Ovarian cancer is a common gynecologic cancer with high mortality but unclear molecular mechanism1,2. Most of ovarian cancers are diagnosed in the advanced stage, which seriously hampers therapy. Mitochondrial changes are a hallmark of human ovarian cancers, and mitochondria are the centers of energy metabolism, cell signaling, and oxidative stress3,4,5,6,7. In-depth insights into the changes of the mitochondrial proteome in ovarian cancers compared to control ovarian tissue will benefit in-depth understanding of the molecular mechanisms of ovarian cancer, and the discovery of effective and reliable biomarkers and therapeutic targets. Mitochondrial metabolism has been proposed and recognized as a target for cancer therapy, and antimitochondrial therapy might ultimately be very beneficial for preventing the recurrence and metastasis of cancer8. Individual metabolic profiling is also already practiced as a useful tool for cancer stratification and predictive strategies9,10.
The long-term goal of this research is to develop and use a quantitative mitochondrial proteomics method to study ovarian cancer for clarification of mitochondrial proteome alterations between ovarian cancer and control ovarian tissues, and their molecular network alterations from a systematic multi-omics angle11,12, which will result in the discovery of mitochondria-targeted molecular biomarkers13 for clarification of the molecular mechanisms of ovarian cancer, prediction, and personalized treatment of ovarian cancer patients. Isobaric tags for relative and absolute quantification (iTRAQ) labeling3,4 are an effective method to quantify the mitochondrial protein changes. Preparation of high-quality mitochondrial samples from human ovarian cancer and control ovarian tissues are the prerequisite for iTRAQ quantitative analysis of mitochondrial proteomes3. Mitochondrial preparation coupled with iTRAQ quantitative proteomics has been successfully used in long-term research programs about the human ovarian cancer mitochondrial proteome, including the establishment of mitochondrial proteome reference maps3, the analysis of differentially expressed mitochondrial profiles4,14 and post-translational modifications, including phosphorylation, which has already resulted in the discovery of important signaling pathway network changes in human ovarian cancers5, including alterations in energy metabolism4, lipid metabolism, and mitophagy pathway-systems3.
Previous studies have found that differential-speed centrifugation in combination with density gradient centrifugation is an effective method to isolate and purify mitochondria from human ovarian cancer and control ovarian tissues3,4,5,14. The iTRAQ labeling coupled with strong cation exchange (SCX)-liquid chromatography (LC)-tandem mass spectrometry (MS/MS) is the key technique to detect, identify, and quantify the proteins from the prepared mitochondrial samples.
Here, detailed protocols for mitochondrial preparation coupled with iTRAQ quantitative proteomics are described. These have been successfully used in the analysis of human ovarian cancer tissue mitochondrial proteomes. The protocols include preparation of samples, differential-speed centrifugation, density gradient centrifugation, quality assessment of mitochondrial samples, protein digestion with trypsin, iTRAQ labeling, SCX fractionation, LC, MS/MS, database searching, and quantitative analysis of mitochondrial proteins. Moreover, this protocol easily translates to analyze other human tissue mitochondrial proteomes.