Biological activity and apoptotic signaling pathway of C11-functionalized cephalostatin 1 analogues

Author(s): Tahtamouni,LH Nawasreh, MM

Statement of the Problem: Cephalostatin 1, a potent anti-cancer agent, is a natural bis-steroidal alkaloid that causes cell death in the subnanomolar to picomolar ranges via an atypical apoptosis pathway. Although cephalostatin 1 is a highly effective anticancer drug, its availability limits its utilization. Previously, our group reported the synthesis of two 12’α-hydroxy derivatives of cephalostatin 1 and their ability to induce cell death by activating the atypical ER stress apoptosis signaling pathway. As part of our continuous investigation into developing active synthetic analogues of cephalostatin 1, we modified position C-11 by functionalization with an -OX group (X = Me, Et, H) of the bis-steroidal dimer 2. Methodology: Six C11-functionalized cephalostatin 1 analogues (CAs) were synthesized and for the cytotoxic compounds, the induced apoptotic pathway was investigated by MTT assay, flow cytometry, clonogenic assay, qRT-PCR, Cytochrome C release assay and Western blotting. Findings: The C11-functionalized cephalostatin 1 analogues 5 and 6 (CA5 and CA6) were found to exhibit cytotoxic activity against K-562 leukemia cells, MCF-7 breast cancer cells and DU-145 prostate cancer cells, while the remaining four analogues did not show anti-tumor activities against any of the cell lines. Our results indicated that CA5 and CA6 induced cell death via the atypical ER-dependent apoptosis pathway; they increased the expression of Smac/DIABLO, an inhibitor of inhibitors of apoptosis (IAPs), which in turn facilitated the activation of different caspases including the ER-caspase 4 without cytochrome c release from mitochondria. Conclusion & Significance: CA5 and CA6 are promising anticancer agents due to their low GI50, the remarkable apoptosis pathway they induce which can overcome chemoresistance, and their very low toxicity to normal cells making them cephalostatin 1 utilizable alternatives.