Cancer cells acquire a common set of properties including; unlimited proliferation, self-sufficiency in growth signals, resistance to cell death and metabolism abnormalities regardless of cellular or tissue origin. While carbohydrate-based metabolism has been addressed in a large number of studies and dysregulation of lipids metabolism is common in cancer cells, lipid-based metabolism in cancer has not been fully addressed. The mitochondrial pro-apoptotic protein SMAC/Diablo (second mitochondria-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low pI) isoform, was originally characterized as a pro-apoptotic mitochondrial intermembrane space (IMS) protein. Following apoptosis induction, SMAC/Diablo is released into the cytosol where it interacts with members of the inhibitor of apoptosis protein (IAP) family (cIAP1, cIAP2, and XIAP) to neutralize the inhibitory effects of IAPs on caspases, and thus, initiates apoptosis.
Unexpectedly, we found that SMAC/Diablo is over-expressed in different types of cancer. Since SMAC/Diablo is a promoter of cell death, we explored how cancer cells tolerate an increase in SMAC/Diablo expression and what are the benefits of SMAC/Diablo over-expression to cancer cells. SMAC/Diablo null mice are viable, grow and mature normally, present embryonic fibroblasts, lymphocytes, hepatocytes without any histological abnormalities and exhibit wild-type responses to all types of apoptotic stimuli. To addressthe paradox of cancer cells overexpressing pro-apoptotic protein, we silenced SMAC/Diablo expression in tumor cell lines and sub-cutaneous lung tumor xenografts in mice using specific siRNA (si-hSMAC). Our results confirmed that SMAC/Diablo silencing reduced cell survival in culture and tumor growth in-vivo. This activity was mediated by decreasing phospholipid and phosphatidylcholine levels and inducing morphological changes such as cell differentiation and re-organization into glandular/alveoli-like structures. Next-Generation Sequencing of tumors treated with si-hSMAC, revealed altered expression of proteins associated with the cell membrane, vesicle formation, ER/Golgi-related proteins and phospholipid transport and synthesis. These novel findings suggest that SMAC/Diablo possesses an additional, non-apoptotic function associated with regulating lipid synthesis essential for cancer growth and development.
- Silencing SMAC/Diablo demonstrated marked inhibited of only cancer cell proliferation but not immortalized non-cancerous cell lines.
- This is the first time that phospholipid synthesis found to be essential for cancer growth is being targeted as an anti-cancer approach.
- Considering that dysregulation of lipid metabolism is common in cancer cells, our findings that over-expressed SMAC possesses an additional, non-apoptotic function associated with regulating lipid synthesis essential for cancer growth and development. As such, SMAC/Diablo represents a promising therapeutic target.
Provisional July 2016, PCT July 2018
Prof. Varda Shoshan-Barmatz, NIBN and the Department of Life Sciences, Ben-Gurion University of the Negev, Israel