Down-regulation of Myc as a Potential Target for Growth Arrest Induced by Human Polynucleotide Phosphorylase (hPNPase^old-35) in Human Melanoma Cells

Terminal differentiation and senescence share several common properties, including irreversible cessation of growth and changes in gene expression profiles. To identify molecules that converge in both processes, an overlapping pathway screening was employed that identified old-35, which is human polynucleotide phosphorylase (hPNPase^old-35), a 3′,5′ -exoribonuclease. We previously demonstrated that hPNPase^old-35 is a type I interferon-inducible gene that is also induced in senescent fibroblasts. In vitro RNA degradation assays confirmed its exoribonuclease properties, and overexpression of hPNPase^old-35 resulted in growth suppression in HO-1 human melanoma cells. The present study examined the molecular mechanism of the growth-arresting property of hPNPase^old-35. When overexpressed by means of a replication-incompetent adenoviral vector (Ad.hPNPase^old-35), hPNPase^old-35 inhibited cell growth in all cell lines tested. Analysis of cell cycle revealed that infection of HO-1 cells with Ad.hPNPase^old-35 resulted in arrest in the G ₁ phase and eventually apoptosis accompanied by marked reduction in the S phase. Infection with Ad.hPNPase^old-35 resulted in reduction in expression of the c-myc mRNA and Myc protein and modulated the expression of proteins regulating G₁ checkpoint and apoptosis. In vitro mRNA degradation assays revealed that hPNPase^OLD-35 degraded c-myc mRNA. Overexpression of Myc partially but significantly protected HO-1 cells from Ad.hPNPase^old-35-induced growth arrest, indicating that Myc down-regulation might directly mediate the growth-inhibitory properties of Ad.hPNPase^old-35. Inhibition of hPNPase^old-35 by an antisense approach provided partial but significant protection against interferon-β-mediated growth inhibition, thus demonstrating the biological significance of hPNPase^old-35 in interferon action.