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Expression dynamics of pro‐ and anti‐apoptotic proteins regulated by cAMP (663.19)

The second messenger cyclic AMP (cAMP) promotes apoptosis in many cell types, including certain lymphoid cells, by poorly understood mechanisms. Murine S49 T‐lymphoma cells are a model system to study cAMP‐mediated apoptosis and have a distinct advantage for such studies: a clonal variant (kin‐) that lacks protein kinase A (PKA) activity and cAMP‐induced apoptosis, thereby facilitating analysis of PKA‐regulated apoptotic components. We have compared protein levels and subcellular localization of pro‐ and anti‐apoptotic Bcl family proteins in wild‐type (WT) and kin‐ S49 cells treated with the cell‐permeable analog CPT‐cAMP (CPT). We found that WT, but not kin‐, cells treated with CPT time‐dependently increase their level of the S, L and EL isoforms of the pro‐apoptotic protein Bim. We also examined PKA‐promoted phosphorylation of Bad, which is anti‐apoptotic when phosphorylated. We detected no phosphorylation of Bad at Ser155 (a PKA target site) in kin‐ cells during 72 hr treatment with CPT. Unexpectedly, WT cells showed strong Ser155 phosphorylation of Bad (and an increased ratio of p‐Ser155 Bad/total Bad) in response to 24‐72 h CPT treatment. Thus, cAMP acts via PKA to enhance both pro‐ and anti‐apoptotic proteins. We speculate that the late increase in pBAD in WT S49 cells may be an attempt by the cells to reverse their ultimate apoptotic response to cAMP/PKA activation.