PTEN is a tumor suppressor that is frequently mutated in cancers and has both lipid phosphatase and protein phosphatase activities. The lipid phosphatase activity is well characterized and serves to terminate or limit signaling through the phosphoinositide 3-kinase (PI3K) pathway. The protein phosphatase substrates and functions are less well understood. Brisbin et al. found that the Caenorhabditis elegans homolog of PTEN (DAF-18) interacted with the receptor tyrosine kinase Eph (VAB-1) in a yeast two-hybrid screen and then showed that the interaction between the two proteins did not require the kinase activity of VAB-1 but was stabilized by phosphatase-inactive mutant forms of DAF-18. In vitro kinase assays with recombinant VAB-1 and DAF-18 or various mutant forms showed that VAB-1 could phosphorylate the C-terminal region of DAF-18 (the part that interacted with VAB-1) when it was expressed as a fragment, but not full-length DAF-18. Additionally, full-length DAF-18 decreased the autophosphorylation of VAB-1 in the kinase assay, but a phosphatase-inactive mutant did not. VAB-1 and DAF-18 were present in worm germline precursor cells, oocytes, and a subset of neurons. Although transcripts for DAF-18 were not affected, the abundance of DAF-18 was lower in embryo lysates or tissues in which VAB-1 was overexpressed; in vab-1 worms, DAF-18 was increased, suggesting that VAB-1 destabilized DAF-18. VAB-1 and DAF-18 both regulated life span, with VAB-1 decreasing life span and DAF-18 increasing it; vab-1 or DAF-18–overexpressing animals exhibited increased life span and sensitivity to dauer formation (a form of worm hibernation). Although this function for DAF-18 most likely is through its lipid phosphatase activity and ability to inhibit PI3K signaling, which is known to regulate life span, DAF-18 appeared to function differently in the reproductive system. Worms lacking VAB-1 function exhibited increased egg laying, and an increased proportion of eggs were unfertilized compared with wild type. Combining vab-1 with daf-18(ok480) rescued the egg-laying phenotypes; however, mutations in genes encoding components of the insulin and PI3K pathway did not show egg-laying phenotypes and failed to rescue vab-1. Instead, VAB-1 and DAF-18 appeared to regulate the activation state of mitogen-activated protein kinase (MAPK) with vab-1 worms exhibiting activated MAPK in more oocytes in the gonad than wild type. In daf-18 mutants or double vab-1, daf-18 mutants, MAPK activity was restricted to at most two oocytes. Thus, PTEN appears to function with Eph, and the two proteins may inhibit each other's activity and thereby regulate either PI3K or MAPK signaling.
S. Brisbin, J. Liu, J. Boudreau, J. Peng, M. Evangelista, I. Chin-Sang, A role for C. elegans Eph RTK signaling in PTEN regulation. Dev. Cell 17, 459–469 (2009). [Online Journal]