Transformation, genomic instability and senescence mediated by platelet/megakaryocyte glycoprotein Ibα

Abstract

GpIba, a subunit of the von Willebrand factor receptor, functions during blood clotting to promote platelet adhesion and activation. GpIba is widely expressed, is positively regulated by c-Myc and is essential for the promotion of c-Myc-mediated chromosomal instability. We now show that GpIba is also a classical oncoprotein in which its deregulated expression leads to transformation, reduced growth factor requirements, increased resistance to apoptosis, and, in primary cells, p53-dependent senescence. Finally, GpIba also promotes double-stranded DNA breaks, and induces profound nuclear dysmorphology, indicating that, in addition to its direct transforming function, it displays genotoxicity at several distinct levels. These findings identify novel functions for GpIba and pathways through which c-Myc mediates transformation and global genomic destabilization.

Modularity of the Oncoprotein-like Properties of Platelet Glycoprotein Ibα

“…Consistent with this finding, cancer cell lines that overexpress c-Myc tend to have extremely high levels of GpIb␣, whereas untransformed cells or primary cells tend to have extremely low or undetectable levels (15). GpIb␣ overexpression also transforms established cells in vitro, promotes tumorigenesis in vivo, reduces growth factor requirements, and enhances survival in the face of growth factor deprivation (15). Moreover, enforced expression of GpIb␣ in primary cells leads to the induction of tetraploidy and doublestranded DNA breaks (DSBs) and to p53-dependent premature senescence reminiscent of that seen in response to many transforming oncogenes (15,(17)(18)(19).…”

“…Specifically, we showed that the GPIBA gene is a direct downstream target for c-Myc and that GpIb␣ is both necessary and sufficient to promote the genomic instability (GI) that typically accompanies c-Myc deregulation (11)(12)(13)(14)(15)(16). Consistent with this finding, cancer cell lines that overexpress c-Myc tend to have extremely high levels of GpIb␣, whereas untransformed cells or primary cells tend to have extremely low or undetectable levels (15). GpIb␣ overexpression also transforms established cells in vitro, promotes tumorigenesis in vivo, reduces growth factor requirements, and enhances survival in the face of growth factor deprivation (15).…”

A c-Myc-MicroRNA functional feedback loop affects hepatocarcinogenesis

“…Data were analyzed by single-histogram statistics. 20,24 For colony assays, 1 Â 10 3 cells of the indicated type were plated in triplicate in soft agar (0.35% low melting point agarose on top of 0.7% bottom agarose) in six-well plates and fed intermittently with DMEM. Colonies were enumerate after 2 weeks by staining with methylene blue after methanol fixation.…”

“…Tumorigenicity assays and tumor volume measurements were performed as previously described. 20 Briefly a total of 1 Â 10 7 indicated cells were suspended in 100 lL serum-free DMEM and injected subcutaneously in the flanks of animals. Tumor growth was monitored every three days for a total period of 30 to 40 days.…”

“…Primary human foreskin fibroblasts (American Type Culture Collection Manassas, VA) were grown as described. 20 Human retinal pigmented epithelium (RPE) cells immortalized with hTERT were cultured in DMEM:F12 medium, whereas HEK293T cells were cultured as described for HCC. Normal human hepatocytes HL-7702 were grown in RPMI1640 (GIBCO) supplemented with 10% fetal bovine serum and 1% penicillin-streptomycin, maintained at 37 C and 5% CO 2 .…”