Researchers at Stanford University School of Medicine have identified a protein critical for the growth of pancreatic cancer. Blocking the expression of the protein slowed or prevented tumour growth in mice and made cultured cancer cells vulnerable to the conditions of low oxygen that occur in solid tumours.
"This research clearly shows that inhibiting the protein inhibits the tumour’s ability to grow," said cancer biologist Amato Giaccia, PhD. "Ultimately, we’d like to be able to specifically knock out the expression of this protein in pancreatic tumours in humans."
Pancreatic cancer is a highly aggressive and deadly disease that accounts for more than 30,000 deaths in the United States annually, and current therapies are largely ineffective.
"Right now, we have very little to offer these patients," said Giaccia. He is the Jack, Lulu and Sam Willson Professor, Professor of Radiation Oncology, and the senior author of the research, which was published 1 February in the journal Cancer Research. Giaccia is also a member of the Stanford Cancer Center.
The researchers studied a protein called connective tissue growth factor, or CTGF. Also known as CCN2, the protein is involved in the abnormal growth of connective tissue in response to injury or disease. It was also thought to be involved in pancreatic tumour progression, although the exact role it played was unknown.
Giaccia and his collaborators found that human pancreatic cancer cells expressing high levels of CCN2 grew robustly when injected under the skin of mice. In fact, in the developing tumour these cells soon out-competed others that expressed lower levels of the protein. Conversely, pancreatic cancer cells in which CCN2 expression was suppressed were either less likely or unable to form tumours when injected into mice.
The researchers observed similar effects when the cancer cells were injected directly into the animals’ pancreases. Cancer cells expressing high levels of CCN2 formed tumours that grew more rapidly and metastasised more aggressively than did those expressing lower levels, and the mice died sooner than others injected with cancer cells expressing less CCN2.
It’s difficult for many types of rapidly growing solid tumours to recruit and build enough blood vessels to keep all the cancer cells adequately oxygenated. Normal cells undergo a process of programmed cell death when oxygen levels drop too far. Overcoming this response to low oxygen levels – a condition called hypoxia – is a critical step in tumour progression.
The researchers wondered if CCN2 played a role in keeping tumour cells alive in hypoxic conditions. If so, this might explain why CCN2-expressing cancer cells are favored during tumour growth. They found that blocking CCN2 expression in cultured pancreatic cancer cells made them significantly more sensitive to hypoxia-induced death than their peers. Additionally, CCN2 was more highly expressed in pancreatic tumour samples from human patients than in neighboring tissue and CCN2 expression seemed to correlate with the expression of another protein expressed by hypoxic cells. Finally, hypoxic conditions themselves cause the pancreatic cancer cells to make CCN2.
Many other cellular conditions can also kickstart CCN2 expression, including the presence of CCN2 itself. The activation of other pathways known to be involved in cancer also increases its expression. As a result, many of the events that occur in a developing tumour act as a kind of perfect storm to support the production of ever larger amounts of CCN2, which then support additional tumour growth and metastasis.
Looking ahead, the researchers would like to know whether people with pancreatic cancer could benefit from therapies targeting CCN2. A phase 1 clinical trial testing the safety of an antibody that binds CCN2 and blocks its activity in a small number of patients began in December at Stanford and Dartmouth-Hitchcock Medical Center. Phase 1 clinical trials are not designed to determine whether a treatment works – only whether it is safe enough for further testing. Albert Koong, MD, PhD, an assistant professor of radiation oncology and a member of the Cancer Center, is the principal investigator for the Stanford arm of the trial.
"We saw a pronounced effect of CCN2 inhibition in these experiments in mice," said Giaccia.
"Our hope is that one day a combination of standard therapy and antibody treatment will have an effect on tumour progression in human patients."
(Source: Stanford University School of Medicine : Giaccia A. The role of tumour cell-derived connective tissue growth factor (CTGF/CCN2) in pancreatic tumour growth. Cancer Research. : February 2009)