Raf-1 and ERK2 kinases are required for phorbol 12,13-dibutyrate-stimulated proliferation of rat lymphoblasts. ERK2 activation precedes Raf-1 hyperphosphorylation

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Abstract
Rat lymphoblasts are arrested in the G1 phase of the cell cycle and can be promoted to proceed up to the S phase, when they are stimulated by phorbol ester. In this work, we have studied some details of the phorbol 12,13‐dibutyrate (PBu2)‐stimulated proliferation. We show that in response to PBu2 at least four different protein kinase C (PKC) isoforms translocate to the membrane. A specific PKC ζ antibody recognizes two bands of 75 and 82 kDa. These two activities are separated using a Mono Q chromatography and we show that p75 is the classical PKC ζ isoform, while p82 might be a related isoform which is PBu2 sensitive. Our data show that there is a correlation between the ability of PBu2 to promote mitogenesis and to activate ERK2 kinase, suggesting that ERK2 kinase might be the limiting step of the process. We also show that ERK kinase activation precedes Raf‐1 kinase hyperphosphorylation, suggesting that Raf‐1 kinase activation is not required for ERK kinase activation. This idea was checked using a Raf‐1 kinase antisense (AS) oligonucleotide. The results obtained with the Raf‐1 AS oligonucleotide indicate that this serine/threonine kinase is dispensable for ERK kinase activation, but needed for the PBu2 mitogenic signaling even as late as 7 h after the delivery of the signal.