Regulation of the retinoblastoma-dependent Mdm2 and E2F-1 signaling pathways during neuronal apoptosis.

We have previously demonstrated that the apoptotic signaling pathway in K(+)-deprived cerebellar granule neurons involves a caspase-dependent cleavage of the retinoblastoma protein (Rb). Here, we have further investigated the functional consequences of this cleavage on two Rb-binding partners: the oncoprotein Mdm2 and the transcription factor E2F-1. A K(+) deprivation time course leads to a caspase inhibitor-sensitive degradation of Mdm2. Experimental blockade of Mdm2 expression with antisense oligodeoxynucleotides (ODN) results in neuronal death, suggesting an active role of Mdm2 in neuroprotection. By contrast, the E2F-1 protein accumulates in a caspase-independent manner following K(+) withdrawal, a consequence of increased gene transcription. This is likely to result from the rapid cyclin-dependent kinase 4 activation observed in LK, that correlates with a transient Rb phosphorylation. Counteracting E2F-1 upregulation with antisense ODNs prevents neuronal loss. Taken together, these data demonstrate that Rb is a central player in regulating both caspase-dependent and -independent events leading to apoptosis. Copyright 2001 Academic Press.