Retinoic Acid Induces Autophagosome Maturation Through Redistribution of the Cation-Independent Mannose-6-Phosphate Receptor

Abstract

Retinoic acids (RAs) have diverse biologic effects and regulate several cellular functions. Here, we investigated the role of RA on autophagy by studying its effects on autophagosome (AUT) maturation, as well as on upstream regulators of autophagosome biogenesis. Our studies, based on the use of pH-sensitive fluorescent reporter markers, suggested that RA promotes AUT acidification and maturation. By using competitive inhibitors and specific agonists, we demonstrated that this effect is not mediated by the classic RAR and RXR receptors. RA did not affect the levels of upstream regulators of autophagy, such as Beclin-1, phospho-mTOR, and phospho-Akt1, but induced redistribution of both endogenous cation-independent mannose-6-phosphate receptor CIMPR and transiently transfected GFP and RFP full-length CIMPR fusion proteins from the trans-Golgi region to acidified AUT structures. Those structures were found to be amphisomes (acidified AUTs) and not autophagolysosomes. The critical role of CIMPR in AUT maturation was further demonstrated by siRNA-mediated silencing of endogenous CIMPR. Transient CIMPR knockdown resulted in remarkable accumulation of nonacidified AUTs, a process that could not be reversed with RA. Our results suggest that RA induces AUT acidification and maturation, a process critical in the cellular autophagic mechanism.

Retinoic acid-induced IgG production in TLR-activated human primary B cells involves ULK1-mediated autophagy

“…In a recent paper it was reported that RA enhances the acidification of autophagosomes in HeLa cells by promoting fusion with endosomes or lysosomes, and that RA in this manner contributes to increase autophagosomal maturation. 38 Our results demonstrating that RA promotes the colocalization between the autophagosomal marker LC3B and the Lyso-ID staining of lysosomes in TLR9-and CD180-stimulated cells, suggest that RA enhances autophagosomal degradation. Whether or not RA-mediated acidification and maturation of autophagosomes is involved in the RA-induced autophagosomal degradation and IgG production is, however, not yet established.…”

“…RA has previously been linked to autophagy in other cell types and under other conditions. 38,49,50 In acute promyelocytic leukemia (APL) cells we found RA to induce autophagy-mediated degradation of the PML-RARA (promyelocytic leukemia/ retinoic acid receptor a) fusion oncoprotein. 49 The effect of RA in the APL cells was mediated via inhibition of the MTOR (mechanistic target of rapamycin [serine/threonine kinase]) pathway, and interestingly, siRNA against ULK1 diminished the autophagy-induced degradation of the PML-RARA fusion oncoprotein.…”

“…[37][38][39] The immunoblot analysis in Figure 5A revealed that RA induced the accumulation of LC3B-II in a dose-dependent manner with notable effects already at 1 nM of RA. The LC3B-II-formation was mediated via RARs, as the RAR agonist TTNPB induced the expression of LC3B-II to a similar or even greater extent than 100 nM of RA, whereas a 500 times excess of the RAR antagonist Ro 41-4253 reversed the LC3B-II-formation induced by 10 nM of RA.…”

Transthyretin—A Key Gene Involved in Regulating Learning and Memory in Brain, and Providing Neuroprotection in Alzheimer Disease via Neuronal Synthesis of Transthyretin Protein

“…Western blot and immunohistochemistry analysis described the increase degradation of 14-3-3ζ protein of lysosome in the absence of TTR, increasing autophagy and memory deficits process [38] [79] [80] [81] [82]. In conclusion, the absence of TTR decreases 14-3-3ζ protein levels in the hippocampus which in turn attributable to increased degradation in the lysosomal compartments, probably by the reduced levels of 14-3-3ζ protein [83] [84].…”

Retinoid receptor signaling and autophagy in acute promyelocytic leukemia

“…ATRA may also have a biologic role in autophagosome maturation through the redistribution of a cation-dependent mannose-6-phosphate receptor (CIMPR) to the developing autophagosome, leading to vesicle acidification. This effect is thought to be independent of nuclear hormone receptors and instead to be mediated by direct binding of ATRA to CIMPR 131 . A recent study indicates that ATO induces autophagy via the induction of MEK/ERK signaling independently of the mTOR autophagy-signaling axis 103 .…”