Signalling pathways involving a cascade of kinase-catalysed phosphorylation reactions are involved in the control of cellular proliferation and tissue growth in lower vertebrates as well as in mammals including humans. Aberrations in these pathways, which have been termed tumour suppressor pathways, may lead to inappropriate tissue growth and thus to cancer. Understanding the often very complex signalling patterns within these pathways may therefore lead to the identification of novel cancer drug targets.

The fruit fly Drosophila has been used as a model organism in genetics for many decades, and studies of its signalling pathways can shed important insights into homologous pathways in mammals. Florence Janody and her colleagues at the Instituto Gulbenkian de Ciência (Gulbenkian Institute), Oeiras, Portugal have now obtained some important insights into one tumour suppressor pathway in Drosophila, termed the Hippo¹ pathway, and its role in regulating the accumulation of the cytoskeletal protein F-actin. Regulation of the growth, assembly and disassembly of actin filaments is an important factor in the control of cell growth in all multi-cellular eukaryotes.

Janody and her co-workers examined developing wings in mutant Drosophila larvae that lack components of the actin-capping protein CP, which binds to the end of actin filaments and so inhibits the addition or removal of further monomers. Wing cells in mutant larvae were previously known to be abnormal in shape, a difference that may result from actin accumulation. The group has now shown that knockdown of CP subunit genes with double-stranded RNA led to a similar phenotype. Cell proliferation and tissue growth was found to be enhanced in the mutant larvae, but only in one part of the wing.

The researchers then tested whether loss of CP gene expression affects the Hippo signalling pathway that is known to regulate tissue growth, and found several genes in this pathway to be under-expressed in the mutant larvae. The pathway is also known to interact with and inactivate a transcription co-activator protein that Drosophila geneticists have named Yorkie, and which triggers the expression of genes involved in cell growth and proliferation. Several known targets of Yorkie were also found to be over-expressed in larval cells lacking expression of the CP genes. Taken together, the results indicated that CP proteins control cell growth by stimulating the Hippo pathway and preventing inactivation of Yorkie.

Janody and co-workers then investigated whether actin accumulation alone could disrupt the normal function of these genes. They studied mutants in which the expression of two other proteins known to control actin accumulation independently of CP, Cofilin and Capulet, had been reduced. The loss of Capulet but not that of Cofilin was found to induce inappropriate Yorkie activity. Furthermore, inhibition of the Hippo pathway was found to cause actin accumulation independently of Yorkie activity, and over-expressing some of the genes in this pathway, including genes named Merlin (Mer) and expanded (ex), could reduce actin accumulation in cells deficient in CP proteins.

The researchers concluded that actin accumulation, and thus cell growth and proliferation, are controlled by a complex interplay between the Hippo pathway that controls Yorkie-mediated transcription, and the actin filament proteins themselves. Maintaining this interplay is necessary for normal cell growth, and any disruption of the human homologs of these proteins may trigger tumour development. Understanding these events in the model organism Drosophila may therefore lead to important insights into carcinogenesis.

Reference

Fernández, B.G., Gaspar, P., Brás-Pereira, C., Jezowska, B., Rebelo, S.R. and Janody, F. (2011) Actin-Capping Protein and the Hippo pathway regulate F-actin and tissue growth in Drosophila Development 138, 2337-2346 doi:10.1242/dev.063545

1. It is worth noting here the imaginative, not to say zany, names that Drosophila geneticists have assigned to many genes. Mammalian orthologs of the Drosophila protein Hippo, for example, are termed MST1 and MST2.