Thirteen years ago, when Lin-Feng Chen joined the Department of Biochemistry, he had his sights set on a specific protein that could both contribute to the growth of cancer and be a major factor in boosting the body’s immune system.
“The protein in question was NF-kappaB (nuclear factor kappa-light-chain-enhancer of activated B cells), a kind of “master controller” of innate and adaptive immune response, and cell survival,” explains Chen.
In healthy cells, it spends most of its life in the cell’s cytoplasm, quietly awaiting orders to assist the immune system. In response to viral and bacterial infection, NF-kappaB moves into the nucleus and triggers the expression of many genes that help to fight against infection. After eliminating the pathogens, NF-kappaB returns to the cytoplasm, awaiting the next order. However, NF-kappaB can work against the body’s best interests, too. Sustained nuclear NF-kappaB is associated with a variety of inflammatory diseases and cancers.
Dr. Chen’s postdoctoral work at the Gladstone Institute of Virology and Immunology at University of California at San Francisco focused on the post-translational regulation of NF- kappaB in immunity and cancer. What chemical modifications were needed to make NF-kappaB dormant, until needed, in healthy cells and run amok in diseased cells?
That work identified acetylation, a chemical tag that can be added to lysine, of NF-kappaB that dictates the transcriptional outcome of NF- kappaB. So the major question when Dr. Chen started his lab at the University of Illinois was to find out how the modification of lysines affected the activity of NF-kappaB.
At that time there were some clues that the acetylated lysine can control the protein’s activity through a signaling partnership with a bromodomain, a pocket-like structure, that can accommodate acetylated lysine and change the properties of the proteins.
“We stared to look for bromodomains that might recognize the acetylated lysine of NF- kappaB and proteins that contain such a domain could be working with NF-kappaB,” said Chen. In 2009, Chen’s group identified bromodomain- containing protein 4 (BRD4) as a co-activator of NF-kappaB, specifically recognizing the acetylated lysine of NF-kappaB via its bromodomains.
This discovery launched a decade of work in Chen’s lab focusing on BRD4’s role in inflammation and cancer.
BRD4 belongs to a class of molecules that can recognize specific chemical tags on other proteins to spur the marked proteins to perform various tasks. Chemical tag "readers" such as BRD4 are important players in the field of epigenetics, which focuses on how specific genes are regulated.
"In epigenetics, there are writers, there are readers, and there are erasers. BRD4 is the reader," Chen said. The writers and erasers add or remove tags to or from proteins, without changing the underlying sequence of the gene that codes for them. Drugs targeting epigenetic regulators have emerged as novel therapies in cancer treatment, and a number of small- molecule inhibitors against epigenetic regulators have already been developed for cancer therapy.
Exploring the role of BRD4 in cancer, Chen’s group found that BRD4 prevents the degradation of nuclear NF-kappaB by binding to acetylated NF-kappaB, and contributing to the sustained presence of NF-kappaB in the nucleus of cancer cells.They also found that a small molecule, JQ1, which blocks the interaction between BRD4 and NF-kappaB, reduced proliferation of cancer cells and suppressed the ability of cancer cells to form tumors.
“These findings opened exciting possibilities for translational research. The interaction between BRD4 and NF-kapapB could be a target for therapies to stop the spread of cancer or inflammatory disease,” said Chen.
Chen’s lab recently discovered that there was a higher expression of BRD4 in gastric cancer cells and in gastric cancer patient samples. They identified an enzyme, PIN1, the expression of which was highly correlated to BRD4 in gastric cancer cells and regulated the tumor-promoting activity of BRD4 in gastric cancer cells. They also found that BRD4 promoted gastric cancer cell proliferation by preventing the cell senescence.
Chen’s latest research is delving even deeper into the physiological role of BRD4 in cancer and inflammation. A recent paper in Oncogene demonstrated that removing BRD4 from specific mouse tissues compromised the mice’s innate immunity to find against bacterial infection and tumor. With these tissue specific BRD4-deficient mice, they have tried to determine the contribution of BRD4 in inflammatory diseases and inflammation-associated cancer.
Various inhibitors targeting BRD4 are undergoing clinical trials for the treatment of cancer and inflammatory diseases. “Our studies will provide new insights for developing therapies targeting BRD4,” said Chen. “This is very translational research.”
