Decreased expression and function of alpha-2 macroglobulin receptor/low density lipoprotein receptor-related protein in photodynamic therapy-resistant mouse tumor cells.

Parental and photodynamic therapy (PDT)-resistant mouse, radiation-induced fibrosarcoma cell lines were evaluated using mRNA differential display in an attempt to identify unique transcripts. We detected one transcript that was consistently present in the parental cells but absent in PDT-resistant cells. The transcript was cloned, sequenced, and identified as alpha-2 macroglobulin receptor/low density lipoprotein receptor-related protein (alpha-2 MR/LRP). Northern and Western immunoblot analysis confirmed that receptor expression was present in the parental cell line but barely detectable in PDT-resistant cells. Functionality of the receptor was evaluated by exposing cells to Pseudomonas exotoxin A. alpha-2 MR/LRP is responsible for Pseudomonas exotoxin A internalization, and only the parental cells exhibited toxin-mediated cytotoxicity. The binding and endocytosis of activated alpha-2 macroglobulin and lipoproteins by alpha-2 MR/LRP are consistent with modulating uptake and localization of photosensitizers. Our results demonstrate that PDT-resistant murine tumor cells exhibit minimal alpha-2 MR/LRP activity and suggest that this receptor plays a role in PDT sensitivity by modulating photosensitizer uptake and/or subcellular localization.