totally 102 SPLs have been identified in the pooled samples, among which six ceramides (d18:1/17:3; d18:1/15:3(OH); d18:1/14:3(OH); d18:2/23:1; d18:0/18:3 and d17:0/13:0(OH)), two ceramide-1-phosphates (d18:1/19:0(OH) and d18:1/12:2), one hexosylceramide (d18:1/20:1), and three sphingosines (d16:3; d15:3 and t19:2) are new SPLs. Sixty-seven out of the 102 SPLs were reported for the first time in A2780 cells.

MS signals might be masked by isomeric, isotopic or isobaric ions. For sphingolipidomic profiling of A2780, our improved sphingolipidomic approach showed great potential in differentiating isomeric and isotopic species as that have been observed in PC12 cells21. A major interference in the identification of SPLs is the isomeric species that have exact identical molecular elemental compositions, thus MS/MS data together with optimized separation are essential for discrimination. For instance, the extracted ion chromatogram of m/z 620.5903 at 5 ppm mass accuracy yielded two peaks at 15.894 and 16.061 min. Targeted MS/MS of m/z 620.6 at respective time points gave distinct product ions corresponding to backbone of Cer (d18:1/22:1) (m/z 264.3) and Cer (d18:2/22:0) (m/z 262.3), providing evidence for the identification of these two species (Fig. 1). The targeted ion pairs together with complete chromatographic separation also enabled subsequent quantification of such isomers by using multiple reaction monitoring (MRM) technique. Notably, 4 pairs of isomeric species (A1–A4) were clearly distinguished in our study (Table 1).

Figure 1. Differentiation of isomeric SPLs by targeted MS/MS.

Figure 1

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(a) Two peaks were observed at m/z 620.59 in extracted ion chromatogram of TOF MS. Accurate MS/MS data confirmed these peaks corresponding to Cer (d18:1/22:1) and Cer (18:2/22:0) due to the characteristic fragment at 264.3 and 262.3 respectively. In MRM mode, target ion pairs consist of the same parent ion (620.6) but different daughter ions [(b) 264.3 for Cer (d18:1/22:1) and (c) 262.3 for Cer (18:2/22:0)] were employed for the accurate quantitation.

Table 1. Identification and quantification of SPLs in A2780/A2780T cells by using UHPLC-Q-TOF and UHPLC-QQQ MS.

Class Name [M + H]+ m/z tR (min) Molecular Formula Measured Mass Calculated Mass Error (ppm) MS/MS Fragments (m/z) MRM transitions
SM d18:1/26:0 843.7315 18.391 C49 H99 N2 O6 P 842.7243 842.7241 0.23 264.2693, 184.0732 843.8 184.1
d18:1/26:1 841.7088 17.057 C49 H97 N2 O6 P 840.7046 840.7084 −4.52 264.2692, 184.0735 841.7 184.1
d18:1/25:0 829.7152 17.641 C48 H97 N2 O6 P 828.7085 828.7084 0.14 264.2638, 184.0733 829.7 184.1
d18:1/25:1 827.6987 16.441 C48 H95 N2 O6 P 826.6898 826.6928 −3.59 264.2655, 184.0736 827.7 184.1
d18:1/24:0 815.7009 16.974 C47 H95 N2 O6 P 814.6936 814.6928 1.01 264.2668, 184.0730 815.7 184.1
d18:1/24:1 [A1] 813.6851 15.807 C47 H93 N2 O6 P 812.6780 812.6771 1.01 264.2697, 184.0735 813.7 184.1
d18:1/24:2 811.6692 14.907 C47 H91 N2 O6 P 810.6618 810.6615 0.35 264.2702, 184.0734 811.7 184.1
d18:1/24:3 809.6528 14.173 C47 H89 N2 O6 P 808.6456 808.6458 −0.3 264.2669, 184.0732 809.6 184.1
d18:1/23:0 801.6848 16.324 C46 H93 N2 O6 P 800.6774 800.6771 0.37 264.2674, 184.0731 801.7 184.1
d18:1/23:1 [A2] 799.6689 15.174 C46 H91 N2 O6 P 798.6615 798.6615 0.01 282.2457, 264.2695, 184.0731 799.7 184.1
d18:1/23:2 797.6507 14.273 C46 H89 N2 O6 P 796.6436 796.6458 −2.84 264.2667, 184.0731 797.6 184.1
d18:1/22:0 787.6692 15.674 C45 H91 N2 O6 P 786.6617 786.6615 0.32 264.2655, 184.0733 787.7 184.1
d18:1/22:1 [A3] 785.6533 14.574 C45 H89 N2 O6 P 784.6455 784.6458 −0.45 264.2688, 184.0731 785.7 184.1
d18:1/22:2 783.6374 13.690 C45 H87 N2 O6 P 782.6291 782.6302 −1.32 264.2700, 184.0726 783.7 184.1
d18:1/21:0 773.6527 15.057 C44 H89 N2 O6 P 772.6453 772.6458 −0.68 264.2674, 184.0729 773.7 184.1
d18:1/21:1 771.6340 14.123 C44 H87 N2 O6 P 770.6270 770.6302 −4.13 264.2679, 184.0728 771.7 184.1
d18:1/20:0 759.6372 14.390 C43 H87 N2 O6 P 758.6300 758.6302 −0.21 264.2734, 184.0731 759.7 184.1
d18:1/19:0 745.6213 13.757 C42 H85 N2 O6 P 744.6138 744.6145 −0.95 264.2689, 184.0726 745.7 184.1
d18:1/18:0 731.6068 13.140 C41 H83 N2 O6 P 730.6005 730.5989 2.16 264.2678, 184.0731 731.6 184.1
d18:1/18:1 729.5906 12.323 C41 H81 N2 O6 P 728.5827 728.5832 −0.75 264.2699, 184.0732 729.6 184.1
d18:1/17:0 717.5911 12.573 C40 H81 N2 O6 P 716.5841 716.5832 1.17 264.2622, 184.0731 717.6 184.1
d18:1/16:0 703.5754 12.023 C39 H79 N2 O6 P 702.5684 702.5676 1.11 264.2694, 184.0731 703.6 184.1
d18:1/16:1 701.5604 11.323 C39 H77 N2 O6 P 700.5530 700.5519 1.53 264.2645, 184.0732 701.6 184.1
d18:1/15:0 689.5595 11.573 C38 H77 N2 O6 P 688.5521 688.5519 0.27 264.2750, 184.0732 689.6 184.1
d18:1/14:0 675.5427 11.190 C37 H75 N2 O6 P 674.5369 674.5363 0.96 264.2676, 184.0732 675.5 184.1
d18:2/24:0 [A1] 813.6848 16.074 C47 H93 N2 O6 P 812.6774 812.6771 0.33 262.2513, 184.0730 813.7 184.1
d18:2/24:3 807.6350 14.590 C47 H87 N2 O6 P 806.6291 806.6302 −1.29 262.2523, 184.0731 807.6 184.1
d18:2/23:0 [A2] 799.6690 15.424 C46 H91 N2 O6 P 798.6615 798.6615 0.09 262.2512, 184.0732 799.7 184.1
d18:2/22:0 [A3] 785.6531 14.757 C45 H89 N2 O6 P 784.6458 784.6458 −0.07 262.2620, 184.0730 785.7 184.1
d18:2/20:0 757.6213 13.490 C43 H85 N2 O6 P 756.6138 756.6145 −0.93 262.2523, 184.0730 757.7 184.1
d18:2/15:0 687.5433 10.906 C38 H75 N2 O6 P 686.5355 686.5363 −1.17 262.2503, 184.0716 687.5 184.1
d18:1/12:0 [IS-1] 647.5133 10.389 C35 H71 N2 O6 P 646.5058 646.505 1.32 264.2699, 184.0732 647.5 184.1
DHSM d18:0/25:0 831.7299 18.324 C48 H99 N2 O6 P 830.7233 830.7241 −0.89 184.0745    
d18:0/24:0 817.7156 17.524 C47 H97 N2 O6 P 816.7082 816.7084 −0.3 184.0726 817.7 184.1
d18:0/23:0 803.6991 16.874 C46 H95 N2 O6 P 802.6919 802.6928 −1.14 266.2781, 184.0724 803.7 184.1
d18:0/22:0 789.6843 16.207 C45 H93 N2 O6 P 788.6769 788.6771 −0.31 184.0734 789.7 184.1
d18:0/20:0 761.6530 14.907 C43 H89 N2 O6 P 760.6456 760.6458 −0.25 184.0724 761.7 184.1
d18:0/19:0 747.6388 14.373 C42 H87 N2 O6 P 746.6310 746.6302 1.08 184.0727 747.6 184.1
d18:0/18:0 733.6222 13.64 C41 H85 N2 O6 P 732.6176 732.6145 4.15 184.0731 733.7 184.1
d18:0/17:0 719.5692 10.673 C39 H79 N2 O7 P 718.5617 718.5625 −1.06 184.0725 719.6 184.1
d18:0/16:0 705.5915 12.457 C39 H81 N2 O6 P 704.5843 704.5832 1.58 184.0735 705.6 184.1
d18:0/15:0 691.5747 11.940 C38 H79 N2 O6 P 690.5685 690.5676 1.35 184.0731 691.6 184.1
d18:0/14:0 677.5576 11.523 C37 H77 N2 O6 P 676.5519 676.5519 0.01 184.0729 677.5 184.1
t18:0/16:0 721.5839 11.423 C39 H81 N2 O7 P 720.5769 720.5781 −1.74 264.2685, 184.0719 721.6 184.1
Cer d18:1/24:0 650.6451 18.458 C42 H83 N O3 649.6379 649.6373 0.86 632.6290, 614.6156, 264.2683 650.7 264.3
d18:1/24:1 648.6293 17.157 C42 H81 N O3 647.6220 647.6216 0.62 630.6170, 612.6100, 264.2690 648.7 264.3
d18:1/24:2 646.6124 16.224 C42 H79 N O3 645.6058 645.6060 −0.25 264.2703 646.7 264.3
d18:1/23:0(OH) 652.6250 16.194 C41 H81 N O4 651.6185 651.6166 0.94 264.2697 652.7 264.3
d18:1/23:0 636.6288 17.707 C41 H81 N O3 635.6213 635.6216 −0.48 264.2689 636.6 264.3
d18:1/23:1 634.6135 16.507 C41 H79 N O3 633.6055 633.6060 −0.73 264.2670 634.6 264.3
d18:1/22:0 622.6136 17.007 C40 H79 N O3 621.6077 621.6060 2.77 264.2700 622.6 264.3
d18:1/22:1 [A4] 620.5983 15.874 C40 H77 N O3 619.5893 619.5903 −1.74 264.2684 620.5 264.3
d18:1/20:0 594.5808 15.600 C38 H75 N O3 593.5735 593.5747 −1.22 264.2674 594.6 264.3
d18:1/18:0 566.5521 14.340 C36 H71 N O3 565.5440 565.5434 1.15 264.2681 566.6 264.3
d18:1/18:1 564.5332 13.167 C36 H69 N O3 563.5251 563.5277 −4.62 264.2660 564.5 264.3
d18:1/17:3 546.4890 10.890 C35 H63 N O3 545.4819 545.4808 2.1 264.2701    
d18:1/16:0 538.5198 13.057 C34 H67 N O3 537.5124 537.5121 0.57 264.2684 538.5 264.3
d18:1/16:1 536.5045 12.243 C34 H65 N O3 535.4974 535.4964 1.74 264.2694 536.6 264.3
d18:1/15:3(OH) 534.4521 13.906 C33 H59 N O4 533.4449 533.4444 0.98 516.4403, 264.2706 534.5 264.3
d18:1/14:3(OH) 520.4367 13.473 C32 H57 N O4 519.4297 519.4288 1.81 502.4256, 264.2679 520.4 264.3
d18:2/23:1 632.5940 15.657 C41 H77 N O3 631.5885 631.5903 −2.86 262.2520 632.6 262.3
d18:2/22:0 [A4] 620.5979 16.074 C40 H77 N O3 619.5906 619.5903 0.45 602.5864, 262.2610 620.5 262.3
d17:1/16:0 524.5045 12.477 C33 H65 N O3 523.4971 523.4964 1.27 250.2520 524.5 250.3
d18:1/12:0 [IS-2] 482.4574 10.990 C30 H59 N O3 481.4501 481.4495 1.3 264.2678 482.5 264.3
DHCer d18:0/24:0 652.6607 19.112 C42 H85 N O3 651.6533 651.6529 0.57 634.6377, 266.2767 652.7 266.3
d18:0/18:3 562.5197 15.924 C36 H67 N O3 561.5122 561.5121 0.25 266.2797    
d18:0/16:0 540.5354 13.507 C34 H69 N O3 539.5273 539.5277 −0.86 266.2833 540.5 266.3
d18:0/17:0(OH) 570.5458 13.408 C35 H71 N O4 569.5383 569.5383 0.05 266.2642    
d18:0/16:0(OH) 556.5300 12.373 C34 H69 N O4 555.5227 555.5227 0.05 266.2857    
d18:0/14:0(OH) 528.4993 11.256 C32 H65 N O4 527.4921 527.4914 1.49 266.2831    
d17:0/13:0(OH) 500.4682 11.323 C30 H61 N O4 499.4608 499.4601 1.5 252.2673    
C1P d18:1/19:0(OH) 676.5279 11.823 C37 H74 N O7 P 675.5200 675.5203 −0.37 264.2673 676.5 264.3
d18:1/12:2 558.3904 9.573 C30 H56 N O6 P 557.3832 557.3845 −2.44 264.2677    
d18:1/12:0 [IS-3] 562.4223 10.006 C30 H60 N O6 P 561.4149 561.4158 −1.72 264.2688 562.5 264.3
HexCer d18:1/26:0 840.7282 18.374 C50 H97 N O8 839.7213 839.7214 −0.11 264.2774 840.7 264.3
d18:1/24:0 812.6975 16.957 C48 H93 N O8 811.6901 811.6901 −0.07 632.6302, 264.2684 812.7 264.3
d18:1/24:1 810.6800 15.807 C48 H91 N O8 809.6728 809.6745 −2.01 630.6137, 264.2676 810.6 264.3
d18:1/23:0 798.6816 16.307 C47 H91 N O8 797.6738 797.6745 −0.85 618.6106, 264.2682 798.7 264.3
d18:1/22:0 784.6656 15.674 C46 H89 N O8 783.6580 783.6588 −1.02 604.6012, 264.2689 784.7 264.3
d18:1/20:1 754.6178 13.173 C44 H83 N O8 753.6105 753.6119 −1.77 264.2691 754.6 264.3
d18:1/16:0 700.5722 12.040 C40 H77 N O8 699.5645 699.5649 −0.6 264.2694 700.6 264.3
d18:1/12:0 [IS-4] 644.5101 10.406 C36 H69 N O8 643.5028 643.5023 0.78 264.2684 644.5 264.3
LacCer d18:1/24:0 974.7501 16.324 C54 H103 N O13 973.7440 973.7429 1.05 794.6828, 614.6288, 264.2674 974.8 264.3
d18:1/24:1 972.7323 15.19 C54 H101 N O13 971.7247 971.7273 −2.65 264.2694 972.8 264.3
d18:1/22:0 946.7175 15.057 C52 H99 N O13 945.7100 945.7116 −1.7 766.3514, 264.2723 946.8 264.3
d18:1/20:0 918.6891 13.990 C50 H95 N O13 917.6815 917.6803 1.22 264.2665    
d18:1/18:0 890.6541 12.660 C48 H91 N O13 889.6463 889.6490 −3.07   890.7 264.3
d18:1/16:0 862.6245 11.623 C46 H87 N O13 861.6175 861.6177 −0.26 844.6130, 520.5112, 502.4944, 264.2688 862.7 264.3
d18:1/12:0 [IS-5] 806.5624 10.189 C42 H79 N O13 805.5549 805.5551 −0.26 464.4472, 264.2683 806.7 264.3
Sa d19:0 316.3202 6.532 C19 H41 N O2 315.3141 315.3137 1.23 298.3106, 280.2984 316.3 298.3
d16:0 274.2742 4.842 C16 H35 N O2 273.2669 273.2668 0.43 256.2604 274.3 256.3
t19:0 332.3166 6.691 C19 H41 N O3 331.3093 331.3086 1.87 314.3032, 296.2776 332.3 314.3
t17:0 304.2855 5.841 C17 H37 N O3 303.2782 303.2773 2.68 286.2729    
Capnine 352.2519 5.075 C17 H37 N O4 S 351.2445 351.2443 0.47 316.1840    
Enigmol 302.3050 6.765 C18 H39 N O2 301.2978 301.2981 −1.66 284.2934 302.4 284.3
Xestoaminol C 230.2481 2.060 C14 H31 N O 229.2408 229.2406 1.01 212.2380 230.3 212.3
d17:0 [IS-6] 288.2901 6.632 C17 H37 N O2 287.2829 287.2824 1.65 270.2794 288.3 270.3
So d19:1 314.3056 10.673 C19 H39 N O2 313.2983 313.2981 0.69 296.3320 314.3 296.3
d18:1 300.2898 6.741 C18 H37 N O2 299.2824 299.2824 −0.08 282.2780, 264.2669 300.3 282.3
d16:1 272.2582 5.418 C16 H33 N O2 271.2505 271.2511 −2.36 254.2849 272.3 254.2
d16:3 268.2277 6.458 C16 H29 N O2 267.2203 267.2198 1.82 250.2529, 238.2509    
d15:3 254.2117 4.358 C15 H27 N O2 253.2045 253.2042 1.08 236.1994
t19:1 330.3009 6.405 C19 H39 N O3 329.2936 329.293 1.9 312.3288 330.3 312.3
t19:2 328.2852 8.274 C19 H37 N O3 327.2777 327.2773 1.13 310.3129 328.3 310.3
t18:1 316.2850 6.874 C18 H37 N O3 315.2776 315.2773 0.88 298.2911 316.3 298.3
t17:1 302.2696 7.024 C17 H35 N O3 301.2624 301.2617 2.21 284.2947 302.3 284.3
N,N-dimethylSo 328.3210 9.473 C20 H41 N O2 327.3136 327.3137 −0.24 310.3091 328.3 310.3
Halaminol A 228.2319 6.891 C14 H29 N O 227.2246 227.2249 −1.17 210.0276 228.2 210.0
d17:1 [IS-7] 286.3106 6.558 C18 H39 N O 285.3034 285.3032 0.74 268.2643 286.3 268.3
Sa1P d17:0 [IS-8] 368.2574 6.774 C17 H38 N O5 P 367.2504 367.2488 4.57   368.3 270.3
So1P d17:1 [IS-9] 366.2406 6.558 C17 H36 N O5 P 365.2331 365.2331 0.03 250.2510 366.2 250.3
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SM, sphingomyelin; DHSM, dihydrosphingomyelin; Cer, Ceramide; DHCer, dihydroceramide; HexCer, hexosylceramide; LacCer, lactosylceramide; Sa, sphinganine; So, sphingosine; C1P, ceramide-1-phosphate; Sa1P, sphinganine-1-phosphate; So1P, sphingosine-1-phosphate. [A1–A4], 4 pairs of isomeric sphingolipids; [IS], internal standard.

The comprehensive profiling of SPLs provided an overall “picture” of the sphingolipidome of A2780 cells. Generally, sphingomyelin (SM) is the most abundant subclass of SPLs in this cell line. Totally 43 SMs, including 31 dehydrosphingomyelins and 12 dihydrosphingomyelins (DHSMs), were identified based on exact mass and characteristic product ions obtained in targeted MS/MS experiments, 31 of which are reported for the first time in A2780 cell line. All the SMs were found to possess a C18 sphingoid base chain, with d18:1 account for the majority, comparing to the d18:0 and d18:2 backbones. The length of N-acyl chain varies from 14 to 26, and the unsaturation degree ranges from 0 to 5. Notably, the N-acyl chains of all the 12 DHSMs are fully saturated. Two highly unsaturated (total unsaturation degree no less than 4) SMs, SM (d18:1/24:3) and SM (d18:2/24:3), have been detected in A2780 cells for the first time.

In A2780 cells, 26 Cers, including 19 dehydroceramides and 7 dihydroceramides (DHCers), were identified based on the MS information and, in some cases, by comparing the retention time with that of SPLs in PC12 cells in our previous study21. Most Cers detected in the sample were with a d18:1 sphingoid backbone, with carbon number of N-acyl chain ranged from 14 to 24. Three dehydroceramides and 4 DHCers with a hydroxyl group on N-acyl chain have been characterized, among which 2 dehydroceramides and 1 DHCer with short N-acyl chain (carbon number less than 16) were reported for the first time to the best of our knowledge. The other 3 new Cers were species with high degree of unsaturation, for instance, Cer (d18:1/17:3), Cer (d18:2/23:1) and a new DHCer (d18:0/18:3). A notable ceramide was DHCer (d17:0/13:0(OH)), which was a very uncommon DHCer with odd carbon number sphingoid backbone.

Due to the limitation of chromatographic separation, galactosylceramide and glucosylceramide cannot be distinguished, thus these two hexose-linked ceramides were represented as HexCer. All C1P, HexCer, and lactosylceramide (LacCer) species exclusively bared a d18:1 sphingoid base backbone. The dominant HexCers and LacCers are d18:1/24:1, d18:1/24:0 and d18:1/16:0. Two novel C1Ps, i.e. C1P (d18:1/19:0(OH)) and C1P (d18:1/12:2), were identified in A2780. The former one has an N-acyl chain with odd carbon number and a hydroxyl group, while the latter one has two degrees of unsaturation on the N-acyl fatty chain.

Eighteen sphingoid bases with carbon number ranging from 14 to 20 were successfully identified. Short chain sphingosines with high unsaturation degree (d16:3 & d15:3) and a sphingosine with 3 hydroxyl groups (t19:2) have been discovered as uncommon species.

Quantitative profiling of sphingolipidome in A2780 cells

Comparing to routine LC-MS based approaches, UHPLC coupled with QQQ mass spectrometer in MRM mode provides more sensitive and accurate quantification with wider dynamic range of SPLs. However, the quantification of SPLs cannot be accomplished accurately in LC-MS/MS analysis with a QQQ analyzer solely, as triple-quadruple cannot distinguish isotopic/isobaric ions within 0.1 Da when selecting the precursor ions. For instance, each unsaturated SPL could generate an isotopic interference on SPLs with less degree of unsaturation as exemplified by SM (d18:1/14:0) and SM (d18:0/14:0) (Fig. 2). In our approach, based on foregoing comprehensive profiling by Q-TOF and the optimized chromatographic separation, all the structurally similar SPLs were accurately quantified with elimination of such isotopic/isobaric interference. With the optimized MRM conditions, 89 species from 9 subclasses out of 102 identified SPLs were quantified by using the UHPLC-QQQ MS method. It was found that A2780 and A2780T share most common sphingolipid molecules, except for HexCer (d18:1/20:1) which is only present in A2780. The amounts of SPLs were measured by using the internal standards previously mentioned, duplicate measurements for each sample yielded consistent results in all cases.

Figure 2. Differentiation of isotopic SPLs by accurate MS together with complete separation.

Figure 2

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(a) SM (d18:1/14:0) (tR = 11.190 min) yields precursor ions at m/z 675.5427, 676.5459 and 677.5484. The last one is the [M + 2] isotopic ion which will interfere with the precursor ion of (b) SM (d18:0/14:0) (tR = 11.523 min) at m/z 677.5576. If the chromatograph cannot separate the two SPLs completely, the quantification result of SM (d18:0/14:0) will be artificially high.

The quantitative results showed that SMs take the highest proportion of all the SPLs, among which SMs with C18 sphingoid base backbone are the dominant species (Fig. 3). In A2780 cells the most dominant species are SM (d18:1/16:0) which corresponding to [M + H]+ at m/z 703, followed by DHSM (d18:0/16:0) (m/z 705), SM (d18:1/16:1) (m/z 701) with less relative abundance. The d18:1 SMs with C16/C18/C22/C24 N-acyl chain showed relative high levels in both A2780 and A2780T. Forty-two out of the 43 SMs were quantified except for DHSM (d18:0/25:0), whose content is lower than the limit of quantitation (LOQ).

Figure 3. Content of marker sphingomyelins in A2780 and A2780T.

Figure 3

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The horizontal and depth axes represent the composition of fatty acid acyl chain and sphingoid base backbone chain, respectively.

A total of 20 Cers have been quantified, but most of them are d18:1 species due to the weak intensity of d18:0 backbone fragment ions (m/z 266.4). According to the finding of Koyanagi et al. in tumors only the content of C16 N-acyl chain ceramide (C16-Cer) are significantly high22, that can explain why other DHCers cannot be quantified exactly. As shown in Fig. 4, the content of individual Cers differ dramatically (at most 500 times), for some common species like d18:1/18:1, d18:1/24:0 and d18:1/24:1, the contents are significantly higher than that of highly unsaturated species d18:1/24:2. In general, the amounts of Cers are significantly higher in A2780 than those in A2780T.

Figure 4. Content of marker ceramides in A2780 and A2780T.

Figure 4

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The horizontal and depth axes represent the composition of fatty acid acyl chain and sphingoid base backbone chain, respectively.

Among all the HexCers and LacCers, only d18:1 sphingoid base backbone type was found. All the 12 HexCers and LacCers showed higher intensity in A2780 cells than that in A2780T. Sphinganine, as the precursor of DHCer, showed decrease in A2780T. The overall content of sphingosines was similar in both cell types, but the expression of individual sphingosine was quite different. Higher level of Cer1P (d18:0/20:0) was detected in A2780T (data not shown). Figure 5 showed the trends of all 7 marker HexCers and LacCers between A2780 and A2780T.

Figure 5. Content of marker HexCer and marker LacCer in A2780 and A2780T.

Figure 5

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The horizontal axis represents the composition of fatty acid acyl chain of the d18:1 HexCer and d18:1 LacCer.

Multivariate analyses of the sphingolipidomic data

Multivariate analyses were further carried out to view the overall differences between A2780 and A2780Ts, and to identify SPL markers that were significantly changed in A2780T. PLS-DA was used to visualize general clustering among A2780, A2780T, and QC groups firstly (Fig. 6A). After auto scaling of data sets, discrimination feature between the profiles were identified for each model by displaying loadings plots. Loading plots and VIP value in PLS-DA model are commonly used for biomarker selection and identification. According to the results, potential SPL markers that were differentially expressed between A2780 and A2780T groups were identified (Fig. 6B and Table 2), suggesting a SPL alternation was involved in A2780T. A total of 52 potential biomarkers were identified according to the VIP value and scattering-plot, among which most of them are sphingomyelins, several highly unsaturated SPLs [SM (d18:1/24:3), SM (d18:1/24:2) and SM (d18:1/22:2)] were also included. LacCer (d18:1/24:1) showed the largest decline in A2780T, whose content decreased by approximately 70 folds, which contributes most significantly to the classification.

Figure 6.

Figure 6

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Partial least squares discriminant analysis projecting scatter plots (A) scores plot (A2780 & A2780T group, n = 20; QC, n = 10), (B) loading plot. Red boxes represent the SPLs which contribute significantly to the classification (VIP > 1).

Table 2. Quantification of SPLs (VIP > 1) in A2780 and A2780T.

SPLs Content (pmol/5*105 cells) Change A2780T vs A2780 p value VIP
A2780 (n = 10) A2780T (n = 10)
SM(d18:2/23:0) 1.91 ± 0.06 1.15 ± 0.03 ↓ <0.001 1.09801
SM(d18:2/22:0) 5.68 ± 0.16 3.19 ± 0.09 ↓ <0.001 1.13525
SM(d18:2/20:0) 1.14 ± 0.03 0.30 ± 0.02 ↓ <0.001 1.16777
SM(d18:1/26:1) 1.23 ± 0.04 0.57 ± 0.02 ↓ <0.001 1.13507
SM(d18:1/26:0) 0.39 ± 0.01 0.85 ± 0.02 ↑ <0.001 1.15741
SM(d18:1/25:1) 1.09 ± 0.03 0.81 ± 0.02 ↓ <0.001 1.01815
SM(d18:1/25:0) 0.69 ± 01 1.16 ± 0.02 ↑ <0.001 1.11518
SM(d18:1/24:3) 0.85 ± 0.04 0.21 ± 0.01 ↓ <0.001 1.14943
SM(d18:1/24:2) 22.5 ± 0.42 4.00 ± 0.09 ↓ <0.001 1.18383
SM(d18:1/24:1) 94.6 ± 1.80 22.5 ± 0.65 ↓ <0.001 1.18238
SM(d18:1/24:0) 35.6 ± 0.64 55.1 ± 1.33 ↑ <0.001 1.13437
SM(d18:1/23:2) 0.72 ± 0.02 0.04 ± 0.00 ↓ <0.001 1.15961
SM(d18:1/23:1) 8.43 ± 0.18 1.27 ± 0.04 ↓ <0.001 1.18228
SM(d18:1/22:2) 0.49 ± 0.03 0.08 ± 0.01 ↓ <0.001 1.13820
SM(d18:1/22:1) 8.00 ± 0.16 0.84 ± 0.03 ↓ <0.001 1.18362
SM(d18:1/21:1) 0.32 ± 0.01 0.05 ± 0.00 ↓ <0.001 1.14304
SM(d18:1/21:0) 1.60 ± 0.04 0.62 ± 0.02 ↓ <0.001 1.16306
SM(d18:1/20:0) 3.89 ± 0.09 2.02 ± 0.07 ↓ <0.001 1.15126
SM(d18:1/19:0) 0.54 ± 0.03 0.15 ± 0.01 ↓ <0.001 1.11029
SM(d18:1/18:1) 31.7 ± 0.03 5.77 ± 0.01 ↓ <0.001 1.18432
SM(d18:1/18:0) 28.7 ± 0.48 7.54 ± 0.31 ↓ <0.001 1.18301
SM(d18:1/17:0) 4.63 ± 0.12 7.27 ± 0.20 ↑ <0.001 1.11640
SM(d18:1/16:1) 27.3 ± 0.47 40.1 ± 0.91 ↑ <0.001 1.12871
SM(d18:1/16:0) 431 ± 20.9 606 ± 40.4 ↑ <0.001 1.11932
SM(d18:1/15:0) 20.0 ± 0.32 14.1 ± 0.26 ↓ <0.001 1.14185
SM(d18:1/14:0) 20.9 ± 0.28 15.6 ± 0.27 ↓ <0.001 1.13645
DHSM(d18:0/24:0) 0.54 ± 0.01 4.96 ± 0.13 ↑ <0.001 1.18082
DHSM(d18:0/23:0) 0.11 ± 0.00 0.81 ± 0.02 ↑ <0.001 1.17627
DHSM(d18:0/22:0) 0.74 ± 0.02 3.35 ± 0.08 ↑ <0.001 1.17865
DHSM(d18:0/20:0) 0.27 ± 0.01 0.49 ± 0.02 ↑ <0.001 1.04670
DHSM(d18:0/18:0) 1.06 ± 0.03 1.87 ± 0.06 ↑ <0.001 1.12766
DHSM(d18:0/16:0) 30.1 ± 0.70 198 ± 5.28 ↑ <0.001 1.17911
DHSM(d18:0/14:0) 0.72 ± 0.02 2.59 ± 0.08 ↑ <0.001 1.17037
DHSM(t18:0/16:0) 0.11 ± 0.01 0.27 ± 0.03 ↑ <0.001 1.07421
Cer(d18:2/22:0) 1.07 ± 0.08 0.23 ± 0.04 ↓ <0.001 1.09314
Cer(d18:1/24:2) 0.24 ± 0.03 0.02 ± 0.00 ↓ <0.001 1.02636
Cer(d18:1/24:1) 14.6 ± 0.59 1.60 ± 0.07 ↓ <0.001 1.17972
Cer(d18:1/24:0) 14.6 ± 0.27 5.97 ± 0.20 ↓ <0.001 1.17416
Cer(d18:1/23:1) 0.26 ± 0.03 0.02 ± 0.00 ↓ <0.001 1.02345
Cer(d18:1/23:0) 1.02 ± 0.04 0.42 ± 0.03 ↓ <0.001 1.12215
Cer(d18:1/23:0(OH)) 0.48 ± 0.03 0.22 ± 0.02 ↓ <0.001 1.02341
Cer(d18:1/22:0) 2.23 ± 0.10 1.16 ± 0.08 ↓ <0.001 1.06470
Cer(d18:1/18:1) 6.39 ± 0.23 4.09 ± 0.25 ↓ <0.001 1.01738
Cer(d18:1/18:0) 1.62 ± 0.13 0.20 ± 0.04 ↓ <0.001 1.10112
Cer(d17:1/16:0) 2.24 ± 0.12 0.47 ± 0.06 ↓ <0.001 1.13502
HexCer(d18:1/24:0) 11.7 ± 0.40 2.70 ± 0.18 ↓ <0.001 1.16555
HexCer(d18:1/23:0) 1.14 ± 0.40 0.21 ± 0.18 ↓ <0.001 1.04684
HexCer(d18:1/22:0) 2.89 ± 0.11 0.85 ± 0.11 ↓ <0.001 1.12835
HexCer(d18:1/16:0) 19.3 ± 0.67 6.11 ± 0.49 ↓ <0.001 1.15137
LacCer(d18:1/24:1) 5.48 ± 0.43 0.08 ± 0.01 ↓ <0.001 1.12942
LacCer(d18:1/24:0) 4.49 ± 0.36 0.84 ± 0.12 ↓ <0.001 1.09150
LacCer(d18:1/16:0) 7.59 ± 0.45 1.93 ± 0.17 ↓ <0.001 1.11980
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Discussion

In order to drive study on the metabolism of sphingolipids, a reliable and informative analytical method for the comprehensive profiling of SPLs is essential. By using a combined analytical strategy, which enables the reliable identification and sensitive quantification, the dynamic distribution and interconversion of SPLs have been comprehensively monitored. Our improved sphingolipidomic analyses on A2780 and A2780T encompassed most of the important SPLs including sphinganine, sphingosine, ceramide-1-phosphase, hexosylceramide, lactosylceramide, dehydroceramide and dihydroceramide as well dehydrosphingomyelin and dihydrosphingomyelin subclasses. It is the most comprehensive sphingolipidomic study on A2780/A2780T cells to date, as evidenced by the identification of up to 102 SPLs including 67 species that are reported in the cell line for the first time. Distinguished from previous studies, this research of SPL took advantage of a well-established LC-MS method, and looked into the content variation of individual SPL species instead of the overall content of each subclass, thus provided much more detailed and useful information for revealing the mechanism of taxol-resistance. Most of the identified SPLs can be the metabolic pathway related biomarkers, especially the low abundance species of which the subtle changes may result in altered biological function like drug resistance23. It’s noted that all the rare SPLs (odd number of carbons/high level of unsaturation) in A2780/A2780T are with the low abundance. Similarly, SPLs with odd number of carbons (C15 and C17) have been reported24, and highly unsaturated SPLs were isolated from halotolerant fungus with poor natural abundance25. Even in A2780 cell line, Cer with C23 and C25 N-acetyl chain have already been found19. Discovery of these rare SPL species is one of the research highlights of this study.

Comparing to taxol sensitive A2780 cells, the most notable alteration in A2780T cells was the overall decrease of Cers. Eleven Cers are recognized as biomarkers of A2780T, along with a 1.5 to 13-fold decrease has been observed. Cer is known as an intracellular messenger that is able to regulate many intracellular effectors mediating activation of the apoptotic process. It has been recognized as a kind of tumor suppressor and has been found to act as a major player in the action of many chemotherapeutic drugs26. Dysregulated metabolism of Cers has been identified as a feature of many drug-resistant cancers27, as well as in taxol resistant human ovarian cancer cell line CABA-PTX28. In A2780T cells the depletion of Cers could potentially help the cells to evade Cer-induced apoptosis, and thereby can be a crucial mechanism responsible for the drug resistance of A2780T.

Totally 34 SMs (including 26 dehydrosphingomyelins and 8 DHSMs) were identified as biomarkers, which took most proportion of the biomarkers. Among all the marker SMs, the content of 6 dehydrosphingomyelins increased in taxol-resistant cells compared to sensitive cells by 0.4–1.2 times. Especially, C16-SMs, a kind of high abundance SPL in both A2780 and A2780T, were found to be significantly higher in the taxol resistant cells than those in the sensitive cells. This leads to the increased total SM level in A2780T, same as previously reported in 2780AD cells29. However, the other 20 dehydrosphingomyelin biomarkers decreased by up to 90%. Individually, the content of all d18:2 SMs, d18:1 SMs with unsaturated double bond(s) at the N-acyl chain, as well as d18:1 SMs with saturated N-Acyl chain of C18 to 23 decreased significantly in A2780T comparing to that of A2780. Whilst the content of d18:1 SMs with saturated N-Acyl chain of C16–C17 and C24–C26, as well as all DHSMs, were found to increase significantly in A2780T comparing to A2780. Of note, the increase of DHSMs in A2780T is high up to 8-fold for most species. Dihydrosphingolipids have received increasing attention. Wang et al. have determined that 4-HPR treated MDR cancer cells displayed elevations in DHCer but not dehydroceramides, together with elevated DHSM species rather than dehydrosphingomyelins30. It indicates that dihydrosphingolipids may fulfil a distinctive role in the metabolic pathway comparing to unsaturated sphingolipids. In A2780T, significant increase of dehydrosphingomyelins and DHSMs concomitant with decrease of corresponding DHCers (which was not identified as markers) have been observed. These variations are consistent with the hypothetical “DHCer - DHSM - dehydrosphingomyelin” pathway, and the activity of related enzymes (dihydroceramide desaturase and dihydroceramide synthase) may be altered31.

Cer plays a central role in the sphingolipid metabolism. All the Cers showed consistent trend of decrease in A2780T, except for some extremely low species (DHCers) whose content cannot meet the limit of quantitation. The overall decrease of Cers and accompanying increase of most SMs in A2780T cells, especially, the decrease of two most abundant Cers [Cer (d18:1/24:0) and Cer (d18:1/16:0)] and concomitant increase of corresponding species of SMs [SM (d18:1/24:0) and SM (d18:1/16:0)], clearly indicated SM-related depletion of Cers in A2780T cells. The roles of SMase and SMS in cancer treatment have been well recognized for decades. Their actions have been defined as one of the main routes for the alteration of Cer8. Sphingomyelinases are key enzymes of sphingolipid metabolism that regulate the formation and degradation of ceramide32. Drugs (including taxol) enhanced ceramide-governed cytotoxic response by activating sphingomyelinase27. While SM is the end product in the SM-Cer related pathway, and inhibition of SMS will result in Cer accumulation with effect solely on SM33. Thus, it can be proposed that in A2780T cells, the decreased level of Cers might be resulted from the down-regulated expression/activity of SMase or up-regulated SMS expression/activity. Similar mechanism has been reported that a decrease of the ceramide level via activation of glucosylceramide synthase (GCS) and SMS was detected in chemoresistant HL-60/ADR human promyelocytic leukemia cells34.

Besides Cer and SM, other SPLs and SPL metabolites also have biological activities that could be responsible for the acquisition of a drug resistance phenotype35. Ceramide glycosylation by the enzyme glucosylceramide synthase, which forms glucosylceramide and has been noted in some drug-resistant cell lines, is an important pathway for bypassing apoptosis36,37. Because SPLs comprised of d18:1 sphingosine backbone are the major species found in mammals38, in A2780T only HexCer and LacCer with d18:1 backbone can be detected and further quantified as markers. Additionally, glucosylceramide is known as an intermediate metabolite in the synthesis and degradation of the more complex gangliosides, and a number of drug-resistant cancer cell lines accumulate this noncytotoxic metabolite27. In our case of A2780T, the decrease of glucosylceramide and LacCer can be explained as “activation of ganglioside pathway”8, which enable cancer cells convert Cer into gangliosides to evade the pro-apoptotic function of Cer. The enzymes related to the “glucosylceramide-lactosylceramide-ganglioside pathway”, including glucosylceramidase, glucosylceramide synthase, and lactosylceramide synthase, could have participated in the biological progress.

In A2780T, reduced syntheses of Cer, HexCer, and LacCer were observed, with the concomitant increase of DHSM and total SM levels, in which C16-SMs contributes the vast majority. These represent the main sphingolipid metabolism pattern in A2780T, which is significantly different from the SPL profiles in similar ovarian cancer cell lines. On one hand, comparing with the sphingolipidome in another taxol resistant human ovarian cell line CABA-PTX28, the level of sphingomyelin in A2780T changed significantly. On the other hand, A2780T also showed different sphingolipidomic profile from A2780 cell lines resistant to other drugs. In sharp contrast to the well-studied MDR A2780 cells29, the rise of cellular HexCer (including galactosylceramide and glucosylcermide) levels was not observed in A2780T. And in A2780/HPR cells the glycosphigolipid-dominated alteration20 is also different from the SPL pattern in A2780T, which possesses a distinctive feature of “SM-related depletion of Cers”. It indicated that the resistant mechanism of A2780T could be different from that of either other taxol-resistant cancer cells (CABA-PTX) or A2780 cells resistant to other drugs (MDR A2780 & A2780/HPR). Such interdisciplinary basic scientific research has close relevance to the medical community and it facilitates the applications in rapid detection and classification of disease type (taxol-resistant or not) and medication direction.

Conclusions

Since the role of sphingolipids in cancer cell has been widely recognized, comprehensive sphingolipidomic study is essential for exploring its drug resistance mechanism. The most comprehensive and accurate method described in this paper fully identified SPLs in A2780 human ovarian cancer cell line and the taxol-resistant cell line A2780T. Most individual species, including some low abundance but biologically important SPLs, have been accurately quantified. It provides more detailed information than general overview of a whole subclass, which is significant for studying the alterations39.

The sphingolipid metabolism in A2780T is oriented toward down-regulation of ceramides. We propose A2780T cells may escape from the ceramide-caused apoptosis mainly via sphingomyelin/ceramide pathway, while SMS was expressed more in A2780T than in the sensitive cell line, or the activity of SMase was inhibited. These enzymes related to the marker SPLs and altered pathways, are the potential targets. Based on the sphingolipidomic study, adjusting the sphingolipid metabolism purposively may represent a winning strategy to overcome taxol resistance and improve cancer therapy. This study facilitates not only development of new drugs against taxol resistance, but also clinical diagnosis of taxol-resistant ovarian cancer.

Methods

Chemicals and solutions

Human ovarian cancer cell line (A2780) and its taxol-resistant strain (A2780T) were purchased from KeyGen Biotech Co., Ltd. (Nanjing, China). The LIPID MAPS internal standard (IS) cocktail in ethanol, composed of 25 μM each of nine sphingolipid standards including SM (d18:1/12:0), Cer (d18:1/12:0), C1P (d18:1/12:0), HexCer (d18:1/12:0), LacCer (d18:1/12:0), Sphinganine (d17:0), Sphingosine (d17:1), Sphinganine-1-Phosphate (d17:0) and Sphingosine-1-Phosphae (d17:1), was purchased from Avanti Polar Lipids (Alabaster, AL, USA). HPLC-grade methanol (MeOH), chloroform (CHCl3) and isopropanol (IPA) were purchased from Merck (Darmstadt, Germany). Ammonium acetate (NH4OAc), potassium hydroxide (KOH), acetic acid (CH3COOH) and formic acid (HCOOH) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Dulbecco’s Modified Eagle’s Medium (DMEM), Roswell Park Memorial Institute (RPMI) 1640 medium, Fetal Bovine Serum (FBS), Penicillin-Streptomycin (PS) were purchased from Gibco, New Zealand. Sodium dodecyl sulfate (SDS) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were purchased from Acros, USA.

A pooled sample equally aliquoted from all samples can provide the most comprehensive information within a specific study. Hence, equivalent amount of A2780T was spiked into A2780 to prepare a pooled quality control (QC) sample.

Cell culture and SPLs Extraction

A2780 human ovarian cancer cell line were cultured in DMEM supplemented with 1% PS and 10% FBS in a humidified 5% CO2 atmosphere at 37 °C. For lipid analysis, cells were seeded into dishes and grown to confluence. Cells were rinsed twice with ice-cold PBS and scraped into a borosilicate glass tube with polytetrafluoroethylene coated top. After adding 0.5 mL of MeOH, 0.25 mL of CHCl3 and 10 μL of internal standards cocktail (2.5 μM) successively, the mixture was sonicated at room temperature for 30 s then incubated at 48 °C for 12 h to extract SPLs. After 75 μL of KOH in MeOH (1M) was added in, the mixture was incubated in a shaking water bath for 2 h at 37 °C to cleave potentially interfering glycerolipids. After cooling and neutralization with acetic acid, a four-step extraction procedure was performed as reported to prepare the SPLs for LC-MS analysis.

In order to verify the taxol resistance in the commercial A2780T cell line, MTT assay was employed. A2780T cells were cultured in RPMI 1640 medium containing 10% FBS, 1% PS and 800 ng/mL taxol solution at 37 °C in a 5% CO2/95% air atmosphere. For assessment of cell viability, A2780 and A2780T were respectively seeded in a 96-well plate at a density of 5 × 103 cells/well and were allowed to adhere for 16 h before treatment. Following incubation for 48 h, MTT solution (10 μL per well, 5 mg/mL solution) was added to each well and incubated for 4 h at 37 °C. Thereafter 100 μL 10% SDS was added for lysing and the plate was maintained overnight at 37 °C in a 5% CO2/95% air atmosphere. The optical densities were determined at 570 nm using a microplate reader. The procedure was repeated three times.

The sensitivity of A2780 and A2780T cell lines to taxol was assayed by using MTT assay. The IC50 for taxol in A2780 and A2780T was 73.16 nM and 149.6 μM, respectively. The result indicates that resistance to taxol of A2780T is at least 1000 fold greater than that of A2780.

LC-MS conditions

Sphingolipid analysis was performed by using our well-established LC-MS method21 with minor optimization, an Agilent 1290 UHPLC tandem 6550 quadrupole time-of-flight (Q-TOF) MS system and an Agilent 1290 UHPLC tandem 6460 triple quadrupole (QQQ) MS system were employed for qualitative profiling and quantitative analysis respectively. Chromatographic separation was performed as described previously, while the injection volume was 10 μL for Q-TOF and 5 μL for QQQ, respectively. An Agilent Eclipse Plus C18 column (100 × 2.1 mm, 1.8 μm) was used to separate the endogenous SPLs. The mobile phase consisted of (A) MeOH/H2O/HCOOH (60:40:0.2, v/v/v) and (B) MeOH/IPA/HCOOH (60:40:0.2, v/v/v), both containing 10 mM NH4OAc. The flow rate was 0.4 mL/min, and the column temperature was maintained at 40 °C for each run. A linear gradient was optimized as follows: 0–3 min, 0% to 10% B; 3–5 min, 10% to 40% B; 5–5.3 min, 40% to 55% B; 5.3–8 min, 55% to 60% B; 8–8.5 min, 60% to 80% B; 8.5–10.5 min, 80% to 80% B; 10.5–16 min, 80% to 90% B; 16–19 min, 90% to 90% B; 19–22 min, 90% to 100% B, followed by washing with 100% B and equilibration with 0% B. A typical data acquisition time was 20 min.

The above UHPLC system was interfaced with an Agilent ultrahigh definition (UHD) 6550 Q-TOF mass spectrometer equipped with an ESI source (Santa Clara, CA, USA). The source parameters were: drying gas (N2) temperature 150 °C, flow rate 15 L/min, nebulizer pressure 25 psi, sheath gas (N2) temperature was set at 200 °C with a flow-rate at 12 L/min. The scan parameters were: positive ion mode over m/z 110–1200, capillary voltage 4000 V, nozzle voltage 300 V, fragmentor voltage 175 V, skimmer voltage 65 V, octopole RF peak 500 V, drying gas 6 L/min at 300 °C. A reference solution was nebulized for continuous calibration in positive ion mode using the reference mass m/z 922.00979800. The acquisition and data analysis were controlled using Agilent Mass Hunter Workstation Software (Agilent, USA).

The UHPLC conditions for quantitative analysis were the same as those mentioned above. The LC system was coupled to an Agilent 6460 triple-quadrupole mass spectrometer (Santa Clara, CA, USA). The ESI parameters were optimized as follows: positive ion mode, drying gas (N2) temperature 325 °C, flow rate 11 L/min, nebulizer pressure 45 psi, capillary voltage 4000 V, nozzle voltage 300 V, sheath gas (N2) temperature was set at 200 °C with a flow-rate at 12 L/min. Data were processed with Agilent Mass Hunter Workstation Software. Further detail of the parameters, such as characteristic transitions, fragmentor and CE voltages optimized for each compound, and the methodology validations are similar as described before.

Data analysis

The screening and identification of SPLs were performed by searching in our personal database, which was built and updated based on the Agilent Mass Hunter Personal Compound Database and Library (PCDL) software and LIPID MAPS information (31643 SPLs until July 08 2015).

The sphingolipidomic approach was applied in qualitative research of SPLs by analyzing a pooled sample equally mixed by A2780 and A2780T. In quantitative research, A2780 cells (models, n = 10) and A2780T cells (models, n = 10) were analyzed in parallel. Supervised multivariate statistical analysis, partial least squares to latent structure-discriminant analysis (PLS-DA) method, was used to differentiate the amounts of SPLs between the two strains. Potential biomarkers were selected according to Variable Importance in Projection (VIP) value and the loading scattering-plot, using SIMCA-P+ software version 13.0 (Umetrics, Umea, Sweden). VIP values higher than 1.00 were considered significant.

Additional Information

How to cite this article: Huang, H. et al. LC-MS Based Sphingolipidomic Study on A2780 Human Ovarian Cancer Cell Line and Its Taxol-resistant Strain. Sci. Rep. 6, 34684; doi: 10.1038/srep34684 (2016).

Acknowledgments

This work was financially supported by the Macao Science and Technology Development Fund, Macau Special Administrative Region (039/2011/A2 to Z.-H. Jiang).

Footnotes

Author Contributions Z.-H.J. and J.-R.W. designed the research; T.-T.T. and L.-F.Y. performed the cell experiments; H.H., C.-Y.C. and J.-N.M. performed LC-MS analysis, H.H., J.-R.W. and Z.-H.J. wrote the manuscript. All authors reviewed and substantially contributed to the manuscript.

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