Dehydroepiandrosterone negatively regulates the p38 mitogen-activated protein kinase pathway by a novel mitogen-activated protein kinase phosphatase

Abstract

Dehydroepiandrosterone-sulfate, the sulfated form of dehydroepiandrosterone, is the most abundant steroid in young adults, but gradually declines with aging. In humans, the clinical application of dehydroepiandrosterone targeting some collagen diseases, such as systemic lupus erythematosus, as an adjunctive treatment has been applied in clinical trial. Here, we report that dehydroepiandrosterone may negatively regulate the mitogen-activated protein kinase pathway in humans via a novel dual specificity protein phosphatase, DDSP (dehydroepiandrosterone-enhanced dual specificity protein phosphatase). DDSP is highly homologous to LCPTP/HePTP, a tissue-specific protein tyrosine phosphatase (PTP) which negatively regulates both ERK and p38-mitogen-activated protein kinase, and is transcribed from the PTPN7 locus by alternative splicing. Although previous reports have shown that the mRNA expression of the LCPTP/HePTP gene was inducible by extracellular signals such as T-cell antigen receptor stimulation, reverse transcribed (RT)-PCR experiments using specific sets of primers suggested that the expression of LCPTP/HePTP was constitutive while the actual inducible sequence was that of DDSP. Furthermore DDSP was widely distributed among different types of human tissues and specifically interacted with p38-mitogen-activated protein kinase. This inducible negative regulation of the p38-mitogen-activated protein kinase-dependent pathway may help to clarify the broad range of dehydroepiandrosterone actions, thereby aiding the development of new preventive or adjunctive applications for human diseases.

Introduction

The mitogen-activated protein kinase (MAPK) family plays a central role in signaling pathways stimulated by extracellular stimuli such as growth factors, cytokines and physical stress. In higher organisms, this kinase family includes the extracellular stimulus-regulated kinases (ERKs) and two stress-stimulated kinase groups, the stress-activated protein kinase/c-JUN N-terminus kinase (SAPK/JNK) and p38-MAPK/p38 High Osmolarity Glycerol response (HOG) 1 [1], [2], [3], [4], [5]. The activation of MAPKs requires phosphorylation of conserved tyrosine and threonine residues within the catalytic domain. This phosphorylation is mediated by dual specificity protein kinases, members of the MAPK kinase family. In contrast, in the absence of a signal the constituents of the MAPK cascade return to their inactive dephosphorylated state, suggesting an essential role for protein phosphatases in the negative regulation of the MAPK cascade. Protein phosphatases are classified into three groups, protein serine/threonine phosphatases, protein tyrosine phosphatases (PTPs) and protein dual specificity serine/threonine/tyrosine phosphatases (DSPs), depending on their phosphoamino acid specificity [6].

Dehydroepiandrosterone-sulfate (DHEA-S), the sulfated form of DHEA, is the most abundant steroids in young adults, but gradually decline with aging. Although the molecular basis of DHEA action still remains to be elucidated, recent findings have suggested modulatory actions of DHEA on the MAPK signal transduction pathway [7], [8], [9]. To date, much data have been accumulated on the biological action of DHEA, although some of these were carried out using rodents in which the P450 C17 activities are extremely low, thus leading to trace or nearly undetectable levels of serum DHEA or DHEAS concentrations. In humans, the clinical application of DHEA targeting hormone replacement therapy [10], [11], [12], [13] has been tested in clinical trials.

We have been interested in the actions of DHEA using both in vitro and in vivo experiments [14] (for review). We have previously reported that a human clonal T lymphocyte, the PEER cell [15], which is stimulated with phorbol-12-myristate-13-acetate (PMA) and calcium ionophore A23187 to mimic the activation of the T-cell antigen receptor (TcR), revealed the specific binding of [3H]-DHEA to its putative receptor. This specific binding was further increased when treated with 100 nM of DHEA itself in addition to the PMA and A23187 treatment [16], while the subcellular localization of the DHEA-bound molecule(s) was not determined. In the experiment, the MAPK cascade was activated by PMA, which bypasses all receptor-induced proximal tyrosine phosphorylation events by directly activating the Raf kinase through protein kinase C [17], [18]. Thus, PEER cells are likely to provide a good model for investigating the cellular phenotypes altered by the DHEA action on the MAPK cascade, or for identifying the putative receptor for DHEA. Here, we report that DHEA negatively regulates the MAPK pathway in humans via a novel MAPK phosphatase, tentatively named DDSP (DHEA-enhanced DSP), which is highly homologous to LCPTP/HePTP [19], [20] not only controlling the activity of MAPKs but also mediating crosstalk between the cAMP system and the MAPK cascade [21].