Johns Hopkins. US: The protein NHE9 sits on the surface of cellular “cargo carriers,” or
endosomes, to keep cargo delivery and removal going at the right rate. In certain forms of autism, NHE9 is blocked, causing the endosomes to remove key proteins too quickly from brain cells. A new study shows that in certain brain cancers, NHE9 is overactive,
producing faulty endosomes that leave deadly, tumor-enhancing proteins
on the cell’s surface for too long.
Applying lessons learned from autism to brain cancer, researchers at The
Johns Hopkins University have discovered why elevated levels of the
protein NHE9 add to the lethality of the most common and aggressive form of brain cancer, glioblastoma. Their discovery suggests that drugs designed to target NHE9 could help to successfully fight the deadly disease.
A summary of their work in human tumor cells and mice will be published on Feb. 9 in the journal Nature Communications.
“My laboratory’s research on cargo transport inside the cells of
patients with autism has led to a new strategy for treating a deadly
brain cancer,” says Rajini Rao, Ph.D.,
a professor of physiology at the Johns Hopkins University School of
Medicine. “This is a great example of the unexpected good that can come
from going wherever the science takes us.”
All animal and human cells contain many “cargo packages” surrounded by
membranes. These so-called endosomes carry newly minted proteins to
specific destinations throughout the cell and haul away old proteins for
destruction. Key to their “shipping speed” is the level of acidity
inside the endosomes. This is controlled by balancing the activity of
protein “pumps” that push protons into endosomes to increase their
acidity with that of protein “leaks,” like NHE9, that remove protons.
Rao says: “Endosomes are like buckets of water that have to be kept
full despite the leaks in them. Altering either the faucet or the leak
rate can dramatically change the water level in the bucket.”
Rao’s research group previously showed that autism-associated defects
in the protein NHE9 are harmful because they “clog the leaks,” leaving
endosomes too acidic and making them race to remove cargo from the cell
membrane, destroying proteins prematurely.
To better understand NHE9, graduate students and postdoctoral fellows
in Rao’s lab searched through patient databases to see if it had other
effects on human health. To their surprise, they found that elevated
levels of NHE9 are associated with resistance to radiation, chemotherapy
and poorer prognoses for patients with glioblastomas.
Teaming up with Alfredo Quinones-Hinojosa, M.D.,
a professor of neurosurgery at Johns Hopkins, the researchers examined
NHE9 in tumor cells from several patients. Cells with low levels of NHE9
grew the slowest, the team found, and those with the highest levels
grew fastest. Similarly, the cells with the most NHE9 traveled fastest
when placed on a surface mimicking that of the brain, suggesting a high
potential for metastasis. And this was confirmed when the tumor cells,
which were manipulated to have high or low NHE9, were transplanted into
the brains of mice.
Based on their autism research, the team suspected that the boost NHE9
gave to glioblastomas was explained by abnormal endosome acidity.
Further studies revealed that, in contrast to autism, NHE9 is overactive
in brain cancer, causing endosomes to leak too many protons and become
too alkaline. This slows down the “shipping rate” of cancer-promoting
cargo and leaves them on the cell surface for too long.
Research from other laboratories suggested that one such cargo protein
is EGFR, which maintains cancer-promoting signals at the cell surface
and helps tumors become more robust so they grow and move faster. It is
also found at elevated levels in more than one-half of patients with
glioblastomas. Drugs targeting EGFR in these patients are sometimes
effective.
As they suspected, the team found that alkaline endosomes slow down the
removal of EGFR from cell surfaces. Lab-grown tumor cells were more
readily killed when treated with both a drug countering NHE proteins and
a drug against EGFR than when treated by the EGFR-targeting drug alone.
Quinones-Hinojosa says: “We are still five to 10 years away from
testing this idea in patients, but these results are encouraging. They
give us a better idea of what to target so that hopefully we can make
this disease less aggressive and less devastating.”
Other authors of the report include Kalyan Kondapalli, Jose Llongueras,
Vivian Capilla-Gonzalez, Hari Prasad, Anniesha Hack, Christopher Smith
and Hugo Guerrero-Cazares of the Johns Hopkins University School of
Medicine.
This work was supported by grants from the National Institute of
Neurological Disorders and Stroke (NS070024), the National Institute of
Diabetes and Digestive and Kidney Diseases (DK054214), the National
Institute of General Medical Sciences (GM62142), the American Heart
Association (11POST7380034), the Johns Hopkins Post-Baccalaureate
Research Education Program, the International Fulbright Science and
Technology Award, and the American Physiological Society’s Porter
Physiology Development Fellowship.