Extracellular signal-regulated kinase suppression is not involved in apoptosis of neuroblastoma N2a cells induced by protein kinase C inhibition.

A model study for the mechanism of cell death in vitro was established on the neuroblastoma N2a cell line. By differential staining of living cells with Hoechst 33258 and propidium iodide, or by Terminal-dUTP-Transferase-Nick-End Labelling (TUNEL), cJun/AP1 immunoreactivity, and DNA laddering, we show that the N2a cell line responded with apoptosis to protein kinase C (PKC) inhibition. Two different classes of PKC inhibitors (staurosporine and Gö6976) at concentrations generally regarded as PKC-selective (10 nM and 50 nM, respectively), significantly increased the number of apoptotic cells in N2a cultures. The cells started to die 2-3 hours after the treatment; then, at 6 and/or 24 hours, approximately 30-40% of the cells acquired the apoptotic feature. This response was dependent on neither cell differentiation nor PKC and bcl2 gene and protein expression, but exclusively on the concomitant withdrawal of serum from growth medium. Furthermore, both of the experimental manipulations (serum deprivation and PKC inhibition) synergically, but to different extents, suppressed the extracellular signal regulated kinase (ERK) pathway. Their pro-apoptotic effect, however, was neither mimicked nor modified by an additional inhibition of MEK/ERK kinase by 50 microM PD 98059, resulting in an 80% inhibition of its initial activity. We conclude, therefore, that apoptosis of N2a cells triggered by PKC-inhibition, as well as its abrogation by serum, is totally independent of concomitant modulations of ERK activity also evoked by the above treatments.