Constitutive activation of extracellular signal-regulated kinase in human acute leukemias: Combined role of activation of MEK, hyperexpression of extracellular signal-regulated kinase, and downregulation of a phosphatase, PAC1

Extracellular signal-regulated kinase (ERK) is an important intermediate in signal transduction pathways that are initiated by many types of cell surf ace receptors. It is thought to play a pivotal role in integrating and tran smitting transmembrane signals required for growth and differentiation. Con stitutive activation of ERK in fibroblasts elicits oncogenic transformation , and recently, constitutive activation of ERK has been observed in some hu man malignancies, including acute leukemia. However, mechanisms underlying constitutive activation of ERK have not been well characterized. In this st udy, we examined the activation of ERK in 79 human acute leukemia samples a nd attempted to find factors contributing to constitutive ERK activation. F irst, we showed that ERK and MEK were constitutively activated in acute leu kemias by in vitro kinase assay and immunoblot analysis. However, in only o ne half of the studied samples, the pattern of ERK activation was similar t o that of MEK activation. Next, by semiquantitative reverse transcriptase-p olymerase chain reaction (RT-PCR) and immunoblot analysis, we showed hypere xpression of ERK in a majority of acute leukemias. In 17 of 26 cases (65.4% ) analyzed by immunoblot, the pattern of ERK expression was similar to that of ERK activation. The fact of constitutive activation of ERK in acute leu kemias suggested to us the possibility of an abnormal downregulation mechan ism of ERK. Therefore, we examined PAC1, a specific ERK phosphatase predomi nantly expressed in hematopoietic tissue and known to be upregulated at the transcription level in response to ERK activation. Interestingly, in our s tudy, PAC1 gene expression in acute leukemias showing constitutive ERK acti vation was significantly lower than that in unstimulated, normal bone marro w (BM) samples showing minimal or no ERK activation (P = .002), Also, a sig nificant correlation was observed between PAC1 downregulation and phosphory lation of ERK in acute leukemias (P = .002). Finally, by further analysis o f 26 cases, we showed that a complementary role of MEK activation, ERK hype rexpression, and PAC1 downregulation could contribute to determining the co nstitutive activation of ERK in acute leukemia. Our results suggest that ER K is constitutively activated in a majority of acute leukemias, and in addi tion to the activation of MEK, the hyperexpression of ERK and downregulatio n of PAC1 also contribute to constitutive ERK activation in acute leukemias . (C) 1999 by The American society of Hematology.