Ovarian cancer is the fifth leading cause of female cancer deaths in the UK and the seventh most common cancer in women in the USA, with more than 25,000 new diagnoses each year. It principally attacks women over the age of 50 and has a high mortality rate due to two key aspects which inhibit successful treatment.
First of all, it is difficult to detect early. Only about 30% of ovarian cancers are detected before they spread beyond the ovaries, often affecting the abdomen where they form secondary tumours. When detection is confirmed, the treatment regimes include surgery and chemotherapy, with paclitaxel (taxol) and the platinum drugs such as cisplatin and carboplatin at the forefront.
This is where the second obstacle arises. As with other cancers, resistance to anticancer drugs is a major obstacle to successful treatment. Indeed, multidrug resistance (MDR) often occurs, in which exposure to one drug somehow builds up the resistance of tumour cells to other structurally unrelated cancer drugs.
The mechanism of MDR is not clearly understood, although a number of factors are believed to be at the core. Published studies have revealed that tumour cells can reduce the intracellular concentrations of anticancer drugs to reduce their efficacy. In addition, the drugs can be diverted to other parts of the cell, or are presented with altered binding sites to render them less active.
In an attempt to provide more information on the molecular basis behind MDR, Chinese scientists have undertaken a proteomics study of ovarian cancer. Da-Zhi Zhang and co-workers from Chongqing Medical University and Central South University, Changsha, examined ovarian cancer COC1 cell cultures and the cisplatin-resistant cells COC1/DDP. These have been shown to develop cross-resistance to other anticancer drugs such as adriamycin and 5-fluorouracil.
Proteins from both cell cultures were extracted and digested with trypsin. The resultant peptides were isotope-labelled with different iTRAQ reagents (Isobaric Tags for Relative and Absolute Quantification). Peptides from the parent cells were treated with the reagent that adds 115 mass units to the peptide and those from the drug-resistant cells were labelled with the 117 reagent, then the two sets of samples were combined.
Following some purification steps, the peptides were analysed by LC-tandem MS with electrospray ionisation. Each peptide was labelled with the 115 or 117 reagent and their relative abundances were determined from the mass spectrometric peak area ratios. The proteins corresponding to peptides of interest were then identified by database searching against a human protein database.
A total of 28 proteins were found to be present in different quantities between the two cultures, 11 being reduced in the drug-resistant cells and 17 increased in abundance. Of these, two proteins were particularly affected. Pyruvate kinase isoform M2 (PKM2), a metabolic enzyme, was significantly decreased in the drug-resistant cells and the chaperone heat shock protein 60 (HSPD1) was markedly increased.
The role of PKM2 was studied by further experiments. Suppression of PKM2 in COC1 cells as well as a second cisplatin-sensitive ovarian cancer cell line conferred cisplatin resistance. In addition, transfection of the PKM2 gene into the COC1/DDP cells increased sensitivity to cisplatin. The research team concluded that PKM2 did seem to be involved in MDR of the cisplatin-resistant cells.
Experiments with HSPD1, which folds key proteins after import into mitochondria, suggested that increased levels of this protein are involved with the inhibition of the cisplatin-induced cell death.
The 28 proteins fell into a number of functional groups, including calcium-binding proteins, chaperones, extracellular matrix proteins, metabolic enzymes, proteins associated with drug detoxification or DNA repair, and those related to cellular structure or signal transduction.
Although their precise roles remain unclear, their implication in MDR should provide some clues to the molecular mechanisms behind the phenomenon and a better understanding of its initiation and persistence. The work might also be useful for studying MDR in other types of cancer.
Related links:- Journal of Cellular Biochemistry 2010, 106, 625-633: "Quantitative proteome analysis of multidrug resistance in human ovarian cancer cell line"
Source: Proteomics and Genomics
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