Hcy-thiolactone, N-Hcy-protein, and Hcy downregulate autophagy-related proteins. In an earlier work, one of us (HJ) has shown that HUVEC can metabolize Hcy to Hcy-thiolactone and N-Hcy-protein (9). We have also shown that these metabolites reduced autophagy flux in mouse neuroblastoma N2A cells (27). To figure out whether each of these metabolites can affect autophagy in human endothelial cells, we treated HUVECs with N-Hcy-protein, Hcy-thiolactone, or Hcy and quantified levels of autophagy-related proteins by Western blotting. We found significantly attenuated levels of regulators of autophagosome assembly BECN1 (Fig. 1A), ATG5 (Fig. 1B), and ATG7 (Fig. 1C) in HUVEC treated with N-Hcy-protein, Hcy-thiolactone, or Hcy, compared with untreated controls. Levels of microtubule-associated protein 1 light chain 3 (LC3-I), were significantly upregulated by Hcy-thiolactone and Hcy and downregulated by N-Hcy-protein (Fig. 1D). In contrast, the lipidated LC3-II was significantly downregulated (Fig. 1E). The LC3-II/LC3-I ratio, an indicator of autophagy flux, was reduced by Hcy-thiolactone and Hcy but unaffected by N-Hcy-protein (Fig. 1F). Protein p62, a receptor for the degradation of ubiquitinated substrates, known to be negatively correlated with autophagy flux (31), was upregulated by N-Hcy-protein but unaffected by Hcy-thiolactone and Hcy (Fig. 1G). Representative images of western blots are shown in Fig. 1H. These findings show that Hcy-thiolactone and Hcy impair autophagy by dysregulating autophagosome assembly and autophagy flux while N-Hcy-protein dysregulates autophagosome assembly but has no effect on autophagy flux.
Hcy-thiolactone, N-Hcy-protein, and Hcy downregulate autophagy-related mRNAs. To elucidate whether effects of Hcy metabolites on the expression of autophagy-related proteins are transcriptional, we quantified by RT-qPCR mRNAs for these proteins in HUVECS treated with N-Hcy-protein, Hcy-thiolactone, and Hcy. We found significantly attenuated levels of BECN1 mRNA (Fig. 2A), ATG5 mRNA (Fig. 2B), and ATG7 mRNA (Fig. 2C) in HUVEC treated with N-Hcy-protein, Hcy-thiolactone, or Hcy, compared to untreated controls. Levels of LC3 mRNA were significantly downregulated by N-Hcy-protein, Hcy-thiolactone, and Hcy (Fig. 2D). p62 mRNA was unaffected by these metabolites (Fig. 2E). These findings show that Hcy metabolites exert transcriptional control over the expression of BECN1, ATG5, ATG7, and LC3 proteins.
Hcy-thiolactone, N-Hcy-protein, and Hcy downregulate autophagy-related mRNAs by upregulating the expression of miR-21, mir-155, miR-216, and miR-320c. To find out whether the transcriptional downregulation of BECN1, ATG5, ATG7, and LC3 caused by Hcy metabolites is mediated by miRs targeting mRNAs encoding these proteins ( https://mirtarbase.cuhk.edu.cn and refs. (32–37)), we quantified miR-21, mir-155, miR-216, and miR-320c in HUVECs treated with Hcy-thiolactone, N-Hcy-protein, or Hcy. We found significantly upregulated miR-21 (Fig. 3A), mir-155 (Fig. 3B), miR-216 (Fig. 3C), and miR-320c (Fig. 3D) levels in HUVECs treated with Hcy-thiolactone or Hcy compared to control. Treatments of HUVECs with N-Hcy-protein significantly elevated miR-21 (Fig. 3A) and mir-155 (Fig. 3B) levels. However, miR-216 (Fig. 3C) and miR-320c (Fig. 3D) were not affected by N-Hcy-protein.
Treatments with miR inhibitors abrogate effects of Hcy-thiolactone, N-Hcy-protein, and Hcy on miR-21, mir-155, miR-216, and miR-320c expression. To verify these findings, we carried out experiments with miR inhibitors. We found that transfections of HUVECs with miR-21 inhibitor significantly reduced miR-21 levels (to 27 ± 4% compared to untreated control, P < 0.0001; Fig. 4A). Transfections with miR-155 inhibitor reduced miR-155 levels (to 24 ± 3% compared to control, P < 0.0001; Fig. 4B). Transfections with miR-216 inhibitor reduced miR-216 levels (to 54 ± 10% compared to control, P < 0.0001; Fig. 4C). Transfections with miR-320c inhibitor reduced miR-320c levels (to 47 ± 12% compared to control, P < 0.0001; Fig. 4D).
We also found that treatments of HUVEC with miR-21 inhibitor or miR-216 inhibitor significantly increased BECN1 mRNA (P < 0.01; Fig. 5A, B) and BECN1 protein level (P < 0.001; Fig. 6A, B). Treatments with inhibitors of miR-21, miR-155, or miR-216 significantly increased LC3 mRNA (P < 0.001; Fig. 5C, D, E), LC3-II protein level (P < 0.01; Fig. 6C, D, E), and LC3-II/LC3-I ratio (P < 0.01; Fig. 6F, G, H), showing reduced autophagy flux. LC3-I protein level was significantly increased by miR-155 inhibitor (Fig. 6I) but was not affected by inhibitors of miR-21 and miR-216 (not shown).
Treatments with miR-21 inhibitor significantly decreased p62 mRNA (P < 0.001; Fig. 5F) and p62 protein level (P < 0.01; Fig. 6J). As p62 mRNA was not affected by treatments with Hcy metabolites (Fig. 2E) and because p62 and LC3 are inversely correlated (31), this effect is most likely due to upregulation of LC3 by the miR-21 inhibitor.
Treatments with inhibitors of miR-155 or miR-216 significantly increased ATG5 mRNA (P < 0.001; Fig. 5G, H) and ATG5 protein level (P < 0.01; Fig. 6K, L) while treatments with mir-320c inhibitor significantly increased ATG7 mRNA (P < 0.001; Fig. 6I) and ATG7 protein level (P < 0.01; Fig. 6M). Representative western blot images for autophagy proteins quantification are shown in Fig. 6N, O, P, Q.
At the same time, inhibitors of miR-21, miR-155, miR-216, or miR-320C abrogated the stimulatory effects of Hcy-thiolactone, N-Hcy-protein, and Hcy on miR-21 (Fig. 4A), mir-155 (Fig. 4B), miR-216 (Fig. 4C), and miR-320c (Figure D) seen in the absence of these inhibitors (Fig. 3A, B, C, and D, respectively).
Inhibitors of miR-21, miR-155, miR-216, or miR-320C also abrogated the inhibitory effects of Hcy-thiolactone, N-Hcy-protein, and Hcy on BECN1, ATG5, ATG7, and LC3 mRNA and protein expression (Fig. 5 and Fig. 6, respectively) seen in the absence of miR inhibitors (mRNA, Fig. 2; protein, Fig. 1).