An additional 5'-upstream exon exists in the human pleiotrophin-encoding gene

Midkine binds to anaplastic lymphoma kinase (ALK) and acts as a growth factor for different cell types

Midkine (MK) is a developmentally regulated, secreted growth factor homologous to pleiotrophin (PTN). To investigate the potential role of MK in tumor growth, we expressed MK in human SW-13 cells and studied receptor binding, signal transduction, and activity of MK. The MK protein stimulates soft agar colony formation in vitro and tumor growth of SW-13 cells in athymic nude mice, as well as proliferation of human endothelial cells from brain microvasculature and umbilical vein (HUVEC) in the low ng/ml range. MK binds to anaplastic lymphoma kinase (ALK), the receptor for PTN, with an apparent K(d) of 170 pm in intact cells, and this receptor binding of MK is competed by PTN with an apparent K(d) of approximately 20 pm. Monoclonal antibodies raised against the extracellular ligand-binding domain of ALK inhibit ALK receptor binding of MK as well as MK-stimulated colony formation of SW-13 cells. Furthermore, MK stimulates ALK phosphorylation in WI-38 human fibroblasts and activates PI3-kinase and MAP kinase signal transduction in WI-38, HUVEC, neuroblastoma (SH SY-5Y) and glioblastoma (U87MG) cells that express the ALK protein. We conclude that MK can act as a growth, survival, and angiogenic factor during tumorigenesis and signals through the ALK receptor.

Molecular forms of human rhodopsin kinase (GRK1)

The G protein-coupled receptor kinases (GRKs) are critical enzymes in the desensitization of activated G protein-coupled receptors. Six members of the GRK family have been identified to date. Among these enzymes, GRK1 (rhodopsin kinase) is involved in phototransduction and is the most specialized of the family. GRK1 phosphorylates photoactivated rhodopsin, initiating steps in its deactivation. In this study, we found that human retina expressed all GRKs except GRK4. Based on results of molecular cloning and immunolocalization, it appears that both rod and cone photoreceptors express GRK1. This conclusion was supported by the cloning of only GRK1 from cone-dominated chicken retina. Human photoreceptors also transcribe a splice variant of GRK1, which differs in its C-terminal region next to the catalytic domain. This novel variant, GRK1b, is produced by retention of the last intron. mRNA encoding GRK1b is exported to the cytosol; however, the level of the protein is relatively low compared with GRK1 (now called GRK1a), and GRK1b appears to have very low catalytic activity. Thus, these studies suggest that rods and cones, express the same form of GRK1.

Regulation of osteoblast-specific factor-1 (OSF-1) mRNA expression by dual promoters as revealed by RT-PCR

OSF-1 (osteoblast-specific factor-1), which is also referred to as p18, HBBM, HB-GAM, HBGF-8, HARP, HBNF, and pleiotrophin, is a 121-amino acid polypeptide that can induce neurite outgrowth in vitro and is highly expressed in several tissues during fetal development but exhibits expression restricted to brain and bone tissues in adults. We have reported the genomic structure of mouse OSF-1 gene, in which the open reading frame spans four exons and at least two additional 5'-UTR exons (upstream exon U2 and downstream exon U1) exist. From analysis of isolated cDNAs, two types of cDNAs were identified: one has a sequence for U1 and U2 and the other has a sequence for an intron (present between U1 and U2) and U1. This suggests that the OSF-1 gene utilizes two alternative promoters, a distal and a proximal promoter, designated promoters II and I, respectively, for the translation initiation site (ATG). Promoter II is thought to exist upstream of the intron, while promoter I is present in the intron. RT-PCR was employed to examine which OSF-1 promoters are used during development and in various cell lines. In adult mice (aged 2 months), usage of promoter I was predominant, and OSF-1 mRNAs were expressed in many organs including brain and bone. At one fetal stage (E-19), promoter I was active in the major organs including brain, liver, kidney, and intestine, while promoter II was active only in the brain. In the cell lines examined, usage of promoter I was frequent, while promoter II was active only in a few cell lines such as MC3T3-E1 (cultured for 7 days) and C3H10T1/2. These findings suggest that OSF-1 may play fundamental roles in differentiation, growth and maintenance of adult organs as well as in embryogenesis, and indicate that the expression of OSF-1 is regulated, at least in part, by the usage of different promoters in the mouse.

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

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 promoter-reporter constructs show that the HERV insert in the human PTN gene generates an additional promoter with trophoblast-specific 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.

Molecular and pharmacologic targeting of angiogenesis factors--the example of pleiotrophin

Polypeptide growth factors contribute to the development and maintenance of normal tissues and are essential for the growth and metastasis of solid tumors. During tumor progression these factors function as autocrine stimulators of tumor cells and/or serve to recruit stromal tissue and blood supply to the expanding tumor. In particular, tumor-induced angiogenesis appears to be significant not only for local tumor growth but also for metastasis to distant organ sites. We purified several years ago the heparin-binding growth factor pleiotrophin (PTN) from the supernatants of human breast cancer cells and demonstrated that PTN can serve as an angiogenesis factor. We found the gene expressed in a number of human tumor cell lines as well as in human tumor tissues. Here we present different approaches to inhibit production and function of this growth factor. Finally we discuss how the experience from this growth factor can be applied to improve our understanding of the role of other factors thought to contribute to tumor angiogenesis.