Figure 8

Overview of the pathway(s) incriminated in cAMP-induced IPC cell apoptosis. (1) The binding of cAMP to both sites on the regulatory subunits of the PKA holoenzyme leads to release of free, active catalytic subunits. (2) Catalytic PKA subunit translocated to the nucleus can phosphorylate the transcription regulatory proteins of the CREB family (CREB, CREM, ATFs), which bind to available CREs in the promoter region of CREB-regulated genes. The Bim-promoter contains four half CRE sites, two of which is conserved and separated by only 13 bp about 500 bp upstream from the transcription start site. The CREM isoform ICER blocks CRE sites and thereby the action of all CRE-binding transcription factors. Phosphorylated CREB recruits the basal transcriptional machinery via CREB-binding protein (CBP/P300) and other co-factors and adaptors. (3) Transcript elongation is stimulated by phosphorylations of the C-terminal domain of RNA polymerase II by members of the CDK family, especially CDK7 and 9, which are sensitive to inhibition by RCV.³⁹ (4) The translation of Bim-mRNA is inhibited by CHX. (5 and 6) The Bim protein can complex Bcl2 and thereby block Bcl2 prosurvival functions. Bim may (dotted line) also induce apoptosis by stimulating BAX/BAK directly. The dotted arrows from PKA to (5 and 6) is based on the observation that cAMP can synergize with DNR to induce apoptosis in IPC_ICER cells (Supplementary Figure S3), which do not die in response to cAMP alone (Figure 1a). The dotted arrows from RCV to (5 and 6) are based on the moderate inhibition of cAMP-induced IPC cell death by dominant negative CDK5,⁶ which might be through inhibition of death processes downstream of Bim mRNA transcription