Screening of tissue microarrays for ubiquitin proteasome system components in tumors

The turnover of key proteins that mediate development, cellular proliferation, and a host of essential biological processes is controlled by the ubiquitin proteasome system (UPS). In several well-studied examples, notably in the cell cycle, regulatory proteins that control ubiquitin-dependent destruction are themselves substrates of the UPS, creating a multilayered system to ensure precise and dynamic control of protein stability. UPS regulators controlled at the level of protein stability--including the F-box protein Skp2 and the VHL protein (substrate adapter proteins for multicomponent E3 ubiquitin ligases)-- seem to be misregulated in tumors. In these cases, especially, measuring levels of critical regulatory and target proteins will often present a more biologically meaningful picture than examining relative mRNA levels, which do not always reflect corresponding protein levels. Tissue microarrays (TMAs) allow simultaneous screening of large numbers of tumors for expression of specific proteins by immunohistochemical staining of a single microscope slide prepared from a TMA paraffin block. Replicate slides prepared from the same block can be immunostained for multiple proteins functioning in a related pathway, and a semiquantitative protein expression profile for a given subset of UPS pathway components, or other subsets of proteins of interest, can be assembled. Protein expression profiles of individual tumors or tissue types can be compared and visualized by hierarchical clustering methods. These expression profiles may be used as screening tools to investigate the relative abundance of components of a biochemical pathway in tumors or other tissues. TMAs have an exciting future as tools for basic research, diagnostic pathology, and drug targeting. In this article, we provide an introduction to the use of TMAs to study the expression of UPS component proteins and substrates in tumors by immunohistochemistry.