BioWorld International Correspondent

LONDON - A team of European researchers identified an enzyme that helps to activate one of the most important oncogenes in human tumors. The molecule could, the researchers predicted, form a potential new target for cancer chemotherapy.

Martin Eilers, professor of molecular biology at the Institute of Molecular Biology and Tumour Research in Marburg, Germany, told BioWorld International, "This enzyme is particularly attractive as a therapeutic target because there is evidence from murine models of cancer that switching off this oncogene can bring about tumor regression, even after metastasis has taken place."

The group is collaborating with a pharmaceutical company on carrying out a high-throughput screen for small molecules that can inhibit the enzyme, which is called HectH9.

In their paper in the Nov. 4, 2005, issue of Cell, Eilers and his colleagues reported their discovery that HectH9 helps activate the product of the oncogene c-myc. That oncogene encodes the transcription factor Myc, which is expressed in a majority of human tumors.

The title of the paper is "The Ubiquitin Ligase HectH9 Regulates Transcriptional Activation by Myc and Is Essential for Tumor Cell Proliferation."

Eilers, working with colleagues in Marburg and collaborators in Italy, Denmark and elsewhere in Germany, investigated the process of protein disposal known as ubiquitination. When a cell needs to dispose a protein, it attaches multiple copies of the protein ubiquitin (known as polyubiquitin) to the target protein, and this "labeling" directs the unwanted molecule to the cellular "dustbin," the proteasome.

Research by other groups already had shown that when polyubiquitin chains, attached to a targeted protein, are linked through the lysine residue at position 48, the protein is destined for degradation in the proteasome. Yet studies also had demonstrated that another type of ubiquitination could take place - in that case, the ubiquitins in the polyubiquitin chains are linked to each other through the lysine residue at position 63.

But Eilers and his team showed that when Myc undergoes the second type of ubiquitination, Myc becomes activated. The group identified the enzyme responsible as an E3 ligase called HectH9.

"The importance of this discovery is twofold," Eilers explained. "First, it is very interesting from the point of view of basic science, to show how the function of this transcription factor Myc is regulated. Secondly, there is now the potential to inhibit the enzyme that is required for this oncogene to function."

Data from animal models have shown, Eilers added, that inhibiting Myc, even at late stages of tumorigenesis, can induce tumor regression.

However, translating that finding into a therapeutic strategy has been difficult, because transcription factors are not readily amenable to inhibition with small molecules.

"If we were able to find an inhibitor of HectH9," Eilers said, "there are indications that such a treatment might be of benefit for even advanced tumors. Although it is not clear that the E3 ligases will be generally inhibited by small molecules, the encouraging news is that there are thousands of ligases in our genomes and each of these is likely to have a very specific biological role."

The team went on to examine expression levels of HectH9 in tumor samples from patients with colon cancer of varying stages. The team found that it was undetectable or expressed at very low levels in normal gut epithelium and in polyps, while it was present at moderate or high levels in up to half of all tumors. Further tests confirmed that it was expressed only in tumor samples.

Analysis also showed that HectH9 was expressed at moderate or high levels in a high percentage of lung and breast carcinomas, but that it was undetectable in normal breast tissue.

A scan of expression databases showed that HectH9 is higher in many different types of solid tumors, including those of lung, breast, prostate and colon, in comparison to normal tissue.

Eilers and his group now are planning to generate a conditional knockout mouse model that will allow them to assess the effect on colon cancer of removing HectH9. That approach will allow researchers to gauge how useful a drug targeted at HectH9 might be.

Eilers said, "One way to do this would be to allow the tumor to develop to an advanced stage in the mouse model, and then flip the switch' so that the gene for HectH9 is inactivated. We would then be able to see if we get tumor regression."