Differential gene expression in premalignant human trophoblast: role of IGFBP-5

Tumorigenesis results from genetic alterations that occur in a stepwise manner giving rise to cells with increasingly cancer-like characteristics. We used in vitro propagated first trimester human extravillous trophoblast (EVT) cells to identify genetic changes responsible for the transition of the EVT from a normal to premalignant stage. The model used consisted of a normal invasive EVT (HTR8) cell line and its premalignant derivative (RSVT2/C) generated by transfection with the SV40 Tag and selected using a forced crisis regimen. RSVT2/C display increased proliferative, migratory and invasive behavior, unresponsiveness to anti-proliferative and anti-invasive signals of TGFbeta and a deficiency in gap junctional intercellular communication. These cells, however, were unable to form colonies on soft agar or tumors in nude mice and are thus defined as premalignant. Differential display revealed 18 gene sequences, 7 with unknown and 11 with known identity, showing altered expression between the normal HTR8 and premalignant RSVT2/C cell lines. The known sequences include the potential tumor suppressors insulin-like growth factor binding protein (IGFBP)-5 and fibronectin (FN) and potential protooncogenes such as chromokinesin (KIF4), alternative splicing factor (SF2), dynein, DNA polymerase epsilon (DNApol epsilon) and NF-kappaB activating kinase (NAK). The role of the remaining 4 genes upregulated in the premalignant EVT is presently unknown and these are FK506 binding protein (FKBP) 25, histone protein (HP1Hs)-gamma, nucleoporin (Nup) 155 and an 82 kDa acidic human protein. The functional role of IGFBP-5 was examined in the control of proliferation, migration and invasiveness of RSVT2/C cells measured in vitro. IGFBP-5 alone had no effect on these properties of RSVT2/C cells. Furthermore, unlike normal EVT cells, RSVT2/C cells exhibited refractoriness to the migration stimulating signals of IGF-II, which was explained by the loss or downregulation of the IGF type 2 receptor (IGF-R2). RSVT2/C cells, however, expressed the IGF type 1 receptor (IGF-R1) and responded to IGF-I by increased proliferation. This response was blocked with increasing concentrations of IGFBP-5. These results suggest that the loss of IGFBP-5 and possibly IGF-R2, both of which can sequester IGF-I from IGF-R1, permits unhindered proliferation of the premalignant EVT in an IGF-I rich environment of the fetal-maternal interface. The functions of the other differentially expressed genes, some of which are essential for cell cycle progression or cell survival require further investigation.