Background: Sorting nexin 19 (SNX19) is important in the localization and trafficking of the dopamine D₁ receptor (D₁R) to lipid raft microdomains. However, the interaction between SNX19 and the lipid raft components caveolin-1 or flotillin-1 and, in particular, their roles in the cellular endocytosis and cell membrane trafficking of the D₁R have not been determined. Methods: Caveolin-1 and flotillin-1 motifs were analyzed by in silico analysis; colocalization was observed by confocal immunofluorescence microscopy; protein-protein interaction was determined by co-immunoprecipitation. Results: In silico analysis revealed the presence of putative caveolin-1 and flotillin-1 binding motifs within SNX19. In mouse and human renal proximal tubule cells (RPTCs), SNX19 was localized mainly in lipid rafts. In mouse RPTCs transfected with wild-type (WT) Snx19, fenoldopam (FEN), a D₁-like receptor agonist, increased the colocalization of SNX19 with caveolin-1 and flotillin-1. FEN also increased the co-immunoprecipitation of SNX19 with caveolin-1 and flotillin-1, effects that were prevented by SCH39166, a D₁-like receptor antagonist. The FEN-mediated increase in the residence of SNX19 in lipid rafts and the colocalization of the D₁R with caveolin-1 and flotilin-1 were attenuated by the deletion of a caveolin-1 (YHTVNRRYREF) (ΔCav1) or a flotillin-1 (EEGPGTETETGLPVS) (ΔFlot1) binding motif. The FEN-mediated increase in intracellular cAMP production was also impaired by the deletion of either the flotillin-1 or caveolin-1 binding motif. Nocodazole, a microtubule depolymerization inhibitor, interfered with the FEN-mediated increase in the colocalization between SNX19 and D₁R. Conclusion: SNX19 contains caveolin-1 and flotillin-1 binding motifs, which play an important role in D₁R endocytosis and signaling.

Vascular smooth muscle cells (VSMCs), the contractile cells in the tunica media of blood vessels, maintain vascular tone. The proliferation of VSMCs is an important feature of vascular remodeling that contributes to the regulation of blood pressure. Autophagy, an intracellular self-degrading process that delivers cytoplasmic constituents to lysosomes, plays a vital role in VSMC proliferation. This is regulated by the dopaminergic and renin–angiotensin systems but their interplay in their regulation of autophagy in VSMCs is not well-understood. In rat VSMCs, fenoldopam (Fen), a dopamine D1-like receptor agonist, increased autophagy, as determined by the increase in the protein expressions of microtubule-associated protein 1 light chain (LC)3-II and beclin-1 (BECN1), in a time- and concentration-dependent manner. Conversely, angiotensin II (Ang II), the endogenous Ang II type 1 receptor (AT₁R) agonist, decreased the protein expression of LC3-II and BECN1, also in a time- and concentration-dependent manner. The production of cyclic adenosine monophosphate (cAMP) and autophagic LC3-II puncta in VSMCs were increased by Fen and decreased by Ang II. Pre-treatment of VSMCs with Rp-cAMPS, a protein kinase A inhibitor, prevented the Fen-mediated increase and the Ang II-mediated decrease in LC3-II protein expression. Fen decreased, whereas Ang II increased the phosphorylation of P70S6K, a direct downstream mammalian target of rapamycin (mTOR). The inhibitory effect of Fen and stimulatory effect of Ang II on P70S6K phosphorylation were prevented by Rp-cAMPS. Ang II also decreased the Fen-mediated increase in cAMP production, while Fen attenuated the Ang II-mediated increase in cell proliferation, a response that occurs downstream of autophagy. Moreover, Ang II prevented the Fen-mediated inhibition of cell proliferation, an effect that was blocked by losartan, an AT₁R antagonist. These results demonstrate that Fen and Ang II counter-regulate autophagy and proliferation of VSMCs via the mTOR pathway, which is cAMP-dependent.