Human trophoblast and choriocarcinoma expression of the growth factor pleiotrophin attributable to germ-line insertion of an endogenous retrovirus

Abstract

Retroviral elements are found in abundance throughout the human genome but only rarely have alterations of endogenous genes by retroviral insertions been described. Herein we report that a human endogenous retrovirus (HERV) type C is inserted in the human growth factor gene pleiotrophin (PTN) between the 5 untranslated and the coding region. This insert in the human genome expands the region relative to the murine gene. Studies with promoterreporter constructs show that the HERV insert in the human PTN gene generates an additional promoter with trophoblastspecific activity. Due to this promoter function, fusion transcripts between HERV and the open reading frame of PTN (HERV-PTN) were detected in all normal human trophoblast cell cultures as early as 9 weeks after gestation (n ‫؍‬ 7) and in all term placenta tissues (n ‫؍‬ 5) but not in other normal adult tissues. Furthermore, only trophoblast-derived choriocarcinoma cell lines expressed HERV-PTN mRNA whereas tumor cell lines derived from the embryoblast (teratocarcinoma) or from other lineages failed to do so. We investigated the significance of HERV-PTN mRNA in a choriocarcinoma model by targeting this transcript with ribozymes and found that the depletion of HERV-PTN mRNA prevents human choriocarcinoma growth, invasion, and angiogenesis in mice. This suggests that the tissue-specific expression of PTN due to the HERV insertion in the human genome supports the highly aggressive growth of human choriocarcinoma and possibly of the human trophoblast.Pleiotrophin (PTN) is a secreted heparin-binding polypeptide growth factor (1) with mitogenic (1-3) and transforming effects (3) on fibroblasts and growth factor activity on epithelial (2, 4, 5) and endothelial (2, 5, 6) cells. Furthermore, PTN induces the release of proteolytic enzymes from endothelial cells (7) and stimulates neurite outgrowth (8) and tube formation by endothelial cells in vitro (5) as well as angiogenesis in the rabbit corneal pocket assay (6). PTN gene expression is regulated in a time-and tissue-specific manner during rodent development and PTN mRNA is found at high levels in the central nervous system during the perinatal period, is downregulated thereafter, and is present at low levels in a few adult tissues (1, 9-11). On the other hand, the PTN gene is upregulated in several human tumor tissues and tumor cell lines (2) but little is known about the regulatory elements in this gene (12).To understand the mechanisms that regulate expression of the human PTN gene, we studied 5Ј ends of PTN mRNA isolated from various human tissues that express the PTN gene. To our surprise we found that all placenta samples, in contrast to brain, expressed PTN mRNA with 5Ј exons that are homologous to a human endogenous retrovirus (HERV) and are spliced onto the intact open reading frame (ORF) of PTN. Upon analysis of human genomic DNA, we located the insertion of an HERV fragment into the intron region upstream of the ORF of the human PTN gene expanding this region relative to the ancestral PTN gene share...

The angiogenic factor midkine is aberrantly expressed in NF1-deficient Schwann cells and is a mitogen for neurofibroma-derived cells

“…The low concentration of MK in conditioned media of NF17/7 Schwann cells and the robust stimulation of endothelial cells suggest that MK is only one among several molecules necessary to switch the angiogenic balance. Interestingly, studies in other systems suggest that shifting the angiogenic balance by eliminating only one stimulator is at least temporarily su cient to suppress tumor growth (Schulte et al, 1996). In addition to angiogenic stimulators, the angiogenic inhibitor TSP-1 was expressed in knock-out but not normal Schwann cells, suggesting a compensatory shift in the angiogenic pro®le.…”

Translocations involving anaplastic lymphoma kinase (ALK)

“…However, pleiotrophin e ects have been demonstrated also in endothelial cells and ®broblasts (Chauhan et al, 1993;Courty et al, 1991;Fang et al, 1992;Li et al, 1990;Wellstein et al, 1992). Elevated pleiotrophin levels in the serum of patients su ering from a variety of solid tumors have been demonstrated Souttou et al, 1998) and animal studies have suggested that pleiotrophin may contribute to tumor growth, invasion and metastasis (Chauhan et al, 1993;Choudhuri et al, 1997;Czubayko et al, 1994Czubayko et al, , 1996Schulte et al, 1996). However, ALK expression has not been detected in any of these di erent cell types in mice or humans.…”

Recruitment of insulin receptor substrate-1 and activation of NF-κB essential for midkine growth signaling through anaplastic lymphoma kinase

“…Overall, PTN and MK induce a variety of effects in normal and transformed cells or tissues including neurite outgrowth, proliferation, angiogenesis, antiapoptosis and tumor metastasis (see e.g. Fang et al, 1992;Czubayko et al, 1996;Schulte et al, 1996; reviewed by Schulte and Wellstein, 1997;Zhang and Deuel, 1999;Muramatsu, 2002).…”