Regulation of p53-dependent apoptosis in Friend erythroleukemia cells by intrinsic survival factors and erythropoietin

The mechanism by which erythropoietin (EPO) acts as a survival factor in the suppression of p53-dependent apoptosis was investigated using a Friend virus-transformed erythroleukemia cell line that expresses a temperature-sensitive p53 allele. EPO treatment rescued cells from death when p53 was activated at 32°C and led to increased expression of Bcl-XL. Suppression of Bcl-XL expression with antisense oligonucleotides inhibited the pro-survival function of EPO. STAT5, ERK, p38 and JNK Mitogen Activated Protein Kinases (MAPKs) were investigated as potential conduits for EPO-mediated survival by exogenous expression of a dominant-interfering STAT5 and chemical inhibition of MAPKs. In all cases EPO was able to promote survival. Moreover, the up-regulation of Bcl-XL by EPO was unaffected by MAPK inhibition. These data indicate a critical role for Bcl-XL in suppressing p53-dependent apoptosis and demonstrate that EPO promotes Bcl-XL accumulation through a mechanism that is independent of STAT5 and MAPKs. The link between EPO/JAK2 and Bcl-XL up-regulation for the survival response of DP16.1/p53ts cells has not been elucidated. Using a 2-dimensional gel based screen, TCP1-alpha and HMGB1 were identified as novel targets of EPO-induced posttranscriptional regulation in DP16.1/p53ts cells. HMBG1 has been implicated in the positive and negative regulation of p53-dependent transcription and it is possible that EPO-modified HMGB1 influences promoter selection by p53.; ERK, p38 and JNK MAPK signalling were found to be constitutively activated in DP16.1/p53ts cells, likely as a result FVP-induced transformation. Signalling through the Ras/MEK pathway was determined to be pro-apoptotic and contributed to p53 phosphorylation on serine residue 15; the possibility that this represents an oncogenic proliferative signal capable of activating p53 is discussed. The inhibition of MEK in DP16.1/p53ts cells redirected the cellular response to p53 activation from apoptosis to GI cell cycle arrest, implicating the Ras/MEK pathway as a potentially important determinant of the cell fate decision after p53 activation. Constitutive p38 and JNK MAPK signalling in DP16.1/p53ts cells was anti-apoptotic and capable of limiting p53-dependent apoptosis at 32°C. p53 half-life was increased in JNK-inhibitor treated cells, indicating that JNK may function in destabilizing p53 in DP16.1/p53ts cells, accounting for the enhanced apoptotic effect of inhibiting this kinase.