Researchers unravel cell death regulation

Researchers at the Walter and Eliza Hall Institute of Medical Research (WEHI) have unravelled some of the interactions between pro-survival and pro-apoptotic proteins, providing clues to the regulatory mechanisms that govern apoptosis.

The process of apoptosis, also known as programmed cell death, has been of great interest to researchers both as a fundamental biological mechanism and as a potential target for the development of anti-cancer therapies. A variety of protein families appear to be involved in the process -- some are pro-survival and act to keep the cell alive until cell damage or other signals trigger the activity of pro-apoptotic proteins, which bind to the pro-survival proteins to inactivate them and start the process of cell death.

In some cancer cells, this mechanism is faulty, leading to damaged cells staying alive and multiplying. But using pro-apoptotic proteins to restore the balance may have unintended side effects on normal cells.

The WEHI researchers, led by David Huang, are focusing on the pro-apoptotic BH3-only proteins, a class of proteins characterised by the so-called BH3-domain, which inserts into a groove formed by BH1, BH2 and BH3 domains of the distantly related pro-survival Bcl-2-like proteins to inactivate them and trigger apoptosis. The eight BH3-only proteins found in mammalian cells are activated by diverse mechanisms, but until now, it was though that they targeted all of the pro-survival Bcl-2-like proteins in more or less the same way.

But an examination of the affinities of the different pro-apoptotic proteins to the five mammalian pro-survival proteins tells a different story, Huang says. In fact there is a wide variation in affinity suggesting that certain pairings of pro-survival and pro-apoptotic proteins may be the norm, reflecting differences in biological function.

"These proteins are functionally distinct -- they don't all act the same way," Huang says. "This gives us a much clearer idea of how cell death initiates and has given us the tools to dissect out the pathways much better."

In fact, only a couple of the BH3-only proteins exhibit 'promiscuous' binding to all of the pro-survival Bcl-2-like proteins, while the remaining members of the family show distinct preferences for a smaller subset of the family. The discovery is backed up by the phenotypes of various knockout mice engineered to disrupt these proteins, which vary considerably.

Using structural studies of the binding sites the researchers have also begun to dissect the precise structural characteristics of the two classes of proteins to understand why one pairing has a higher affinity than another.

The studies have another purpose -- they may assist with the design of mimetic drugs that can specifically inactivate pro-survival targets, Huang says. Individual cancers often over-express a specific pro-survival protein, such as Bcl-2 itself, and it may be possible to target these proteins using highly selective drugs, based on the blueprint provided by the highest affinity BH3-only protein ligand.

"It means we could selectively target the relevant over-expressed protein in a tumour," Huang says.

The study, which was supported by a variety of funding agencies including the NHMRC; the US-based Leukemia and Lymphoma Society; the Australian Cancer Research Foundation (ACRF); the Cancer Council of Victoria; the Viertel Foundation; the Leukaemia Foundation of Victoria; and the Marsden Fund (New Zealand), has just been published in the journal Molecular Cell.

Reference: Lin Chen, Simon N Willis, Andrew Wei, Brian J Smith, Jamie I Fletcher, Mark G Hinds, Peter M Colman, Catherine L Day, Jerry M Adams, and David CS Huang (2005). 'Differential Targeting of Prosurvival Bcl-2 Proteins by Their BH3-Only Ligands Allows Complementary Apoptotic Function', Molecular Cell, Vol 17, 393-403.