Insights will likely be forthcom ing when more is learned about the cellular actions of PHLDA2. The activities of PHLDA2 may be linked to its pleckstrin homology domain and ability to bind phos phoinositides and could include an intracellular signal Bioactive compound transduction function. Some differences in the behavior of mouse tropho blast stem cells and Rcho 1 trophoblast stem cells are noteworthy. Elf5, a member of the ETS transcription factor family and a player in the derivation and main tenance of mouse trophoblast stem cells is not among the trophoblast stem cell associated genes of the Rcho 1 trophoblast stem cell model. This may relate to differences in the requirements for exogenous factors to maintain trophoblast stem cell populations.

Mouse trophoblast stem cells are dependent upon fibroblast growth factor 4 FGF receptor 2 sig naling, whereas maintenance of Rcho 1 tropho blast stem cells does not require FGF4. Evidence indicates that ELF5 may be a downstream effector of FGF4 signaling needed to sustain activation of Cdx2 and Eomes genes and the trophoblast stem cell state. The requirement for Elf5 must in some way be circumvented in Rcho 1 trophoblast stem cell maintenance. In addition to Rcho 1 trophoblast stem cells other recently derived trophoblast cell lines from the rat and common vole also grow in the absence of exogenous FGF4. These observations do not reflect a fundamental species difference in the regula tion of trophoblast stem cells. FGF4 dependent tropho blast stem cell lines can be established from the rat blastocyst.

Instead, the FGF4 independence of the tropho blast stem cell populations is probably the conse quence of genetic and or epigenetic modifications and in vitro selection. Several trophoblast stem cell associated genes were not shared with mouse trophoblast stem cells. Among these genes were Mif and S1pr1. Mif encodes a pro inflammatory cytokine implicated in the regulation of angiogenesis, the migration and adhesion of monocytes, and modulation of uterine natural killer cell cytolytic activity. S1pr1 encodes a Gi protein coupled receptor for sphingosine 1 phosphate. S1P has been implicated in a range of functions, including controlling cell proliferation and differentia tion. In human trophoblast, S1P inhibits differen tiation.

Activation of some of the trophoblast stem cell associated genes may represent a develop mental progression beyond the trophoblast stem cell state exhibited by mouse trophoblast GSK-3 stem cells or alternatively may provide Rcho 1 cells with their tumorigenic features. Trophoblast differentiation associated genes Differentiation associated genes possess a broader range of functions than noted for the trophoblast stem cell associated gene cluster. Many of these genes are characteristic of the trophoblast giant cell phenotype.