Down-regulation of Myc as a Potential Target for Growth Arrest Induced by Human Polynucleotide Phosphorylase (hPNPaseold-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 G 1 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.