The latter cells express high levels of GpIb, as determined by immunoblotting (16). activity are not necessary for c-Myc-like functions. Specifically, the L-701324 six C-terminal amino acids of the cytoplasmic L-701324 domain name, which mediate vWFR signaling, are entirely dispensible for the c-Myc-like functions of GpIb. Instead, these require a more membrane-proximal filamin-binding domain name. Also important is the GpIb transmission peptide, which, in the absence of other vWFR subunits, directs GpIb to the endoplasmic reticulum rather than the membrane. Together, these results provide strong evidence that this domains of GpIb mediating c-Myc-like functions are modular, genetically distinct, and impartial of those involved in vWFR signaling. Glycoprotein Ib (GpIb)2is a Type I trans-membrane glycoprotein that is expressed on the surface of megakaryocytes and platelets, where it associates with three other trans-membrane proteins, glycoprotein Ib (GpIb), glycoprotein V (GpV), and glycoprotein IX (GpIX), to form the von Willebrand factor receptor (vWFR) (13). GpIb is usually in the beginning synthesized as a 627-amino acid precursor, with L-701324 a 610-amino acid membrane-bound mature form resulting from cleavage of the N-terminal transmission peptide. vWF is usually exposed on damaged vascular endothelium, which allows for the attachment, aggregation, and activation of vWFR-bearing platelets during the initial stages of hemostasis. In addition to its role in hemostasis, GpIb also regulates bone marrow megakaryocyte ploidy and proliferation (46). These functions, as well as those associated with platelet aggregation and activation, require the extreme C terminus of the GpIb cytoplasmic domain name, which interacts with signaling proteins, such as 14-3-3, c-Src, and phosphatidylinositol 3-kinase (48). In addition, a more membrane-proximal cytoplasmic domain name communicates indirectly with the actin cytoskeleton via its association with filamin. This so-called filamin-binding domain name serves to stabilize the entire vWFR and facilitate its transport to the cell surface (9,10). Recently, we showed that GpIb is usually more widely expressed than previously appreciated and participates in aspects of c-Myc oncoprotein function that are unrelated to and impartial of its role in platelet physiology. Specifically, we showed that theGPIBAgene is usually a direct downstream target for c-Myc and that GpIb is usually both necessary and sufficient to promote the genomic instability (GI) that typically accompanies c-Myc deregulation (1116). Consistent with this obtaining, malignancy cell lines that overexpress c-Myc tend to have extremely high levels of GpIb, whereas untransformed cells or main cells tend to have extremely low or undetectable levels (15). GpIb overexpression also transforms established cellsin vitro, promotes tumorigenesisin vivo, reduces growth factor requirements, and enhances survival in the face of growth factor deprivation (15). Moreover, enforced expression of GpIb in main cells leads to the induction of tetraploidy and double-stranded DNA breaks (DSBs) and to p53-dependent premature senescence reminiscent of that seen in response to many transforming oncogenes (15,1719). Taken together, these unanticipated oncoprotein-like aspects of GpIb show that it plays an integral role in mediating a number of unique c-Myc phenotypes. These unexpected properties of GpIb raise a number of important questions. For example, which regions of the molecule are necessary for these multiple behaviors, and are they genetically separable? How do these regions relate to those that execute the traditional vWFR-dependent functions of GpIb? Finally, if GpIb is usually expressed more widely than previously recognized, are GpIb, GpV, and GpIX also similarly expressed, and, if so, is usually their association with GpIb required for its oncoprotein-like properties? In the current work, we have investigated these and other questions pertaining to the means by which GpIb exerts its oncoprotein-like functions. Our findings provide evidence that cells other than megakaryocytes do not coordinately express all vWFR subunits, that GpIb need not be expressed at the cell surface in order for it to carry out its c-Myc-like functions, that certain of these functions are genetically separable, and that these regions may be quite different from Rabbit polyclonal to PAX2 those that mediate vWFR functions. Our results also imply that, in nonmegakaryocytic cells, GpIb plays functions that are unique from those related to hemostatasis. == EXPERIMENTAL PROCEDURES == Cell Lines and Culture ConditionsRat1a fibroblasts, amphotropic Phoenix retroviral packaging cells, human foreskin fibroblasts, and BJ/TTR cells (14,15) were produced in Dulbecco’s altered Eagle’s minimal essential medium made up of 10% fetal calf serum and supplemented with 2 mmglutamine, 100 models/ml penicillin G, and 100 mg/ml streptomycin. HL60 promyelocytic cells, HeLa, MCF7 breast malignancy cells, and CaLu1 lung malignancy cells were produced as explained previously (20,21). The Mo7e megakaryocyte collection was.