Every cellular signaling mechanism needs an ‘off switch’ to limit the risk of damage or cancerous growth. This is especially true for signaling factors such as the multi-protein NF-κB complex, which regulates a diverse range of activities within the cell and needs to be shut off as rapidly as it gets turned on.
NF-κB activation is reasonably well understood: external stimuli trigger the release and degradation of inhibitor factors bound to the complex, freeing NF-κB to undergo additional chemical modifications that enable it to activate its target genes. However, scientists are still working to understand deactivation.
“The NF-κB pathway is found deregulated in a multitude of human disorders ranging from inflammation to cancer,” explains Vinay Tergaonkar of the Institute of Molecular and Cell Biology (IMCB) of A*STAR, Singapore. “Clearly it is a multi-billion [dollar] drug target and identifying novel players—especially inhibitors—in the pathway is important both academically and pharmaceutically.”
As with many other signaling proteins, NF-κB activation depends on the addition of phosphate chemical groups; deactivation of such signals typically relies on the removal of these phosphates by specialized enzymes called phosphatases. Biochemical and genetic screening experiments performed recently by Tergaonkar’s laboratory, in collaboration with two partner laboratories at the IMCB and a team from A*STAR’s Singapore Immunology Network, revealed one phosphatase—WIP1—that inhibits expression of numerous NF-κB-activated target genes. Their data demonstrate that WIP1 constrains NF-κB activity by at least two mechanisms: indirectly, via inhibition of NF-κB activator p38; and directly, by dephosphorylation of one of the NF-κB subunits.
This latter mechanism is especially notable, as it represents the first demonstration of direct inhibition of NF-κB by a phosphatase. “Excess NF-κB activity can cause havoc in cells, as its target genes include cytokines like TNFα, which are potent killers,” says Tergaonkar. “WIP1 can negatively regulate NF-κB by removing a phosphorylation that makes NF-κB a potent transcription factor.”
Accordingly, the team noted elevated expression levels for various inflammation-related signaling factors in cells lacking WIP1. Likewise, WIP1-deficient mice produced significantly increased levels of these inflammatory factors in response to bacterial toxins relative to their wild-type counterparts, and spleen cells from these animals showed clear evidence of increased NF-κB signaling.
These intriguing findings may just be the tip of the iceberg, as the screen that revealed WIP1 also spotlighted several other promising candidates. “We have been working on this for the past four years,” says Tergaonkar, “and our plans now are to characterize the other NF-κB regulators identified in our screens.”