Novel Discovery Points to New Strategy for Treating Drug Resistance in Cancer Cells

Drug resistance—when diseases have become tolerant to therapies—is a major hurdle to effective cancer treatment. It is responsible for most of the relapses of cancer patients that occur after seemingly successful therapy. To overcome this enormous problem, researchers are seeking ways to sensitize drug-resistant cancer cells to targeted therapies and to prevent such cells from forming.

Researchers at the National Cancer Institute (NCI)-designated Montefiore Einstein Cancer Center (MECC) have now identified a promising approach to overcoming treatment resistance in acute myeloid leukemia (AML), an aggressive blood cancer.

Based on years of laboratory research, they have developed a molecule that prevents AML cells from evading the damage and death that current chemotherapeutic treatments are intended to cause. The results of that research (a collaboration with researchers at New York University Grossman School of Medicine and Rutgers, The State University of New Jersey) were published online on April 24 in Cancer Discovery.

“Cancer cells have a range of tools at their disposal to resist the effects of powerful chemotherapy drugs,” said Evripidis Gavathiotis, PhD, Co-Leader, MECC Cancer Therapeutics Program, Professor, Biochemistry and Medicine, Albert Einstein College of Medicine, and co-corresponding author of the paper. “Based on our extensive study of apoptosis, also known as programmed cell death, we’ve developed a new strategy for making existing treatments against AML more effective.”

Killing Cells by Puncturing Mitochondria

Chemotherapy and radiation therapy both work by damaging cancer cells severely enough to make them undergo apoptosis. When cancers become resistant to therapy, impaired apoptosis is almost always the reason.

The body relies on apoptosis to get rid of unwanted cells—excess cells pruned during embryological development, for example. Apoptosis is a highly choreographed sequence of molecular events that requires damage to mitochondria, the critical intracellular components that produce the energy that enables cells to function. Each cell contains hundreds of mitochondria, and their elimination—caused by a protein that punctures them—is the key step in mediating cell death via apoptosis.

For optimal functioning, mitochondria depend on their ability to fuse with each other—something they do when they need to produce more energy or to function under stressful conditions. Fusion is regulated by two specialized proteins, called mitofusins, which reside on the outer membrane of mitochondria. Health problems can occur when mitofusins are defective. “We had no way to alter mitofusin activity, making it difficult to understand how they influence mitochondria,” said Dr. Gavathiotis.

A Focus on Fusion

With his expertise in structural and chemical biology, Dr. Gavathiotis designs small molecules to serve as “tools” for biological research or as prototypes for new drugs. In a study published last July in Nature Communications, his research team synthesized small molecules that enabled them to either activate or inhibit mitofusins. Their mitofusin activator stimulated mitochondria to fuse and improved their function; in contrast, their mitofusin inhibitor prevented mitochondria from fusing with each other and impaired their function.

“The results we observed with our mitofusin inhibitor, MFI8, were especially intriguing,” Dr. Gavathiotis recalled. “MFI8 not only prevented mitochondrial fusion but also—by fragmenting cells’ mitochondria—primed cells to undergo apoptosis.”

Dr. Gavathiotis reasoned that MFI8 might be able to sensitize cancers to the apoptosis-promoting chemotherapies to which they had become resistant. He decided to test his hypothesis on AML, which has a dismal five-year overall survival rate of only 28%.

Finding Why Resistance Occurs

In 2018, the U.S. Food and Drug Administration (FDA) approved a promising two-drug therapy for treating AML. While early results were promising, prolonged treatment with the drug combination has led to drug resistance.

“To determine whether our mitofusin inhibitor MFI8 could help in treating AML, we first had to discover how AML becomes resistant to therapy, including to the recent FDA-approved treatment,” Dr. Gavathiotis said. His colleagues found that resistance could be explained, in part, by a quality-control system that cells use to keep their mitochondria healthy.

When mitochondria undergo extreme stress (from exposure to chemotherapy, for example), the stress activates a process called mitochondrial autophagy, or mitophagy, that targets and digests damaged mitochondria.

The researchers found that mitophagy in drug-resistant human AML cells happened at a higher rate than in drug-sensitive AML cells. “With their increased rate of mitophagy, the drug-resistant AML cells were able to maintain a healthy and functional population of mitochondria,” Dr. Gavathiotis said. “In those cancer cells, chemotherapy wasn’t able to cause enough mitochondrial damage to induce the cells to undergo apoptosis and die.”

Dr. Gavathiotis and colleagues found that drug-resistant AML cells needed more than just revved-up mitophagy to evade apoptosis. The cells also required help from mitofusin 2 (MFN2)—one of the two proteins that increase mitochondrial fusion. They found that MFN2 is a crucial enabler of mitophagy, with experiments revealing higher levels of MFN2 in chemotherapy-resistant AML cells compared with chemotherapy-sensitive cells.

Sensitizing Cells to Chemotherapy

MFN2 proved to be indispensable for helping AML cells become resistant to apoptosis,” Dr. Gavathiotis said. “This suggested that MFI8, the mitofusin inhibitor that we had developed, might work as a supplement to current therapies for treating AML. It might be able to slow down mitophagy, allowing sufficient numbers of chemo-damaged mitochondria to accumulate in AML cells so that apoptosis could occur.”

In experiments described in the Cancer Discovery paper, the team of investigators generated models of drug-resistant AML by taking AML cells from patients whose disease had become resistant to chemotherapy and implanting them into a mouse model. These AML patient cells failed to respond when the mice were treated with AZD5991, an experimental drug specifically designed to induce apoptosis in cancer cells. However, when AZD5991 was combined with MFI8, the two agents acted synergistically resulting in enhanced death of the AML cells.

“Our research suggests that the best way to overcome drug resistance in AML—and possibly in other types of cancer—is to couple current pro-apoptotic chemotherapies with an agent like MFI8 that curbs mitophagy,” Dr. Gavathiotis said. “By crippling cancer cells’ ability to dispose of damaged mitochondria, you give chemotherapy a much better chance of killing those cells by inducing apoptosis.”

Dr. Gavathiotis acknowledged that MFN2-inhibiting drugs could transiently compromise mitochondria in normal cells. “Future studies,” he said, “should definitely assess the impact of MFN2 targeting on normal cells and tissues.”

The study is titled “Mitophagy Promotes Resistance to BH3 Mimetics in Acute Myeloid Leukemia.” The paper’s co-first author is Emmanouil Zacharioudakis, PhD, Associate, Biochemistry, Albert Einstein College of Medicine, and in the laboratory of Dr. Gavathiotis.

Additional authors are Christina Glytsou, Xufeng Chen, Wafa Al-Santli, Hua Zhou, Bettina Nadorp, Soobeom Lee, Audrey Lasry, Zhengxi Sun, Dimitrios Papaioannou, Michael Cammer at New York University, Tomasz Zal, Malgorzata Anna Zal, Bing Z. Carter and Jo Ishizawa at MD Anderson Cancer Center, Raoul Tibes at AstraZeneca, Aristotelis Tsirigos, Michael Andreeff and Iannis Aifantis.

Mitochondrial fusion (pictured above) can help mitigate the effects of stress.