Role of the Hippo pathway in cell proliferation and organ size control. Disorders of the pathway in cancer diseases

The Hippo pathway (also known as SWH--Salvador/Warts/Hippo), discovered for the first time in Drosophila melanogaster, is responsible for cell proliferation and organ size control in mammalian systems. The components of the pathway are two kinases and their adaptor proteins which inhibit the transcription co-activator YAP by phosphorylation. When the pathway is inactive (as an effect of upstream component gene expression disorders), activated YAP is translocated to the nucleus where it cooperates with TEAD transcription factor and promotes expression of genes that regulate cell proliferation and apoptosis. YAP acts generally as an oncogene, although there are some reports describing its role as a tumor suppressor. Since all of the core components are well known, the latest reports provide mostly information about upstream components of the Hippo pathway or its interaction with other biochemical pathways. Because of the Hippo pathway's role in the cell cycle, it has become a very attractive object for studies of the genetic background of cancer. The under- or overexpression of genes involved in the Hippo pathway has been described in many different types of cancers. Moreover, it has been shown that there is a strong connection between cancer cell phenotype and highly activated YAP presence in the nucleus. This paper reviews the most important data about Hippo pathway regulation in Drosophila and mammals, including its numerous disorders and their implications for cell function.