Two-photon probes for pH: Detection of human colon cancer using two-photon microscopy.

607 Background: In cancer cells, lysosomal pH decreases along with a concomitant increase in lysosomal volume and cathepsin expression levels. Lysosomes also play crucial roles in cancer progression following their release into the extracellular space. Since cancer cells invade a tissue by secreting degradative enzymes, the extracellular pH of tumor tissues becomes acidic. However, to date, there has been no report on the use of multi-photon microscopy (MPM) probes to image human colon cancer tissues. Methods: We have developed multi-photon (MP) pH-sensitive probes (BH-2 and BHEt-1) that exhibit absorption and emission maxima at 370 and 466 nm, and TP absorption cross-section values of 51 and 61 GM (1 GM = 10−50 cm4 s/photon), respectively, at 750 nm and pH 3.0 in a universal buffer (0.1 M citric acid, 0.1 M KH2PO4, 0.1 M Na2B4O7, 0.1 M Tris, 0.1 M KCl)/1,4-dioxane (7/ 3) solution. Results: The TPM images of CCD-18co (a normal colon cell line) and HCT116 cells (a colon cancer cell line) labeled with BH-2 were too dim to be distinguished. When the same cells were labeled with BHEt-1, however, the MPM image of the HCT116 cells was much brighter than that of CCD-18co cells, and the relative proportion of the acidic vesicles (Pacid) of the former was 5-fold larger than that of latter. BHEt-1 could also differentiate HepG2 cells (a human liver cancer cell line) from LX-2 cells (a human hepatic stellate cell line) with a 6-fold larger P acid value. Human colon cancer tissues labeled with BHEt-1 showed similar results, demonstrating much brighter MPM images and 6-fold larger Pacid values compared to normal tissue. Conclusions: These results suggest the potential utility of BHEt-1 for molecular image analysis of colon cancer tissues using MPM.