Ubiquitin ligase SYVN1/HRD1 facilitates degradation of the SERPINA1 Z variant/α-1-antitrypsin Z variant via SQSTM1/p62-dependent selective autophagy

SERPINA1/AAT/alpha-1-antitrypsin (serpin family A member 1) deficiency (SERPINA1/AAT-D) is an autosomal recessive disorder characterized by the retention of misfolded SERPINA1/AAT in the endoplasmic reticulum (ER) of hepatocytes and a significant reduction of serum SERPINA1/AAT level. The Z variant of SERPINA1/AAT, containing a Glu342Lys (E342K) mutation (SERPINA1(E342K)/ATZ), the most common form of SERPINA1/AAT-D, is prone to misfolding and polymerization, which retains it in the ER of hepatocytes and leads to liver injury. Both proteasome and macroautophagy/autophagy pathways are responsible for disposal of SERPINA1(E342K)/ATZ after it accumulates in the ER. However, the mechanisms by which SERPINA1(E342K)/ATZ is selectively degraded by autophagy remain unknown. Here, we showed that ER membrane-spanning ubiquitin ligase (E3) SYVN1/HRD1 enhances the degradation of SERPINA1(E342K)/ATZ through the autophagy-lysosome pathway. We found that SYVN1 promoted SERPINA1(E342K)/ATZ, especially Triton X 100-insoluble SERPINA1(E342K)/ATZ clearance. However, the effect of SYVN1 in SERPINA1(E342K)/ATZ clearance was impaired after autophagy inhibition, as well as in autophagy-related 5 (atg5) knockout cells. On the contrary, autophagy induction enhanced SYVN1-mediated SERPINA1(E342K)/ATZ degradation. Further study showed that SYVN1 mediated SERPINA1(E342K)/ATZ ubiquitination, which is required for autophagic degradation of SERPINA1(E342K)/ATZ by promoting the interaction between SERPINA1(E342K)/ATZ and SQSTM1/p62 for formation of the autophagy complex. Interestingly, SYVN1-mediated lysine 48 (K48)linked polyubiquitin chains that conjugated onto SERPINA1(E342K)/ATZ might predominantly bind to the ubiquitin-associated (UBA) domain of SQSTM1 and couple the ubiquitinated SERPINA1(E342K)/ATZ to the lysosome for degradation. In addition, autophagy inhibition attenuated the suppressive effect of SYVN1 on SERPINA1(E342K)/ATZ cytotoxicity, and the autophagy inducer rapamycin enhanced the suppressive effect of SYVN1 on SERPINA1(E342K)/ATZ-induced cell apoptosis. Therefore, this study proved that SYVN1 enhances SERPINA1(E342K)/ATZ degradation through SQSTM1-dependent autophagy and attenuates SERPINA1(E342K)/ATZ cytotoxicity.