Nuclear injection of anti-pigpen antibodies inhibits endothelial cell division

Endothelial cell proliferation is required for angiogenesis in both embryonic and adult tissues. In rat brain tumors, it has recently been shown that the nuclear protein pigpen is expressed selectively in endothelial cells of developing microvasculature but not in the established peritumoral vessels (Blank, M., Weinschenk, T., Priemer, M., and Schluesener, H. (2001) J. Biol. Chem. 276, 16464-16468). This finding suggests that pigpen may be important for promoting the undifferentiated, or "angiogenic" endothelial cell phenotype. Our studies show that pigpen protein and mRNA are expressed in actively dividing endothelial cells and down-regulated as they become confluent. Protein distribution is regulated in a cell cycle-dependent manner. We conclude that this expression pattern is important for and not simply ancillary to proliferation because nuclear microinjection of anti-pigpen Fab fragments inhibited endothelial cell division. Moreover, expression of the proliferating cell marker Ki67 was inhibited in antibody-injected cells. The absence of Ki67 suggests exit from rather than arrest within (for example, at the G(1)/S interface) the cell cycle. Together with earlier observations on the structure and expression of this molecule, our data support the hypothesis that pigpen helps regulate endothelial cell differentiation state.