Most human cancers show inactivation of both pRB- and p53-pathways. While retinoblastomas are initiated by loss of the RB1 tumor suppressor gene, TP53 mutations have not been found. High expression of the p53-antagonist MDM2 in human retinoblastomas may compromise p53 tumor surveillance so that TP53 mutations are not selected for in retinoblastoma tumorigenesis. We previously showed that p14ARF protein, which activates p53 by inhibiting MDM2, is low in retinoblastomas despite high mRNA expression.
In human fetal retinas, adult retinas, and retinoblastoma cells, we determined endogenous p14ARF mRNA, ARF protein, and miR-24 expression, while integrity of p53 signalling in WERI-Rb1 cells was tested using an adenovirus vector expressing p14ARF. To study p14ARF biogenesis, retinoblastoma cells were treated with the proteasome inhibitor, MG132, and siRNA against miR-24.
In human retinoblastoma cell lines, p14ARF mRNA was disproportionally high relative to the level of p14ARF protein expression, suggesting a perturbation of p14ARF regulation. When p14ARF was over-expressed by an adenovirus vector, expression of p53 and downstream targets increased and cell growth was inhibited indicating an intact p14ARF-p53 axis. To investigate the discrepancy between p14ARF mRNA and protein in retinoblastoma, we examined p14ARF biogenesis. The proteasome inhibitor, MG132, did not cause p14ARF accumulation, although p14ARF normally is degraded by proteasomes. miR-24, a microRNA that represses p14ARF expression, is expressed in retinoblastoma cell lines and correlates with lower protein expression when compared to other cell lines with high p14ARF mRNA. Transient over-expression of siRNA against miR-24 led to elevated p14ARF protein in retinoblastoma cells.
In retinoblastoma cells where high levels of p14ARF mRNA are not accompanied by high p14ARF protein, we found a correlation between miR-24 expression and low p14ARF protein. p14ARF protein levels were restored without change in mRNA abundance upon miR-24 inhibition suggesting that miR-24 could functionally repress expression, effectively blocking p53 tumor surveillance. During retinal tumorigenesis, miR-24 may intrinsically compromise the p53 response to RB1 loss.