ERK8 is a novel HuR kinase that regulates tumour suppressor PDCD4 through a miR-21 dependent mechanism

Programmed cell death 4 (PDCD4) is a tumour suppressor implicated in cancer development and progression and was recently identified as a repressor of cap-independent translation of specific genes involved in the regulation of apoptosis. We show that the RNA-binding protein HuR binds to the PDCD4 3′UTR to protect it from miR-21-induced silencing. However, following H(2)O(2) treatment, PDCD4 mRNA is degraded via miR-21 binding. Importantly, we identify HuR as a novel substrate of the ERK8 kinase pathway in response to H(2)O(2) treatment. We show that phosphorylation of HuR by ERK8 prevents it from binding to PDCD4 mRNA and allows miR-21-mediated degradation of PDCD4.