Potassium-induced apoptosis in rat cerebellar granule cells involves cell-cycle blockade at the G1/S transition

The role of regulators controlling the G1/S transition of the cell cycle wa s analyzed during neuronal apoptosis in post-mitotic cerebellar granule cel ls in an attempt to identify common mechanisms of control with transformed cells. Cyclin D1 and its associated kinase activity CDK4 (cyclin-dependent kinase 4) are major regulators of the G1/S transition. Whereas cyclin D1 is the regulatory subunit of the complex, CDK4 represents the catalytic domai n that, once activated, will phosphorylate downstream targets such as the r etinoblastoma protein, allowing cell-cycle progression. Apoptosis was induc ed in rat cerebellar granule cells by depleting potassium in presence of se rum. Western-blot analyses were performed and protein kinase activities wer e measured. As apoptosis proceeded, loss in cell viability was coincident w ith a significant increase in cyclin D1 protein levels, whereas CDK4 expres sion remained essentially constant. Synchronized to cyclin D1 accumulation, cyclin-dependent kinase inhibitor p27(Kip1) drastically dropped to 20% nor mal values. Cyclin D1/CDK4-dependent kinase activity increased early during apoptosis, reaching a maximum at 9-12 h and decreasing to very low levels by 48 h. Cyclin E, a major downstream target of cyclin D1, decreased concom itantly to the reduction in cyclin D1/CDK4-dependent kinase activity. We su ggest that neuronal apoptosis takes place through functional alteration of proteins involved in the control of the G1/S transition of the cell cycle. Thus, apoptosis in post-mitotic neurons could result from a failed attempt to re-enter cell cycle in response to extracellular conditions affecting ce ll viability and it could involve mechanisms similar to those that promote proliferation in transformed cells.