Ornithine decarboxylase (ODC) is the first and usually rate-limiting
enzyme in the
polyamine biosynthetic pathway. Under normal physiological conditions,
polyamine content and ODC
enzyme activity are highly regulated. However, the induction of ODC activity is an early step in neoplastic transformation. The studies described here use normal mouse keratinocytes (C5N cells), and spindle
carcinoma cells (A5 cells) to explore the regulation of ODC in nonmelanoma
skin cancer development. Previous results have shown that induction of ODC activity is both necessary and sufficient for the promotion of skin
tumors. We see a marked increase in ODC
enzyme activity in A5 cells compared with C5N keratinocytes, which correlates with a 4-fold stabilization of ODC
mRNA. These data suggest that ODC is post-transcriptionally regulated in skin
tumor development. Thus, we sought to investigate whether the ODC transcript interacts with the
RNA-binding protein HuR, which is known to bind to and stabilize its target mRNAs. We show that HuR is able to bind to the ODC 3'-UTR in A5 cells but not in C5N cells. Immunofluorescence results reveal that HuR is present in both the nucleus and cytoplasm of A5 cells, whereas C5N cells exhibit strictly nuclear localization of HuR. Knockdown experiments in A5 cells showed that when HuR is depleted, ODC
RNA becomes less stable, and ODC
enzyme activity decreases. Together, these data support the hypothesis that HuR plays a causative role in ODC up-regulation during nonmelanoma
skin cancer development by binding to and stabilizing the ODC transcript.