α2B-Adrenergic receptors activate MAPK and modulate proliferation of primary cultured proximal tubule cells

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Abstract
In the rat proximal tubule, the α2B-adrenergic receptor (α2B-AR) enhances Na+reabsorption by increasing the activity of Na+/H+exchanger isoform NHE3. The mechanisms involved are unclear, and inhibition of cAMP production remains controversial. In this study, we reinvestigated α2B-AR signaling pathways using rat proximal tubule cells (PTC) in primary culture and LLC-PK1cells permanently transfected with the RNG gene (rat nonglycosylated α2-AR). Binding experiments indicated that PTC express substantial amounts of α2B-AR (130 fmol/mg protein), and only RNG transcripts were detected. In both cell types, the α2B-AR is coupled to G protein, and its stimulation by dexmedetomidine, but not by UK-14304, provoked a significant inhibition of the accumulation of cAMP induced by forskolin or parathyroid hormone. Exposure to α2-agonists increased arachidonic acid release and caused extracellular signal-regulated kinase (ERK)1/2 phosphorylation, which correlated with enhanced mitogen-activated protein kinse (MAPK) activity and nuclear translocation. MAPK phosphorylation was blunted by pertussis toxin but not by protein kinase C desensitization, and it coincided with transient phosphorylation of Shc. Finally, treatment with UK-14304 accelerated cell growth. Further studies will be necessary to clarify the precise mechanism of MAPK activation, but the present data suggest that α2B-AR may play a positive role during tubular regeneration.