Protein kinase D, ubiquitin and proteasome pathways are involved in adenosine receptor-stimulated NR4A expression in myeloid cells

Adenosine is a purine nucleoside pivotal for homeostasis in cells and tissues. Stimulation of the adenosine receptors (AR) has been shown to regulate the nuclear orphan receptor 4A (NR4A1-3) family, resulting in attenuation of hyper-inflammatory responses in myeloid cells. The NR4A1-3 orphan receptors are early immediate response genes and transcriptional regulators of cell and tissue homeostasis. The signal transduction and transcriptional mechanism(s) of how AR-stimulation promotes NR4A expression in myeloid cells is unknown and is the focus of this study. We confirm that adenosine and the stable analogue, 5'-N-Ethylcarboxamidoadenosine (NECA), enhance NR4A1-3 expression in THP-1 cells. Pharmacological approaches identified that protein kinase D (PKD) mediates AR-stimulated NR4A expression in myeloid cells and reveals no involvement of PKA nor PKC. The role of NF-kappa B, a principal regulator of NR4A expression in myeloid cells, was examined as a possible transcriptional regulator downstream of PKD. Utilising BAY11-7082 and MG-132, inhibitors of the respective ubiquitin and proteasome pathways essential for NF-kappa B activation, suggested a prospective role for NF-kappa B, or more specifically signalling via IKK alpha/beta. However, biological interventional studies using overexpression of I kappa B alpha in myeloid cells and MEF cells lacking IKK alpha and IKK beta(IKK alpha/beta(-/-)) revealed the NF-kappa B pathway is not utilised in mediating AR-stimulated NR4A expression. Thus, this study contributes mechanistic insight into how AR signalling modulates the expression of NR4A receptors, pivotal regulators of inflammatory responses in myeloid cells. (C) 2021 The Authors. Published by Elsevier Inc.

Automated summary

Adenosine receptors play a crucial role in regulating NR4A expression in myeloid cells, with PKD mediating this process. NF-kappa B also plays a role in regulating NR4A expression, but may not be the central pathway through which AR-stimulation affects NR4A expression.