Virtually every cell in the human body has an internal clock. These clocks take their cues from a central clock in the brain. In a normal, biological process called synchrony, the central clock coordinates daily rhythms around the body, so that every cell and tissue recognizes the same external time of day.
Knowing local time helps our bodies to regulate essential processes, including when to sleep and wake, when to eat and what temperature to maintain, among many other important functions.
But a deadly interloper is keeping time the same way.
Glioblastoma is an aggressive, incurable brain cancer that is the most common malignant brain tumor in adults. New research from Washington University in St. Louis shows that glioblastoma has an internal clock and syncs its daily rhythms to match — and take advantage of — the rhythms of its host. In this way, brain tumors grow in response to the host’s daily release of steroid hormones like cortisol.
Blocking circadian signals dramatically slowed glioblastoma growth and disease progression, the WashU scientists discovered. This process worked both in cells in a dish and in tumor-bearing animals, according to the study published Dec. 12 in Cancer Cell.
“Glioblastoma takes its cues from hormones released by the same central clock in the host that establishes the body’s regular daily rhythms,” said Erik D. Herzog, PhD, the Viktor Hamburger Distinguished Professor and a professor of biology in Arts & Sciences, senior author of the study. “Blocking the daily surge in glucocorticoid signaling desynchronizes circadian rhythms in glioblastoma from the host and dramatically slows disease progression in tumor-bearing mice.”
“Our previous research helped us to see a pattern,” said Maria F. Gonzalez-Aponte, PhD, first author of the study. “Whether we were looking at clinical data, or patient-derived cells or mice with model glioblastoma tumors, chemotherapy treatment always worked best around normal waking time. That’s what led us to think that these tumors knew the time of day outside.”
“This study provides yet another example of how important contextualizing research in real-life biology is to improving cancer treatment. It was possible to extend survival by synchronizing treatment to circadian time. No new drug was required,” said Joshua B. Rubin, MD, PhD, a professor of pediatrics and of neuroscience at WashU Medicine, a longtime collaborator with the Herzog laboratory and a co-author on the paper. Herzog and Rubin are research members of Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine.
The findings are important in part because they affect the way that glioblastoma tumors respond to a drug called dexamethasone (DEX), a synthetic steroid that is commonly given to glioblastoma patients to reduce brain edema after radiation and surgery. This study finds that giving DEX in the morning promotes tumor growth in mice, while giving it in the evening suppresses growth.
“For many years, the use of DEX for glioblastoma has remained controversial because of studies showing either growth-promoting or growth-suppressing effects,” Gonzalez-Aponte said. “Knowing that glioblastoma has daily rhythms, we immediately asked if time of day of DEX administration could explain these different findings, and it seems like it does.”
“The interaction between brain tumors and the circadian system is now a targetable mechanism to optimize treatments,” Herzog said.
Resetting the clock
Every day, just before a person or animal wakes up — in response to light and other environmental cues — the brain sends a signal to the adrenal glands to deliver a surge of steroid hormones called glucocorticoids. These hormones are involved in the well-recognized fight-or-flight response. But they also regulate a variety of more essential biological processes, including metabolism and immunity.
“Under normal conditions, glucocorticoid levels increase dramatically each day prior to waking,” Gonzalez-Aponte said. She and Herzog hypothesized that glioblastoma responds to this reliable daily glucocorticoid blast to set its clock in sync with its host.
To test this idea, Gonzalez-Aponte first set out to see if she could disrupt a tumor’s sense of timing by resetting its host’s daily rhythms.
She placed tumor-bearing mice in cages that could be made light or dark using a timer. By shifting when she turned on the lights, Gonzalez-Aponte coaxed the mice into adopting a flipped schedule. She could tell it was working by observing when each day the mice started running in their wheels.
“Mice run on their wheels more during the night than during the day,” Gonzalez-Aponte said. “When we reverse the light and dark schedule, it’s basically like flying from St. Louis to India. We’re forcing them to resynchronize.”
As the mice took to their new, flipped schedules, the scientists monitored the cancer cells in the tumors in their brains for changes. They used a novel method to image clock gene expression in cancer cells in the freely behaving mice — collecting data every minute for multiple, continuous days. The scientists observed that two clock genes in the cancer cells, Bmal1 and Per2, changed their schedules as the mice changed their schedules.
“What we found was that Bmal1 and Per2 do the same thing as the mouse does in the wheel. That is, the cancer cells are resynchronizing their daily rhythms as the mouse resynchronizes its locomotor activity,” Gonzalez-Aponte said.
Similarly, the tumors remained synchronized to the host in conditions where the mice woke and slept according to their own circadian cycles in the absence of any environmental timing cues.
More than a wake-up signal
Glucocorticoids are just one of the circadian signals that have been shown to synchronize clocks in cells around the body. But glucocorticoids are important in the context of cancer care because synthetic versions of these steroid hormones are sometimes used in high doses to treat symptoms that cancer patients experience after surgery and treatment.
DEX is one of these synthetic glucocorticoids. It is often administered in addition to chemotherapy and can be given to glioblastoma patients to reduce cerebral edema that comes after surgery and radiation. But despite its widespread use, doctors and scientists continue to report mixed results with DEX. Some studies have shown DEX has tumor suppressive effects, while others have shown that DEX promotes glioblastoma cell proliferation.
Gonzalez-Aponte and Herzog suspected that if glioblastoma has its own reliable circadian rhythms, then its response to DEX — a synthetic glucocorticoid hormone — could vary based on the time of day when DEX was administered.
They designed an additional set of experiments that showed that glucocorticoids promote or suppress glioblastoma cell growth depending on the time of day. In mice with glioblastoma brain tumors, the scientists found that tumor size increased significantly if DEX was administered in the morning, as compared to evening or control applications.
These findings, in mice, have implications for the use of glucocorticoids like DEX in the clinic, Gonzalez-Aponte said. Additional research is needed to determine if there are times of the day when DEX can be used to reduce cerebral edema without promoting glioblastoma growth.
“As we continue to study this brain tumor — how it grows, how it interacts with other cells in the brain and how it responds to therapies — it is important to acknowledge that timing is an essential variable,” Gonzalez-Aponte said.
Taking advantage of data from a publicly available cancer database, the researchers found that glioblastoma patients tend to live 60% longer if their tumor expressed less glucocorticoid receptor. This encourages them to pursue clinical trials aimed at avoiding DEX treatments in the morning.
“To critically evaluate the potential for chronotherapy in different cancers, we must consider how daily rhythms arise and synchronize in specific tissues,” Herzog said.
“It’s important to understand how circadian rhythms regulate tumor biology in a cell- and tissue-specific context,” Herzog said. “We believe that this tractable and translatable approach will ultimately personalize patient care by determining when therapies should be given to cancer patients, depending on their individual circadian rhythms.”
Funding: This work was supported by National Institutes of Health (NIH) grants NINDS R21NS120003 and NCI R01NS134885 and Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine.
