Apoptotic and autophagic responses to Bcl-2 inhibition and photodamage

Abstract

Among the cellular responses to photodamage initiated by photodynamic therapy (PDT) are autophagy and apoptosis. While autophagy is a reversible process that can be both a survival and a death pathway, apoptosis is irreversible, leading only to cell death. In this study, we followed the fate of mouse leukemia L1210 cells after photodamage to the endoplasmic reticulum (ER) using a porphycene photosensitizer, where Bcl-2 was among the PDT targets. In wild-type cells, we observed a rapid wave of autophagy, presumed to represent the recycling of some damaged organelles, followed by apoptosis. Using shRNA technology, we created a Bax knockdown line (L1210/Bax − ). In the latter cell line, we found a marked decrease in apoptosis after photodamage or pharmacologic inactivation of Bcl-2 function, but this did not affect PDT efficacy. Loss of viability was associated with a highly-vacuolated morphology consistent with autophagic cell death. Previous studies indicated pro-survival attributes of autophagy after low-dose PDT, suggesting that autophagy may be responsible for the 'shoulder' on the dose-response curve. It appears that attempts at extensive recycling of damaged organelles are associated with cell death, and that this phenomenon is amplified when apoptosis is suppressed.

mTHPC-based photodynamic therapy induction of autophagy and apoptosis in cultured cells in relation to mitochondria and endoplasmic reticulum stress

“…Inactivation or photodamage of Bcl-2 was also reported to increase autophagy induced by PDT (10,19). However, although autophagy was enhanced when apoptosis was impaired, Kessel et al have reported that autophagy accompanied apoptosis in photosensitized leukaemia cells (8).…”

ROS-mediated mechanisms of autophagy stimulation and their relevance in cancer therapy

“…For example, when Bcl-2 is a principal target of photodamage after 9-capronyloxytetrakis (methoxyethyl)porphycene (CPO)-mediated PDT, the consequent loss of Bcl-2 function is proposed to stimulate autophagy, likely by disrupting its association with the proautophagic protein Beclin 1. 128 On the other hand, overexpression of Bcl-2 influences only apoptosis but not autophagic flux after PDT with silicon phtalocyanine (Pc4), 66 thus suggesting that this is not a general mechanism launching autophagy in photosensitized cells. When the ER is the main organelle damaged after PDT with hypericin, 125 the rapid shutdown of the AktmTOR pathway, which may promote a proautophagic environment, has been observed (Dewaele et al unpublished results).…”

Effects of HSP27 downregulation on PDT resistance through PDT-induced autophagy in head and neck cancer cells

“…Autophagy enhances the survival of cells showing low levels of photodamage (10), thus serving as a pro-survival response by recycling damaged organelles. In contrast, autophagy induces the death of cells treated with high-dose PDT (11,12). Some studies have shown that PDT limits autophagy and apoptosis (13).…”