A parasporin from Bacillus thuringiensis native to Peninsular India induces apoptosis in cancer cells through intrinsic pathway
Parasporins, a class of non-insecticidal crystal proteins of Bacillus thuringiensis (Bt) are being explored as promising anticancer agents due to their specific toxicity to cancer cells. The present study has identified 25 Bt isolates harbouring parasporin genes from Western Ghats region, the hotspot of biodiversity in India. Among these, the isolate, KAU 41 (Kerala Agricultural University isolate 41) contained non-hemolytic homogenous crystals showing specific cytotoxicity towards cancer cells. SDS-PAGE analysis of this crystal, isolated by aqueous biphasic separation, revealed a 31 kDa sized peptide. The N-terminal sequence deciphered in BLAST analysis showed homology to a hypothetical Bt protein. Upon proteolysis, a 29 kDa active peptide was generated which exhibited heterogenic cytotoxic spectrum on various cancer cells. HeLa cells were highly susceptible to this peptide with IC50 1 µg/mL and showed characteristics of apoptosis. RT-qPCR analysis revealed the overexpression of APAF1, caspase 3 and 9 by 14.9, 8 and 7.4 fold, respectively which indicates the activation of intrinsic pathway of apoptosis. However, at higher concentrations of peptide (>3 µg/mL), necrotic death was prominent. The results suggest that the 31 kDa protein from Bt isolate, KAU 41 is a parasporin that may have high therapeutic potential.