In contrast with protein kinase Cα (PKCα) and PKC∊, which are better known for promoting cell survival, PKCΔ is known for its pro-apoptotic function, which is exerted mainly through a caspase-3-dependent proteolytic activation pathway. In the present study, we used the rat GH3B6 pituitary adenoma cell line to show that PKCα and PKC∊ are activated and relocalized together with PKCΔ when apoptosis is induced by a genotoxic stress. Proteolytic activation is a crucial step used by the three isoforms since: (1) the catalytic domains of the PKCα, PKC∊ or PKCΔ isoforms (CDα, CD∊ and CDΔ respectively) accumulated, and this accumulation was dependent on the activity of both calpain and caspase; and (2) transient expression of CDα, CD∊ or CDΔ sufficed to induce apoptosis. However, following this initial step of proteolytic activation, the pathways diverge; cytochrome c release and caspase-3 activation are induced by CD∊ and CDΔ, but not by CDα. Another interesting finding of the present study is the proteolysis of PKCΔ induced by CD∊ expression that revealed the existence of a cross-talk between PKC isoforms during apoptosis. Hence the PKC family may participate in the apoptotic process of pituitary adenoma cells at two levels: downstream of caspase and calpain, and via retro-activation of caspase-3, resulting in the amplification of its own proteolytic activation.
Positive feedback of protein kinase C proteolytic activation during apoptosis
