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

Abstract

Extracellular signal-regulated kinase (ERK) is an important intermediate in signal transduction pathways that are initiated by many types of cell surface receptors. It is thought to play a pivotal role in integrating and transmitting transmembrane signals required for growth and differentiation. Constitutive activation of ERK in fibroblasts elicits oncogenic transformation, and recently, constitutive activation of ERK has been observed in some human malignancies, including acute leukemia. However, mechanisms underlying constitutive activation of ERK have not been well characterized. In this study, we examined the activation of ERK in 79 human acute leukemia samples and attempted to find factors contributing to constitutive ERK activation. First, we showed that ERK and MEK were constitutively activated in acute leukemias by in vitro kinase assay and immunoblot analysis. However, in only one half of the studied samples, the pattern of ERK activation was similar to that of MEK activation. Next, by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblot analysis, we showed hyperexpression 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 leukemias suggested to us the possibility of an abnormal downregulation mechanism of ERK. Therefore, we examined PAC1, a specific ERK phosphatase predominantly expressed in hematopoietic tissue and known to be upregulated at the transcription level in response to ERK activation. Interestingly, in our study, PAC1 gene expression in acute leukemias showing constitutive ERK activation was significantly lower than that in unstimulated, normal bone marrow (BM) samples showing minimal or no ERK activation (P = .002). Also, a significant correlation was observed between PAC1 downregulation and phosphorylation of ERK in acute leukemias (P= .002). Finally, by further analysis of 26 cases, we showed that a complementary role of MEK activation, ERK hyperexpression, and PAC1 downregulation could contribute to determining the constitutive activation of ERK in acute leukemia. Our results suggest that ERK is constitutively activated in a majority of acute leukemias, and in addition to the activation of MEK, the hyperexpression of ERK and downregulation of PAC1 also contribute to constitutive ERK activation in acute leukemias.

Molecular basis of MAP kinase regulation

“…3,4 The importance of protein phosphorylation by MAPKs is well-illustrated by the many inherited or acquired human diseases that stem from abnormalities in MAPK signaling pathways, including Parkinson's, inflammatory disorders and cancer. 4,5 As might be expected, MAPKs are ubiquitously expressed, making them conceivable but difficult targets for drug treatments. 6 However, MAPK activation is finely tuned in a cell-type specific and temporal manner, making MAPK regulators much more attractive targets for drug development.…”

Cellular Signalling Pathways: New Targets in Leukaemia Therapy

“…The contribution of the serine/threonine signalling pathways to leukaemogenesis is complex, involving effects on various cellular processes that are not as yet clear (McCubrey et al , 2000). For example, it is known that the constitutive activation of ERKs in fibroblasts induces oncogenic transformation, and that activated ERKs are expressed in acute leukaemias; however, the effects of such activation have not been elucidated in the leukaemias (Towatari et al , 1997; Kim et al , 1999). In addition, mutations and inappropriate activation of Ras have been described in a wide variety of human leukaemias (Janssen et al , 1987; Ahuja et al , 1990; Hirsch‐Ginsberg et al , 1990; Imamura et al , 1993; Sawyers et al , 1995; Beaupre & Kurzrock, 1999).…”

Beyond Kinase Activity: ERK5 Nucleo-Cytoplasmic Shuttling as a Novel Target for Anticancer Therapy

“…In particular, mitogen-activated protein kinases (MAPK) are a group of proteins able to translate environmental signals elicited by a plethora of stimuli, including growth factors and stresses, into different biological responses such as survival, apoptosis, proliferation and differentiation [3,4]. The importance of MAPK is underlined by the abnormal signaling conveyed by members of MAPK family in a number of human diseases, including Parkinson's disease, inflammatory disorders and cancer [4,5]. There are several MAPK in mammals.…”