Throughout this application various publications are referred to in superscripts. Citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications are hereby incorporated by reference in their entireties into the subject application to more fully describe the art to which the subject application pertains.
Autophagy is the process by which intracellular components undergo degradation in lysosomes1,2, contributing in this way to the maintenance of cellular homeostasis and to cellular quality control. In addition, autophagy is upregulated as a mechanism of cellular defense against aggressors or to allow cellular adaptation to changing environmental conditions3. Alterations of the autophagic process have been described in multiple pathological conditions and underlie the pathogenesis of severe human diseases such as neurodegeneration, cancer and metabolic disorders1,4.
The best characterized autophagic pathways are macroautophagy and chaperone-mediated autophagy (CMA)1. The distinctive characteristic of CMA is that the specific subset of cytosolic proteins degraded by this pathway are directly translocated across the lysosomal membrane into the lysosomal lumen for degradation5. Substrates for this pathway all bear in their amino acid sequence a targeting motif6 that once recognized by the cytosolic chaperone hsc70, mediates substrate delivery to the surface of lysosomes7. Once there, substrates bind to the lysosome-associated membrane protein type 2A (LAMP-2A) and promote its multimerization into a high molecular weight complex, required for substrate translocation8. A variant of hsc70 resident in the lysosomal lumen assists substrates to achieve complete translocation inside lysosomes.
CMA is maximally activated in response to stressors such as prolonged nutritional deprivation, oxidative stress, hypoxia or exposure to different toxic compounds5. Malfunctioning of CMA has been described in neurodegenerative conditions such as familial forms of Parkinson's disease9,10 and certain tauopathies11, in metabolic disorders such as diabetes12 and in different lysosomal storage disorders13. Furthermore, the gradual decline in the activity of this pathway with age has been proposed to act as an aggravating factor in different age-related disorders14. In fact, if the reduction in CMA activity with age is prevented in vivo, through genetic manipulation in a mouse model, cellular homeostasis and organ function can be preserved until late in life15. These findings, along with the growing number of connections between CMA and human diseases, justify the growing interest in developing efficient chemical modulators of this autophagic pathway.
Chemical compounds shown to have an effect on CMA until now lack selectivity for this pathway16. For example, inhibition of protein synthesis or of lysosomal proteases results in reduced CMA degradation, but it also affects many other intracellular processes16. Inhibition of glucose-6-phosphate dehydrogenase or of the cytosolic chaperone hsp90 lead to higher CMA activity in some cell types but not in others16; and in fact, later studies demonstrated that the effect was not direct but a consequence of compensatory upregulation of other CMA components8. One of the limitations for the future development of CMA modulators has been the lack of information on the cellular signaling mechanisms that activate this pathway.
Retinoic acid receptors (RARs) act as transcriptional activators and repressors of a broad subset of genes, contributing thus to modulate cellular processes in which CMA has also been involved, such as differentiation, proliferation and control of cellular homeostasis17. Furthermore, RAR loss or aberrant function has been described in many oncogenic processes, where CMA upregulation is a common feature required to sustain cancer cell growth18. The three types of RARs identified in mammals, RARα, RARβ and RARγ, are coded by three different genes17. In contrast to the complex tissue-dependent expression of RARβ and RARγ, RARα is ubiquitously expressed.
RAR are attractive druggable targets because their natural substrates, all-trans-retinoic acid (ATRA) and similar retinoids have been well characterized19. Their efficient trafficking across lipid bilayers, due to their small size and hydrophobic character20, along with the growing understanding of the chemical modifications that the different regions of retinoid derivatives can undergo intracellularly21,22 explains why ATRA by-products and derivatives are being already explored for therapeutic purposes.
The present invention address the need for compounds that affect retinoic acid receptor (RAR) signaling and CMA activity and the use of these compounds in treatment of diseases and conditions associated with loss of CMA activity.