Onzin, a c-Myc-repressed target, promotes survival and transformation by modulating the Akt–Mdm2–p53 pathway

Abstract

The c-Myc oncoprotein is a general transcription factor whose target genes dictate the c-Myc phenotype. One such target of c-Myc, 'onzin', is normally expressed at high levels in myeloid cells and is dramatically downregulated in response to c-Myc overexpression. We show here that short hairpin interfering RNA-mediated knockdown of endogenous onzin results in a reduced growth rate and a proapoptotic phenotype. In contrast, onzin overexpression in fibroblasts is associated with an increased growth rate, resistance to apoptotic stimuli, loss of the G2/M checkpoint, and tumorigenic conversion. Onzin-overexpressing cells fail to induce p53 in response to apoptotic stimuli and contain higher levels of the active, phosphorylated forms of Akt1 and, more strikingly, of Mdm2. Using yeast two-hybrid and coimmunoprecipitation assays, we show that onzin directly interacts with both proteins. Green fluorescent protein tagging also confirms directly that Akt1 and Mdm2 colocalize with onzin, although the precise subcellular distribution of each protein is dependent on its relative abundance. Collectively, our results identify onzin as a novel regulator of several p53-dependent aspects of the c-Myc phenotype via its dramatic effect on Mdm2. This is reminiscent of the c-Mycp19 ARF -| Mdm2 pathway and might function as a complementary arm to ensure the proper cellular response to oncogenic and/or apoptotic stimuli.

The downregulation of onzin expression by PKCɛ-ERK2 signaling and its potential role in AML cell differentiation

“…7 However, our results showed that the forced expression or downregulation of onzin had no significant effect on apoptosis induction by etoposide or tunicamycin in either U937 or NB4 cells. We speculated that this is possibly because p53 is mutated in U937 cells 47 and inactivated in NB4 cells.…”

“…Earlier, it was shown that c-Myc is a negative regulator of onzin expression. 7 However, PMA treatment also decreased c-Myc expression in leukemic and solid tumor cells tested (Supplementary Figures S2 and S3), which should exclude the possibility that the suppression of onzin by PMA is caused by the negatively regulating effect of c-Myc on onzin expression. The above-described fact, that the suppression of onzin expression by PMA was cell typeindependent, proposed that the common mechanism(s) is involved in this event.…”

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

“…Because HFFs are resistant to serum deprivationmediated apoptosis (Rogulski et al, 2005a), we were unable to study this property. However, we were able to determine effects on proliferation.…”

“…Rat1a and primary human foreskin fibroblasts (ATCC, Manassas, VA, USA) and 32D cells were propagated as described (Rogulski et al, 2005a). All reagents were from Gibco-BRL (Grand Island, NY, USA) unless otherwise stated.…”

Impaired Host Defense in Mice Lacking ONZIN

“…Attenuation of Onzin expression in the myeloid cell line 32D using short-hairpin RNA was reported to decrease growth and increase apoptosis after cytokine withdrawal and after exposure to Adriamycin (43). To further address the possibility that Onzin expression limits the apoptosis of cells of myeloid lineage, we examined the survival of Onzin Ϫ/Ϫ and wild-type neutrophils isolated from bone marrow.…”