Thyroid hormones (TH) promote differentiation of astrocytes. We have previously reported that a downstream role β-adrenergic receptor (β-AR) system in such effects of TH. Although evidences indicate strong interaction between TH and the β-ARs, the underlying mechanism is poorly understood. In the present study, we further explored the influence of TH on β-AR signaling during the differentiation process. Unlike β1-AR, binding of (125)I-pindolol to β2-AR in cell membranes was significantly decreased at 2 h of exposure to TH which came back to control values after 24 h. The initial decrease in β2-AR in membranes resulted in a concomitant increase in β2-AR levels in the cytosol, suggesting that TH may induce endocytosis of the receptor. qRT-PCR as well as Western blot analysis demonstrated that unlike β-adrenergic receptor kinase (β-ARK)1 and β-ARK2, the messenger RNA (mRNA) and protein levels of β-arrestin-1 in the astrocyte cultures increased on exposure to TH. Knockdown of β-arrestin gene suggested requirement of both β-arrestin-1 and β-arrestin-2 isoforms during endocytosis of β2-AR, thereby facilitating cell differentiation. Endocytic inhibitors blocked the delayed but sustained activation of p-extracellular signal-regulated kinase (ERK) observed during cell differentiation. Observations suggest that TH upregulate β-arrestin-1 in astrocytes to facilitate endocytosis of β2-AR, required for endosomal ERK activation to drive the differentiation process.
Keywords: Astrocytes; Endocytosis; Thyroid hormone; siRNA; β-arrestin; β2-adrenergic receptor.