Up-regulation of the Angiotensin II Type 1 Receptor by the MAS Proto-oncogene Is Due to Constitutive Activation of Gq/G11 by MAS

Coexpression of the MAS proto-oncogene with the angiotensin II type 1 (AT(1)) receptor in CHO-K1 cells has been reported to increase the number of [(3)H]angiotensin II-binding sites, although MAS does not bind [(3)H]angiotensin II. In HEK293 cells stably expressing AT(1) receptor-cyan fluorescent protein (CFP), MAS-yellow fluorescent protein (YFP) expression from an inducible locus caused strong up-regulation of AT(1) receptor-CFP amounts and [(3)H]angiotensin II binding levels. The time course of AT(1) receptor-CFP up-regulation was also markedly slower than that of induction of MAS expression. These effects were not mimicked by induced expression of I138D MAS-YFP, a mutant unable to cause constitutive loading of [(35)S]guanosine 5'-O-(thiotriphosphate) onto the phospholipase Cbeta-linked G protein Galpha(11). Protein kinase C (PKC) inhibitors and the selective Galpha(q)/Galpha(11) inhibitor YM-254890 fully blocked MAS-induced up-regulation of AT(1) receptor-CFP amounts, whereas the PKC activator phorbol 12-myristate 13-acetate produced strong up-regulation of AT(1) receptor-CFP without induction of MAS-YFP expression and in the presence of I138D MAS-YFP. The C-terminal tail of the AT(1) receptor is a known target for PKC-mediated phosphorylation. In cells stably expressing a C-terminally truncated version of the AT receptor, induction of MAS expression did not up-regulate the truncated construct levels. These data demonstrate that the ability of MAS to up-regulate AT(1) receptor levels reflects the constitutive capacity of MAS to activate Galpha(q)/Galpha(11) and hence stimulate PKC-dependent phosphorylation of the AT(1) receptor.