RB activation and repression of C-MYC transcription precede apoptosis of human prostate epithelial cells

The retinoblastoma gene (Rb) encodes a transcriptional regulatory protein that functions in the regulation of cellular growth, differentiation, and survival. Many of the target genes of Rb with respect to growth regulation and differentiation have been identified. However, the identities of the Rb target genes involved in the regulation of cell survival and cell death (apoptosis) are unknown. We sought to determine whether the c-myc oncogene, a known target of Rb activity in cell cycle control, is also recruited in an apoptotic pathway uniquely regulated by Rb in prostate epithelial cells.

We previously described a cell culture model to study the apoptosis of prostate cancer cells in which the human prostate cancer line, LNCaP, will undergo apoptosis after inducible expression and activation of the alpha isozyme of protein kinase C (PKC) or after exposure to low concentrations of the PKC activator TPA. Rb protein and c-myc mRNA and protein were evaluated in the Rb+/+ LNCaP and in the Rb-/- DU145 prostate cancer cells.

TPA-induced apoptosis in LNCaP cells was preceded by the rapid depletion of c-myc mRNA. The DU145 cultures were resistant to TPA-induced apoptosis and the c-myc mRNA levels remained elevated. The examination of Rb protein in the LNCaP cells revealed rapid dephosphorylation preceding both c-myc protein depletion and apoptosis. Additionally, pretreatment of LNCaP cells with staurosporine, a potent inhibitor of several isozymes of the PKC family, inhibited apoptosis in these cells and completely blocked activation of Rb and repression of c-myc.

On the basis of these studies, we suggest that induction of PKC-mediated apoptosis of Rb+/+ prostate cancer cells occurs by means of an intracellular pathway that involves the activation of Rb and repression of c-myc transcription.