Cellular uptake of 9-hydroxypheophorbide-α and its photoactivation to induce ER stress-related apoptosis in human cervical cancer cells

The 9-hydroxypheophorbide-alpha (9-HPbD) is a chlorophyll derivative and was found to be very effective for photodynamic therapy of tumor cells. The current study investigates uptake, retention, and intracellular localization of 9-HPbD by HeLa, human cervical cancer cells via fluorescence spectrophotometry and confocal laser scanning microscopy, and its photodynamic effect against human cervical carcinoma cell. HeLa cells exposed to 9-HPbD exhibited a linear uptake of photosensitizer during the first 12 h, and after removal of 9-HPbD, cell fluorescence was observed to decrease gradually over the next 12 h. Cells treated with 9-HPbD and stained with a panel of organelle-specific fluorescence probes (MitoTracker, LysoTracker, and ER-Tracker) revealed an intracellular fluorescence distribution restricted to cytoplasmic compartments with no detectable fluorescence in the nucleus. The 9-HPbD showed cytotoxicity effect against HeLa cells in 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay. Endoplasmic reticulum (ER) disruption and cellular calcium dynamics also showed a photoactivation followed by cell death. The apoptotic effect of 9-HPbD was confirmed by caspase 3 activity study and immunofluorescence study of caspase 12. Morphological observation through the transmission electron microscopy and scanning electron microscopy also confirmed that 9-HPbD can induce apoptosis in HeLa cells. Therefore, it can be concluded that maximum uptake and clearance time of 9-HPbD was 12 h with endoplasmic reticulum as the major organelle site in cellular uptake, and 9-HPbD can induce apoptosis in HeLa cells through ER stress-related pathways via activation of caspase 12.