Spotlight: Tubacin overcomes leukemia drug resistance

Of all the chemical modifications proteins undergo, we think that acetylation — the addition of a small acetyl group — is one of the most interesting. The classic example of an acetylated protein is histone, one of the proteins that makes up the scaffolding on which DNA is held and regulates which genes are turned "on" or "off." Inhibiting the function of proteins that remove acetyl groups (the so-called histone deacetylases or HDACs) results in overacetylation of the histones and has effects on gene expression, cell-cycle arrest, cell differentiation, and cell death: all issues in cancer cell physiology. In fact, there are many HDAC inhibitors in clinical cancer trials and dozens more in preclinical development.

Histones are important, but other cellular proteins are also targets of HDACs. One particularly interesting acetylated protein is α-tubulin, the basic building block of a cell's microtubules, the skeletal structures as well as intracellular "railroads" for moving molecules around the cell. Knowing that problems with cellular transport and structure are hallmarks of many different diseases, we decided to look for specific inhibitors of α-tubulin deacetylation.

After performing a cell-based screen of a library of over 7000 small molecules, we identified tubacin, and showed that it was a specific inhibitor of HDAC6, one of 11 known human HDACs. Besides deacetylating tubulin, HDAC6 has a fascinating function: It binds both polyubiquinated proteins (i.e., proteins marked for degradation by attached ubiquitin "flags") and dynein (a protein motor associated with microtubules), bringing the two together to help the cell clean house. This takes place through a recently described cell structure called the aggresome.

In addition to the aggresome, cells can also clean up through another structure called the proteasome, although it is unclear why there are two pathways for protein removal. Perhaps having two different disposal systems provides necessary backup in case one fails, as the accumulation of junk protein can cause cell death. But it can also give unfair advantage to a cell that insists on living and thriving at the expense of its neighbors: the basic definition of cancer. It would only make sense to figure out a way to shut down both the aggresome and proteasome disposal systems in cancer cells, leaving them to fill up with waste and die.

Indeed, there are experimental cancer drugs that inhibit the proteasome, resulting — to some degree — in cancer cell death. One such drug, bortemazib, has shown some promise in multiple myeloma, a particularly vicious form of leukemia. But more than half of multiple myeloma patients are resistant to bortemazib. Could it be because the aggresome pathway is picking up the slack from the inhibited proteasome? Can adding tubacin to bortemazib overcome this resistance?

Jay Bradner (left) and Steve Haggarty

Indeed it can. Recent work by Dana-Farber and Broad scientists, published this month in the Proceedings of the National Academy of Sciences, describes treating multiple myeloma cells with both bortemazib and tubacin. This one-two punch resulted in rapid accumulation of intracellular protein "junk" and cell death. The next step is bringing tubacin, or an optimized analog, into animal models of multiple myeloma and possibly a clinical trial.

As exciting as this is, tubacin's potential doesn't stop with cancer. There is growing evidence to support the relevance of HDAC6 and its non-histone substrates as a target for other cancers, including breast cancer. In addition, HDAC6 also targets another cellular protein known as Hsp90, a protein critical in helping other proteins achieve their correct conformation, including proteins implicated in many forms of cancer. Finally, tubacin could also prove useful in neurodegenerative disorders associated with mutations in polyglutamine-encoding tracts like Huntington's disease and other psychiatric diseases in which protein acetylation and deacetylation is known to play a role.

In short, tubacin is a remarkable molecule, not only for its amazing potential in treating disease but for helping us unravel previously unknown cell biology. It deserves to be in the Spotlight.