Apoptosis in cancer cells may be easier to unleash than previously thought, according to new research led by scientists at Umeå University and collaborators. That finding could open up more cancers to treatment with anti-apoptotic drugs. The team used neutron reflectometry (NR) and ATR-FTIR to detail communication between proteins in and around the mitochondrial outer membrane (MOM).
“We use neutrons as a kind of ‘x-ray’ magnifying glass to study how various proteins talk to each other inside the cell,” Gerhard Gröbner, PhD, told Inside Precision Medicine. He is a professor at Umeå University and senior author of a new study that looks at the role of the Bax protein in apoptosis. The findings appear in ACS Chemical Biology.
Apoptosis is a form of programmed cell death that removes old or damaged cells, enabling the immune system to function properly. When apoptosis does not work as it should, as in many cancers, cells can divide uncontrollably and form tumors.
Many cancer therapies (e.g. drugs and radiation) are designed to trigger apoptosis in tumors. But there are also many aggressive and often incurable cancers that current anti-apoptotic therapies do not work on due to these tumors’ intensive use of survival proteins, such as Bcl-2 and its relatives, which can stop apoptotic death.
“Finding new drugs to inhibit Bcl-2 and its relatives in a wider sense would thus help treat more cancers. Currently only one Bcl-2 drug is available, and it is used for very specific leukemia,” said Gröbner.
“Going forward we will test a range of potential drug candidates to block Bcl-2 to release cell-killing proteins like Bax again to improve therapies,” he added.
The cell‑killing protein Bax protein is one of the most important proteins controlling apoptosis. Once activated, Bax can initiate apoptosis by forming pores in the membranes of mitochondria. Another key protein from the same family, the cell‑protective protein Bcl‑2, instead prevents Bax from killing tumor cells. In nearly half of all human cancers, one of the underlying problems is an increased production of Bcl‑2, which promotes tumor growth and often leads to poor response to therapy.
“In our research, we have used advanced neutron experiments to show how Bcl‑2 protects cancer cells by blocking the death‑inducing proteins that are most often activated by therapy,” said Gröbner.
The team used NR and ATR-FTIR to elucidate the molecular communication between those proteins in and around the mitochondrial outer membrane (MOM). The spatial and temporal changes across model MOM surfaces were resolved during the interaction of Bax with Bcl-2. The NR-derived membrane surface Bax distributions suggested that Bcl-2 mediated Bax sequestration through both Bcl-2/Bax heterodimerization and Bax/Bax oligomerization. Kinetic analysis revealed a two-step process: rapid formation of Bcl-2/Bax heterodimers, followed by slower Bax oligomerization on these complexes
The experiments show that Bcl‑2 can capture and bind several Bax proteins at the same time. This makes the inhibition of cell death more efficient than previously thought. Cancer cells do not need to produce extremely large amounts of Bcl‑2 to protect themselves—even a moderate increase can be sufficient.
The researchers also investigated how the composition of the mitochondrial membrane affects the interaction between the proteins. They found one particular lipid, cardiolipin, can promote apoptosis and help Bax form pores in the membrane. However, even in membranes containing cardiolipin, a sufficiently high level of Bcl‑2 can still prevent cell death.
“In the longer term, this type of knowledge could open up new opportunities for cancer treatment, for example by targeting Bcl‑2 and its protective function,” says Gröbner.
The study was carried out in collaboration between researchers from Umeå University, Lund University, the European Spallation Source (ESS) in Lund, the ISIS Neutron and Muon Source and Diamond Light Source in the United Kingdom, and the Institut Laue‑Langevin (ILL) in France.
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