Cancer evolves through a multistep process that occurs by the temporal accumulation of genetic mutations mediated by intracellular and extracellular cues. Researchers from the University of Texas Southwestern Medical Center observe that exosomes isolated from pancreatic cancer cells, but not normal pancreatic cells, can initiate the first step of malignant cell transformation. Injection of exosome-initiated transformed cells into mice results in aggressive tumor growth. Using proteomic profiling and DNA sequencing of exosome-treated and transformed cells, they show that cancer cell exosomes act as a classic initiator by causing random genetic changes in recipient cells. These studies provide new insight into a function of cancer cell exosomes and how they might specifically contribute to orchestrated local cell transformation.
Pancreatic cancer cell exosomes function as an initiator in malignant cell transformation
(A) In the classic 2 – stage cell transformation assay used, NIH/3T3 cells are treated with the tumor initiator MCA for three days (Days 3 – 6) and the tumor promoter TPA for two weeks (Days 8 – 21). After 42 days, 5 Cells are fixed with methanol and stained with Crystal Violet for malignant foci counting. (B) Representative images of stained cells showing foci formation (arrows) fromun treated cells and cells treated with MCA/DMSO,DMSO/TPA,MCA/TPA,orCapan-2 exosomes (ExC)/TPA(initiator-promoter).(C)Quantification of foci formed after cell transformation. The average foci/well were determined via double-blind counting as described in methods. The red dashed line represents the 10 established level of background foci present in untreated cells. Only MCA/TPA(p=0.008) and ExC/TPA(p=0.0002) treatments resulted in increased foci formation above background. Asterisks indicate significant differences from either control treatment or MCA/TPA treatments as determined by unpaired, two-tailed t-test with Welch’s correction (*p<0.05; **p<0.01; ***p<0.001; ****p<0.0001). Red (+) = initiator; purple (+) = promoter; HPDE exosomes (Ex HPDE)(D)Subcutaneous injections of NIH/3T3 cells, MCA/TPA transformed cell foci, or ExC/TPA transformed cell foci into mice (n=5) at a concentration of 2.5x106cells. Tumor growth was tracked by measuring tumor volume 2x/week for 37 days post injection (Post TCI).