Fig 2: AMPK inhibits PARP1 activity to attenuate cholesterol-induced toxicity. Cholesterol-stimulated INS-1 cells were treated with AICAR or PARP-1 alone or co-treated with AICAR and PARP-1 and examined for (A) cell viability by MTT assay; (B) cell apoptosis by Flow cytometry; (C) insulin secretion by insulin ELISA kit; (D) intracellular lipid deposition by Oil Red O and BODIPY staining. (E) The cholesterol efflux from INS-1 cells by BODIPY-cholesterol assay. *P < 0.05, **P < 0.01 compared with control group, #P < 0.05, ##P < 0.01 compared with AICAR group.

Fig 3: The pancreatic islet morphology and ABCA1 and GLP-1 protein levels. Sprague-Dawley (SD) rats were randomly assigned into five groups: non-treatment control group (n = 6), streptozotocin (STZ)-induced T2DM group (n = 6), T2DM plus sham surgery group (n = 7), T2DM plus RYGB surgery group (n = 9), and T2DM plus EX-4 treatment group (n = 8). (A) The histopathology of rats’ pancreatic tissues was examined by H&E staining. n = 6. (B,C) The protein contents and distribution of ABCA1 and PARP-1 were examined by IHC staining in rats’ pancreatic tissues. n = 6, *P < 0.05, **P < 0.01 compared with NC group, ##P < 0.01 compared with T2DM + sham group, &&P < 0.01 compared with T2DM + RYGB group.

Fig 4: GLP-1 regulates ABCA1 expression through AMPK-dependent inhibition on PARP1 activity. (A) Cholesterol-stimulated INS-1 cells were pre-treated with AMPK activator AICAR or AMPK inhibitor Compound C for 4 h before the addition of GLP-1 and incubated for another 24 h. The protein levels of PAR in INS-1 cell lysates were determined by immunoblotting. (B) PARP-1 knockdown was generated in INS-1 cells by the transfection of si-PARP-1. The transfection efficiency was confirmed by immunoblotting. (C) Cholesterol-stimulated INS-1 cells were transfected with si-PARP-1 or treated with PARP-1 inhibitor 3-AB and examined for the protein levels of ABCA1 by immunoblotting. (D) Cholesterol-stimulated INS-1 cells were treated with AMPK activator AICAR or PARP-1 alone or co-treated with AICAR and PARP-1 and examined for the protein levels of p-PARP-1 and ABCA1 by immunoblotting. **P < 0.01 compared with si-NC group or control group, ##P < 0.01 compared with AICAR group.

Fig 5: PARP-1 inhibits LXR-induced ABCA1 transcription in INS-1 cells. (A) Confirmation of the interaction between LXR and PARP-1 in INS-1 cells using coimmunoprecipitation (Co-IP) assay. (B) LXRa poly(ADP-ribosyl)ation was detected in INS-1 by Poly(ADP-ribosyl)ation Assay. (C,D) INS-1 cells were transfected with si-PARP-1 in the presence or absence of LXR receptor agonist T0901317 and examined for the mRNA expression and protein levels of ABCA1 by real-time PCR and immunoblotting. n = 3 for each experiment. **P < 0.01 compared with the si-NC + DMSO group, #P < 0.05 compared with the si-NC + T0901317 group.