Fig 2: Model: CDO1 antagonizes the growth and survival of KEAP1MUT cells by producing toxic products and depleting NADPH.(Left) Elevated intracellular cysteine (CYS) stabilizes CDO1, leading to the production of CSA and SO32-. In turn, SO32- depletes cystine via sulfitolysis to produce CYS-SO3-, further limiting cysteine availability and utilization. SO2 (sulfur dioxide) exists in equilibrium with SO32- and is a gas that may diffuse in and out of cells. The continual reduction of (CYS)2 to CYS depletes cellular NADPH, limiting its availability for cellular processes. (Right) CDO1 silencing promotes the accumulation of intracellular CYS and NADPH, and prevents wasting of CYS as CSA and toxic SO32-. GSR, glutathione reductase. TXNRD1, thioredoxin reductase 1. Fe-S, iron sulfur cluster.

Fig 3: Sulfitolysis is not required for the inhibition of proliferation by CDO1.(a) Western blot analysis of GOT1 and ß-ACTIN expression in parental (P) A549 and H460 cells, and GOT1 KO clones #1 and #2 for each cell line expressing empty pMXS (GOT1 -) and reconstituted with pMXS-GOT1 (GOT1 +). (b) Contribution of sulfitolysis to (CYS)2 consumption by CDO1. CDO1-dependent (CYS)2 consumption, and CSA, SO32-, and CYS-SO3- production were determined as cysteine molar equivalents. N = 3 replicates/group. (c) Analysis of the proliferation of GOT1 KO A549 cells from (a) expressing CDO1Y157F (Y157F) or CDO1WT (WT). Cells were collected on the indicated days, stained with crystal violet and their absorbance at 600 nm determined. N = 3 replicates/group. (d,e) Quantitation of glutathione (GSH) total (12C + 13C) levels (d) and 13C-labeling only (e) from 13C-cystine in cells from (A) following expression of CDO1Y157F (Y157F) or CDO1WT (WT). 13C-label is shown in red, while 12C-label is gray. Cells were labeled for 1 hr. N = 3 replicates/group. (f) Analysis of Coenzyme A (CoA) labeling from 13C, 15N-cystine in cells from (a) expressing CDO1Y157F (Y157F) or CDO1WT (WT). 13C,15N-label (M + 3) is shown in red, while 12C, 14N-label is gray. Cells were labeled for 4 hr. N = 3 replicates/group. (g) Analysis of protein synthesis rates with azidohomoalanine labeling in cells from (a) expressing CDO1Y157F (Y157F) or CDO1WT (WT). Cells treated with 50 µg/mL cyclohexamide (C) were used as a positive control for translation inhibition. N = 4 replicates/group. For b–g,) cells were treated with 0.25 µg/ml doxycycline for 2 days prior to and during the assay.10.7554/eLife.45572.032Figure 6—source data 1.Sulfitolysis is not required for the inhibition of proliferation by CDO1.

Fig 4: Nrf2 promotes the accumulation of cysteine dioxygenase (Cdo1) to promote entry of cysteine into the taurine synthesis pathway.(a) Western blot analysis of Cdo1 and ß-Actin levels following Nrf2 stabilization in wild-type (WT) vs. homozygous Keap1R554Q/R554Q (R554Q) MEFs. (b) Real-time PCR analysis of Cdo1 and Nqo1 mRNA levels following Nrf2 stabilization in wild-type (WT) vs. homozygous Keap1R554Q/R554Q (R554Q) MEFs. mRNA expression was normalized to ß-Actin expression, followed by normalization to WT. N = 3. (c) Western blot analysis of Nrf2, Cdo1, Nqo1 and ß-Actin levels following expression of sgControl (sgCON) or Cdo1 deletion (sgCdo1 #2 and #3) with CRISPR/Cas9 in primary wild-type (WT) and homozygous Keap1R554Q/R554Q (R554Q) MEFs. (d–i) Quantitation of cysteine (CYS, (d), glutathione (GSH, (e,f), cystine ([CYS]2,(g), and hypotaurine (HTAU, (h,i) total levels and 13C-labeling from 13C-cystine in cells from (c). 13C-label is shown in red, while 12C-label is gray. Cells were labeled for 4 hr. N = 3.10.7554/eLife.45572.008Figure 2—source data 1.Nrf2 promotes the accumulation of cysteine dioxygenase (Cdo1) to promote entry of cysteine into the taurine synthesis pathway.

Fig 5: CDO1-dependent cystine reduction limits NADPH availability for cellular processes.(a) The NADPH/NADP + ratio was assayed following expression of CDO1Y157F (Y157F) or CDO1WT (WT) in A549 and H460 GOT1 KO cells expressing empty pMXS (GOT1 -) or reconstituted with pMXS-GOT1 (GOT1 +). N = 3 replicates/group. (b) Quantitation of reduced glutathione (GSH) and oxidized glutathione (GSSG) in KEAP1C273S-expressing cells from Figure 4k,l following expression of CDO1Y157F (Y157F) or CDO1WT (WT). N = 3 replicates/group. (c) A549 and H460 GOT1 KO cells expressing empty pMXS (dashed line, open circle) or reconstituted with pMXS-GOT1 (solid line, solid circle), followed by expression of CDO1Y157F (gray) or CDO1WT (red), were treated with 0–25 µM cumene hydroperoxide (CuH2O2) for 24 hr. Cell numbers were analyzed using crystal violet and normalized to untreated cells. *=GOT1+ CDO1WT vs. CDO1Y157F, #=GOT1- CDO1WT vs. CDO1Y157F. N = 3 replicates/group. (d) Analysis of proline (M + 5) labeling from L-[U]-13C-glutamine in GOT1 KO A549 and H460 cells expressing empty pMXS (Control) or reconstituted with pMXS-GOT1 (GOT1), followed by expression of CDO1Y157F or CDO1WT. Proline M + 5 abundance was normalized to the abundance of its precursor glutamate M + 5, and then CDO1WT levels were normalized to CDO1Y157F. Cells were labeled for 1 hr in proline-free media. N = 3 replicates/group. (e,f) Mass isotopomer analysis of citrate labeling in GOT1 KO A549 (d) and H460 (e) cells expressing empty pMXS (Control) or reconstituted with pMXS-GOT1 (GOT1) following expression of CDO1Y157F or CDO1WT cultured with L-[U]-13C-glutamine for 4 hr. N = 3 replicates/group. For (a–f), cells were treated with 0.25 µg/ml doxycycline for 2 days prior to and during the assay.10.7554/eLife.45572.035Figure 7—source data 1.CDO1-dependent cystine reduction limits NADPH availability for cellular processes.