Relationship between internalization and mRNA decay in down-regulation of recombinant type 1 angiotensin II receptor (AT1) expression in smooth muscle cells

In vascular smooth muscle cells, the hormone angiotensin II is thought to cause internalization of the seven-transmembrane domain type 1 angiotensin II receptor (AT1-R) but it also suppresses expression of the receptor mRNA. As for similarly regulated members of this gene superfamily, the relative roles of these processes in receptor down-regulation are not well understood. In this study a recombinant AT1-R mRNA was synthesized in A7r5 vascular smooth muscle cells from a tetracycline-suppressible promoter using a retroviral vector system. Angiotensin II induces a profound internalization of the cell surface AT1-R protein but has no effect on steady-state AT1-R mRNA levels. Shortly after either bolus or prolonged dosing with angiotensin II, cell surface AT1-R expression recovers, indicating the existence of a significant restorative externalization pathway. The extent of this recovery is attenuated markedly when transcription of the recombinant AT1-R gene is suppressed by cotreatment of the cells with anhydrotetracycline. Although agonist-stimulated internalization appears to contribute directly to a loss of AT1-R protein, these observations provide direct evidence that a reduction in AT1-R mRNA content plays a significant role in sustained AT1-R down-regulation.