Inhibition of Chemoresistant Human Oral Squamous Carcinoma by Pseudomonas Exotoxin

Objectives: Advances in the understanding of bacterial toxins have produced new strategies for treatment of cancers. HSP70 is expressed in response to a wide variety of physiological and environmental insults, thus allowing the cell to survive. This study was performed to investigate the effect of Pseudomonas aeruginosa exotoxin A (PEA) on apoptosis and HSP70 expression in chemoresistant Oral squamous carcinoma (OSC) cells. Methods: Cell viability assay, Hoechst stain, Immunocytochemistry, and Western blot were used. Results: While YD-9 cells are strongly resistant to anticancer drugs such as 5-FU and etoposide, PEA significantly decreased the viability of YD-9 cells. HSP70 was overexpressed in YD-9 cells compare to human gingival fibroblast (HGF-1) and inhibited by PEA treatment. After downregulation of HSP70 with anti-sense oligonucleotides, treatment of 5-FU and etoposide decreased the viability of YD-9 cells, suggesting that HSP70 is in charge of resistance to anticancer drugs in YD-9 cells. Apoptotic manifestations were evidenced by changes in nuclear morphology and generation of DNA fragmentation. PEA treatment induced caspase-3, -6 and -9 cleavage, and activation. These events preceded proteolysis of the caspase substrates poly (ADP-ribose) polymerase (PARP), DNA fragmentation factor 45 (DFF45), and lamin A in YD-9 cells. Conclusions: Taken together, PEA inhibits the expression of HSP70 which is in charge of resistance to anti-cancer drugs, and thereby induces apoptosis in chemoresistant YD-9 cells via activation of caspases.