Sylvester researchers have discovered exactly how one critical regulator affects a transcription factor that is consistently turned on in most cancers. The findings of Edward W. Harhaj, Ph.D., assistant professor of microbiology and immunology and member of the Viral Oncology Program at Sylvester Comprehensive Cancer Center at the University of Miami Miller School of Medicine, and Noula Shembade, Ph.D., research assistant professor of microbiology and immunology, have been published in the February 26 issue of the prestigious journal Science.
Transcription factor nuclear factor kappa-B (NF-ΚB) regulates hundreds of genes that are involved in a wide variety of different functions such as inflammation, cell development and cell death. When NF-ΚB is functioning normally, for instance during an infection, certain cytokines will be produced and they will activate NF-ΚB for a brief period of time. During that time, NF-ΚB activates specific genes and then it’s shut off. If NF-ΚB is not tightly regulated, it stays on continuously which can lead to auto-immune diseases and unregulated cell growth that can become cancer.
The zinc finger protein A20 has an essential role in limiting the strength and duration of NF-ΚB signaling. The process works like an auto-regulatory loop in that NF-ΚB activates, turning on the expression of A20 which in turn shuts down NF-ΚB, resetting the system.
The NF-ΚB pathway is regulated by ubiquitin, a small molecule that attaches itself to proteins and either triggers their degradation or changes how that protein works. Ubiquitination occurs in a 3-step enzymatic cascade consisting of E1, E2 and E3 enzymes. Ubiquitin is first activated by a ubiquitin-activating enzyme (E1) and transferred to a ubiquitin-conjugating enzyme (E2). In the final step, the ubiquitin ligase (E3) transfers ubiquitin to a protein substrate.
There are several important proteins in the NF-ΚB pathway that function as E3s, including TRAF2 and TRAF6. In a normal process, these proteins work by binding to Ubc13 and UbcH5c, both E2 ubiquitin-conjugating enzymes. Harhaj and Shembade discovered that “A20 works by disrupting the interactions between the TRAF proteins and Ubc13 and UbcH5c. It disrupts the binding of E2 to E3 enzymes.”
By shutting down this linear cascade, explains Harhaj, A20 in effect tells NF-ΚB to stop working. In certain cancers such as lymphomas, if A20 is not present, these E3 ligases are persistently turned on leading to constitutive NF-ΚB activation and cell survival. Harhaj explains that this finding could lead scientists to “create drugs that mimic the action of A20, that inhibit E2, E3 interactions when A20 is not expressed.”
Joseph Rosenblatt, M.D., professor of medicine, microbiology and immunology and interim director of Sylvester, says Harhaj’s discovery “provides us with unique insight into the regulation of the NF-ΚB pathway. His research will lead to new discoveries that can be applied to develop new therapies for our patients.”