A POX on Renal Cancer Cells
The introduction of a miR-23b*-inhibiting microRNA molecule (Antagomir) into cultured cancer cells caused a significant decrease in miR 23b* expression and a simultaneous increase in POX protein, which produced reactive oxygen species (ROS) and led to cellular self-destruction called apoptosis.
Proline oxidase, or POX, is an enzyme responsible for metabolizing the amino acid proline. POX contributes to the regulation of cell death that occurs when cellular systems malfunction, a process called apoptosis. Previous studies have determined that levels of POX are reduced in several types of human cancer. Likewise, many cancer cells become resistant to apoptosis, suggesting a link between POX and cancer cell survival.
Wei Liu, Ph.D., a postdoctoral fellow in the CCR Laboratory of Comparative Carcinogenesis, working under James Phang, M.D., has been studying the link between POX and resistance to apoptosis in renal, or kidney, cancer cells. Early studies failed to elucidate the mechanism by which cancer cells reduce the level of POX. A recent paper in Oncogene described how Dr. Liu and her colleagues were successful in determining how POX levels are controlled by small RNA molecules, called microRNAs, that are able to bind to mRNA encoding POX and prevent the production of POX protein.
Based on the sequence of the POX mRNA, the researchers were able to identify 91 microRNA molecules that could potentially influence POX protein expression. Renal cancer cells from cultures were then examined for the presence of each microRNA molecule. One microRNA, miR-23b*, was present at higher levels in cancer cells than in normal kidney cells, and when artificially introduced into noncancerous cultures of kidney cells, miR-23b* caused a 53% decrease in POX protein.
The link between miR-23b* and POX was further evidenced by the introduction of a miR-23b*-inhibiting molecule, called antagomir into cultured cancer cells, which caused a significant decrease in miR 23b* expression and a simultaneous increase in POX protein. The inverse correlation between the levels of miR-23b* and POX was also observed in renal cancer cells harvested directly from patients. Importantly, the amount of mRNA coding for POX was not affected by the presence or absence of miR-23b*, suggesting that the observed changes were in POX protein levels as a result of binding of miR23b* to POX mRNA rather than changes in gene expression.
The scientists then investigated whether miR-23b* could control the initiation of apoptosis through POX to influence cell survival. Artificially increasing the expression of POX in normal cells leads to an increase in cell death. However, introduction of miR-23b* into cells that normally produce large amounts of POX reduced the level of POX protein and killed fewer cells. A similar effect was observed in cancer cells. When the high levels of miR-23b* normally found in cultures of renal cancer cells was inhibited by the addition of miR23b* antagomir, cells undergoing apoptosis increased from 6% to 16%.
The confirmation of reduced expression of POX in renal cancer cells by Dr. Liu and her colleagues reinforces its potential role as a tumor suppressor since cancer cells that are unable to undergo apoptosis can grow unchecked and promote tumor development. Identification of miR-23b* in the regulation of POX protein levels suggests that the microRNA may be a target for the treatment of renal cancer or be useful in the development of a diagnostic tool to identify cancers that are resistant to apoptosis, a mechanism tumors employ to escape cancer treatments.Summary Posted: 07/2010
Oncogene 2010 Jun 21 [Epub ahead of print] PubMed Link