Stopping Liver Cancer's Rogue COP

Representative in vivo imaging (left) of animals with HCC cells growing in the liver and treated with either control or COP1 siRNA. Brighter coloring represents more cell growth. Photos of whole livers and sections of liver tissue (right) on day 28 after cell transplantation and treatment with either control or COP1 siRNA.

Representative in vivo imaging (left) of animals with HCC cells growing in the liver and treated with either control or COP1 siRNA. Brighter coloring represents more cell growth. Photos of whole livers and sections of liver tissue (right) on day 28 after cell transplantation and treatment with either control or COP1 siRNA.

Liver cancer is the fourth most common cancer type and the third leading cause of cancer death worldwide. Many liver tumors are actually metastases, tumors seeded in the liver by cancer cells from another organ, but hepatocellular carcinomas (HCCs), the most common liver tumors, are a heterogeneous family of cancers that arise in hepatocytes, the functional cells of the liver. HCCs are often associated with cirrhosis or liver scarring. Because of the variation in tumor phenotypes, the poor understanding of the molecular origins of these tumors, and the increasing number of diagnoses especially in the US, HCC is a major clinical challenge.

To begin filling in these missing pieces in the HCC field, Yun-Han Lee, Ph.D., a fellow in CCR’s Laboratory of Experimental Carcinogenesis working with Snorri S. Thorgeirsson, M.D., Ph.D., and their colleagues set out to identify commonly altered pathways in HCC tumors. One protein they often found over-expressed was COP1. While its biological role has not been fully defined, COP1 can target the tumor suppressor protein p53 for destruction, and COP1 expression was previously shown to predict HCC patient survival, suggesting COP1 may be an important regulator of these tumors.

The researchers tested the effects of reducing COP1 levels in several HCC cell lines using small inhibitory RNAs (siRNAs), which silence a target messenger RNA to inhibit protein production. A loss of COP1 protein led to a significant decrease in cell proliferation with a corresponding increase in programmed cell death or apoptosis. Interestingly, these effects were only observed in p53-expressing cells, implicating COP1’s ability to regulate p53 in its growth promoting activity.

Since p53 is a known regulator of gene transcription, the scientists investigated changes in gene expression with or without COP1 using microarray analysis. A number of known p53-regulated genes were altered with COP1 expression. When COP1 protein levels were reduced, a decrease in pro-growth genes and an increase in anti-proliferation and pro-apoptotic genes, such as the tumor suppressor GLIPR1, were observed. In fact, GLIPR1-mediated regulation of reactive oxygen species may be a common growth inhibitory pathway induced by COP1 knock-down.

As a critical test of the effectiveness of decreasing COP1 protein levels to halt the growth of HCC, the researchers used their siRNA in two animal models. HCC tumor cells were placed under the skin, and, once a tumor formed, siRNA molecules were directly injected into the tumor. In agreement with their hypothesis, the scientists observed a dose-dependent decrease in tumor mass. In another model that more closely reflects HCC in patients, HCC cells were allowed to form tumors in the liver. Systemic treatment with lipid-coated siRNA particles allowed for delivery to the liver. Ten days following the last siRNA treatment, the tumors had shrunk 9-12 fold compared to controls.

In summary, Drs. Lee and Thorgeirsson and their colleagues demonstrated that COP1 plays a critical role in promoting the growth of p53-expressing HCC cells. Inhibiting COP1 protein levels induces the expression of apoptosis-promoting genes and even shrinks tumors in vivo. Future studies will need to examine the normal regulation of COP1 and how COP1 over-expression occurs in HCC. Additionally, it will be important to determine whether targeting COP1 in HCC patients provides the same positive effects on tumor growth observed in the animal models.

Summary Posted: 10/2010

Reference

Cancer Res. 2010 October 19 PubMed Link