DRD3 (dopamine receptor D3) but not DRD2 activates autophagy through MTORC1 inhibition preserving protein synthesis.

ABSTRACT

Growing evidence shows that autophagy is deficient in neurodegenerative and psychiatric diseases, and that its induction may have beneficial effects in these conditions. However, as autophagy shares signaling pathways with cell death and interferes with protein synthesis, prolonged use of autophagy inducers available nowadays is considered unwise. The search for novel autophagy inducers indicates that DRD2 (dopamine receptor 2)-DRD3 ligands may also activate autophagy, though critical aspects of the action mechanisms and effects of dopamine ligands on autophagy are still unknown. In order to shed light on this issue, DRD2- and DRD3-overexpressing cells and drd2 KO, drd3 KO and wild-type mice were treated with the DRD2-DRD3 agonist pramipexole. The results revealed that pramipexole induces autophagy through MTOR inhibition and a DRD3-dependent but DRD2-independent mechanism. DRD3 activated AMPK followed by inhibitory phosphorylation of RPTOR, MTORC1 and RPS6KB1 inhibition and ULK1 activation. Interestingly, despite RPS6KB1 inhibition, the activity of RPS6 was maintained through activation of the MAPK1/3-RPS6KA pathway, and the activity of MTORC1 kinase target EIF4EBP1 along with protein synthesis and cell viability, were also preserved. This pattern of autophagy through MTORC1 inhibition without suppression of protein synthesis, contrasts with that of direct allosteric and catalytic MTOR inhibitors and opens up new opportunities for G protein-coupled receptor ligands as autophagy inducers in the treatment of neurodegenerative and psychiatric diseases. ABBREVIATIONS AKT/Protein kinase B thymoma viral proto-oncogene 1; AMPK AMP-activated protein kinase; BECN1 beclin 1; EGFP enhanced green fluorescent protein; EIF4EBP1/4E-BP1 eukaryotic translation initiation factor 4E binding protein 1; GPCR; G protein-coupled receptor; GFP green fluorescent protein; HEK human embryonic kidney; MAP1LC3/LC3 microtubule-associated protein 1 light chain 3; MAP2K/MEK mitogen-activated protein kinase kinase; MAPK1/ERK2 mitogen-activated protein kinase 1; MAPK3/ERK1 mitogen-activated protein kinase 3; MDA malonildialdehyde; MTOR mechanistic target of rapamycin kinase; MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; PPX pramipexole; RPTOR/raptor regulatory associated protein of MTOR, complex 1; RPS6 ribosomal protein S6; RPS6KA/p90S6K ribosomal protein S6 kinase A; RPS6KB1/p70S6K ribosomal protein S6 kinase B1; SQSTM1/p62 sequestosome 1; ULK1 unc-51 like autophagy activating kinase 1; WT wild type.