Cancer

Inhibiting telomerase, an enzyme that rescues malignant cells from destruction by extending the protective caps on the ends of chromosomes, kills tumor cells but also triggers resistance pathways that allow cancer to survive and spread, scientists report in Cell.

"Telomerase is overexpressed in many advanced cancers, but assessing its potential as a therapeutic target requires us to understand what it does and how it does it," said senior author Ronald DePinho, M.D., president of The University of Texas MD Anderson Cancer Center.

"We exploited the experimental merits of mice to model and study more precisely telomere crisis, telomerase reactivation and telomerase extinction in cancer development, progression and treatment," DePinho said. "This elegant model exposed two mechanisms, including one unexpected metabolic pathway, used by cancer cells to adapt to loss of telomerase.

"These findings allow us to anticipate how tumor cells might respond to telomerase inhibition and highlight the need to develop drug combinations that target telomerase and these adaptive resistance mechanisms," DePinho said.

Researchers evaluated telomerase as a therapeutic target in experiments that originated in DePinho's lab at the Dana-Farber Cancer Institute in Boston. He became MD Anderson's fourth full-time president in September.

Telomerase activity is low or absent in normal cells, which have segments of repeat nucleotides called telomeres at the ends of their chromosomes that protect DNA stability during cell division, said first author Jian Hu, Ph.D., an instructor in MD Anderson's Department of Cancer Biology.

With each division the telomeres shorten, leading eventually to genomic instability and cell death, a period termed "telomere crisis," Hu said. In cancer, telomerase becomes active during telomere crisis and rescues the genomically abnormal cells by lengthening telomeres.

In a series of experiments in a lymphoma mouse model, the team found:

• Telomerase reactivation in malignant cells after genomic instability causes cancer progression.
• Inhibiting telomerase caused tumor cell death but also led to alternative lengthening of telomeres (ALT) independent of telomerase.
• ALT-positive cells increase both the expression and copy number of a gene called PGC-1ß, a key regulator of mitochondrial function, to compensate for mitochondrial and reactive oxygen species defense deficiencies.
• Targeting PGC-1ß to weaken mitochondria function enhances anti-telomerase therapy.

To study the impact of reactivated telomerase, DePinho and colleagues genetically modified mice to develop T cell lymphomas and to have telomerase reactivated when the mice are treated with 4-hydroxytamoxifen (4-OHT). They crossed the mice for several generations, treating some with 4-OHT and others with a control vehicle that did not trigger TERT activation.

Telomerase reactivation causes aggressive cancer

Third- and fourth-generation mice with telomerase activated by 4-OHT had a median survival of 30 days and more frequent tumor infiltration to the spleen, kidney, liver, lung, bone marrow and brain than did control-treated mice, 70 percent of which lived beyond 50 days. Tumor cells in control-treated mice were more likely to be detected and destroyed by tumor-suppressing p53 signaling.

"These findings are consistent with telomere crisis leading to genomic instability during early stage cancer, with reactivated telomerase protecting malignant cells later to ensure tumor progression," Hu said.

Telomere dysfunction causes cancer-promoting genetic changes

Later-generation mice with activated telomerase had 4,928 amplified genes and 2,297 deletions. The team compared these changes to those in human lymphoma tumors and found 565 matching amplified genes and 300 deletions.

These cross-species copy number alterations included several known tumor-suppressing genes and oncogenes, suggesting that initial telomere dysfunction not only drives primary tumor development but also confers malignant traits such as invasiveness.

Telomerase extinction works - at first

The team then took tumor cells from late-generation mice with activated telomerase - the aggressive tumors - and passaged them four times through groups of mice treated with either 4-OHT to trigger telomerase production or the control vehicle that leaves the enzyme off.

During the first two rounds, survival for the two groups was about the same. In the third round, the control mice had a major improvement in survival over the telomerase arm, indicating that telomere erosion had allowed cellular defense mechanisms to pick off genomically unstable cells.

However, in the fourth passage, survival of the control-treated mice fell back toward that of mice with active telomerase. The tumors had become resistant without relying on telomerase.

Alternative lengthening of telomeres rescues cancer cells

An analysis showed that telomere lengths of tumor cells with active telomerase remained largely unchanged across the four passages. Telomeres shortened in cells lacking telomerase through the first two passages followed by a sharp increase during the third and fourth passages.

Other molecular evidence pointed to alternative lengthening of telomeres in telomerase-absent cells.

They found that ALT-positive tumors had different gene expression patterns - 891 genes with increased expression, 1,345 with decreased - compared to telomerase-positive tumors.

Key gene in mitochondrial pathway active in ALT cells

Many genes were found in networks regulating mitochondrial biology and oxidative stress regulation. PGC-1ß was the only gene in both pathways with increased expression and copy number gain in the ALT-positive tumors.

PGC-1ß is a master regulator of both pathways, which turn out to be dysfunctional in ALT-positive tumors. When the researchers knocked down PGC-1ß, mice with ALT-positive tumors survived much longer than those with intact PGC-1ß and than those with activated telomerase in their tumors.

In normal cells, power-generating mitochondria process fatty acids to produce ATP, a molecule that serves as the major energy source for the cell. Cancer cells generally rely more on sugar processing to generate energy. However, DePinho and colleagues note their genetic evidence suggests that mitochondria play a role supporting cancer cells

Combining gemcitabine with MRK003, an experimental drug, triggers a chain of events leading to pancreatic cancer cell death, researchers from Cambridge reported in the Journal of Experimental Medicine. The researchers explained that when the two drugs are combined, the effect of each one is multiplied, thus intensifying the destruction of pancreatic cancer cells.

Professor David Tuveson, from the Cambridge Research Institute, UK, and team demonstrated in animal studies that MRK003, an experimental medication, when combined with chemotherapy medication gemcitabine, set off a domino effect which ultimately destroyed the malignant cells.

The drug combo is being used in a human study, a clinical trial, which is being managed by Cambridge University Hospitals Foundation Trust, together with Cancer Research UK's Drug Development Office.

MRK003 is a gamma secretase inhibitor. It inhibits, or blocks a crucial cell signaling pathway in both pancreatic cancer cells and the cells in the lining of blood vessels that supply the tumor with vital nourishment (endothelial cells).

The researchers found that when MRK003 is added to gemcitabine, the chemotherapy drug's ability to destroy tumors was significantly enhanced. Gemcitabine is a nucleoside analog, which is marketed as Gemzar by Eli Lilly. Gemcitabine is commonly used in pancreatic cancer therapy, as well as non-small cell lung cancer, bladder cancer and breast cancer.

Following a week-long meeting of international experts, the World Health Organization's (WHO's) cancer panel has classified diesel engine exhaust as carcinogenic or cancer-causing to humans, more than 20 years after it was classified as "probably carcinogenic to humans".

The International Agency for Research on Cancer (IARC) told the press on Tuesday that it had based its decision on "sufficient evidence that exposure is associated with an increased risk for lung cancer".

The new decision follows an IARC Monographs Meeting that took place from 5 to 12 June in Lyon, France, and places diesel engine exhaust in Group 1, alongside more than 100 other agents such as tobacco products, asbestos, benzene, UV rays and secondhand smoke.

The panel also noted that there was a link between exposure to diesel engine exhaust and higher risk of bladder cancer.

In 1988, IARC classified diesel exhaust as "probably carcinogenic to humans" (Group 2A). An advisory group said in 1998 that this classification should be reviewed.

Concerns about the cancer-causing potential of diesel exhaust have been rising, particularly after the publication of a number of epidemiological studies of workers in various settings.

The most recent evidence came in March 2012, when the Journal of the National Cancer Institute (JNCI) published a large study by the US National Cancer Institute and the National Institute for Occupational Safety and Health of underground miners' exposure to diesel engine emissions, that showed an increased risk of death from lung cancer in exposed workers.

Dr Christopher Portier, who chaired the IARC working group that reviewed the evidence, said:

"The scientific evidence was compelling and the working group's conclusion was unanimous: diesel engine exhaust causes lung cancer in humans."

"Given the additional health impacts from diesel particulates, exposure to this mixture of chemicals should be reduced worldwide," said Porter, who is Director of the National Center for Environmental Health and the Agency for Toxic Substances and Disease Registry at the US Centers for Disease Control and Prevention (CDC).


People are exposed to diesel engine exhaust every day, whether at work or just through ambient air, says the IARC. These fumes come not only from cars and buses but also from other engines such as those in diesel trains, ships and power generators.

The panel also reviewed the evidence on gasoline (petrol) exhaust and concluded it should remain in the "possibly carcinogenic to humans" category (Group 2B), where it has been since 1989.

The IARC says there is now sufficient evidence for governments and decision makers to formulate environmental standards for diesel exhaust emissions and to work with engine and fuel manufacturers to attain these targets.

Dr Christopher Wild, Director of IARC, said the new ruling "sends a strong signal" for action to protect public health. He said action is "needed globally, including among the more vulnerable populations in developing countries where new technology and protective measures may otherwise take many years to be adopted".

More developed countries in North America, Europe and elsewhere have already been tightening up emission standards for diesel and gasoline/petrol engines, and as standards become more stringent, so the technology improves, which in turn allows standards to tighten further. Such changes have reduced sulfur content in fuel, increased efficiency in diesel engines, and led to reductions in exhaust emissions.

But fuels and engines that don't have these modifications are still around, and it may be many years before they are replaced, particularly in less developed countries with less stringent standards, said the IARC panel, noting that many developing countries don't have any regulation at all

Substituting smokeless tobacco products can save smokers' lives, and there is a scientific foundation that proves it.

That is the message Brad Rodu, D.D.S., professor of medicine at the University of Louisville (UofL) School of Medicine and the Endowed Chair in Tobacco Harm Reduction at UofL's James Graham Brown Cancer Center, delivered at the Annual Meeting of the American Association for the Advancement of Science. Rodu spoke at the session, "Harm Reduction: Policy Change to Reduce the Global Toll of Smoking-Related Disease."

"Quit or die: That's been the brutal message delivered to 45 million American smokers, and it has helped contribute to 443,000 deaths per year, according to statistics from the Centers for Disease Control and Prevention," Rodu said. "The truth, however, is that total nicotine and tobacco abstinence is unattainable and unnecessary for many smokers."

Rodu's presentation, "Transforming Tobacco Use: The Potential of Tobacco Harm Reduction," was based on his almost 20 years of research. His work shows that smokers can greatly reduce their risk of disease and death by replacing smoking products with e-cigarettes or modern, spit-free smokeless tobacco. These products provide a much safer alternative for those smokers who are unable or unwilling to quit smoking because they continue to deliver nicotine without the harmful effect of smoking.

"Nicotine is addictive, but it is not the cause of any smoking-related disease. Like caffeine, nicotine can be used safely by consumers," Rodu said.

Decades of epidemiologic research bear out Rodu's findings. While no tobacco product is completely safe, smokeless products have been shown to be 98 percent safer than cigarettes. In the United Kingdom, the Royal College of Physicians reported in 2002 that smokeless tobacco is up to 1,000 times less hazardous than smoking, and in 2007, further urged world governments to seriously consider instituting tobacco harm reduction strategies as a means to save lives.

To see the proof of what tobacco harm reduction can do, look to Sweden, Rodu said. "Over the past 50 years, Swedish men have had Europe's highest per capita consumption of smokeless tobacco as well as Europe's lowest cigarette use. During the same time, they also have the lowest rate of lung cancer than men in any other European country."

In the United States, steps have been made to document the value of tobacco harm reduction. In 2006, a National Cancer Institute-funded study estimated that if tobacco harm reduction was "responsibly communicated" to smokers, 4 million would switch to smokeless tobacco. The American Council on Science and Health - which organized Rodu's session at the AAAS Annual Meeting - concluded in the same year that tobacco harm reduction "shows great potential as a public health strategy to help millions of smokers."

Rodu is well aware of the controversy his research findings generate. Opponents of any use of nicotine delivery products maintain that smokeless tobacco puts the user at great risk for oral cancer, a position not supported by research.

"The risk of mouth cancer among smokeless tobacco users is extremely low - certainly lower than the risk of smoking-related diseases among smokers," he said. "The annual mortality rate among long-term dry snuff users is 12 deaths per 100,000 and the rate among users of more popular snus, moist snuff and chewing tobacco is much lower. For perspective, the death rate among automobile users is 11 per 100,000 according to a 2009 report from the National Highway Traffic Safety Administration. Compare those to the rate among smokers: more than 600 deaths per 100,000 every year"

"The data clearly show that smokeless tobacco users have, at most, about the same risk of dying from mouth cancer as automobile users have of dying in a car wreck.