The ZIP8/SIRT1 axis regulates alveolar progenitor cell renewal in aging and idiopathic pulmonary fibrosis

Abstract

Type 2 alveolar epithelial cells (AEC2s) function as progenitor cells in the lung. We have shown previously that failure of AEC2 regeneration results in progressive lung fibrosis in mice and is a cardinal feature of idiopathic pulmonary fibrosis (IPF). In this study, we identified deficiency of a specific zinc transporter, SLC39A8 (ZIP8), in AEC2s from both IPF lungs and lungs of old mice. Loss of ZIP8 expression was associated with impaired renewal capacity of AEC2s and enhanced lung fibrosis. ZIP8 regulation of AEC2 progenitor function was dependent on SIRT1. Replenishment with exogenous zinc and SIRT1 activation promoted self-renewal and differentiation of AEC2s from lung tissues of IPF patients and old mice. Deletion of Zip8 in AEC2s in mice resulted in impaired AEC2 renewal, increased susceptibility to bleomycin injury, and development of spontaneous lung fibrosis. Therapeutic strategies to restore zinc metabolism and appropriate SIRT1 signaling could improve AEC2 progenitor function and mitigate ongoing fibrogenesis.

Keywords: Adult stem cells; Fibrosis; Pulmonology; Stem cells.

Figure 1. Selective loss of ZIP8 in IPF AEC2s.

(A) UMAP plots of flow-enriched EpCAM⁺CD31^–CD45^– cells from healthy (11,381 cells, n = 6) and IPF lungs (14,687 cells, n = 6). (B and C) Clusters of epithelial cell types (B) and distribution of epithelial cell types (C) in healthy and IPF lungs. PNEC, pulmonary neuroendocrine cells. (D) Expression of AEC2 marker genes in cells from healthy and IPF lungs. (E) SFTPC expression in healthy (n = 6) and IPF AEC2s (n = 4) by qPCR (*P < 0.05). (F) Expression of the zinc transporter gene SLC39A8 in AEC2s from healthy and IPF lungs in the present scRNA data set. (G) Expression of SLC39A8 in AEC2s from healthy (control) and IPF lungs from the recently published data sets GSE135893, GSE132915, GSE132771, and GSE128033. (H and I) qPCR for SLC39A8 mRNA expression in AEC2s freshly isolated from lung tissues (n = 4 each, ***P < 0.001) (H) and derived from 3D-cultured organoids (healthy n = 7, IPF n = 8, ****P < 0.0001) (I). (J and K) Flow cytometry of cell-surface ZIP8 levels and percentage of ZIP8⁺ cells in healthy (n = 11) and IPF (n = 7) AEC2s (****P < 0.0001). (L) SLC39A8 expression in healthy (n = 108) and IPF lung tissues (n = 160) (****P < 0.0001). Data are shown as mean ± SEM. (M) Correlation of SLC39A8 expression and lung function as DLCO (% predicted DLCO) in healthy control (n = 97) and IPF (n = 145) lung tissues (r = 0.6277). (N) Immunofluorescence staining for the AEC2 marker HTII-280 and ZIP8. Arrows indicate examples of HTII-280⁺ cells. Scale bars, 100 μm. Staining was performed with lung sections from 3 IPF patients and 3 healthy donors. E, H, I, K, and L: unpaired 2-tailed Student’s t test; M: nonparametric Spearman’s correlation analysis.

Figure 2. ZIP8-dependent zinc metabolism is required for AEC2 renewal.

(A) Representative plots of intracellular zinc in gated healthy and IPF AEC2s by flow cytometry. (B) Percentage of zinc-positive AEC2s within the gated AEC2 population from healthy (n = 5) and IPF lungs (n = 11) (**P < 0.01). (C) Flow cytometry plots of gated ZIP8⁺ and ZIP8^– AEC2s. (D) CFE of flow-sorted ZIP8⁺ and ZIP8^– AEC2s from healthy (n = 6) and IPF lungs (n = 4) (***P < 0.001, ****P < 0.0001). (E) PDPN expression in AEC2s derived from 3D-cultured organoids (n = 3, *P < 0.05). (F) CFE of AEC2s from healthy and IPF lungs with and without ZnSO₄ (100 μM) treatment (n = 5–7, ****P < 0.0001). (G) CFE of ZIP8⁺ and ZIP8^– AEC2s with and without ZnSO₄ (100 μM) treatment (n = 3 each, ****P < 0.0001). (H) CFE of AEC2s with and without ZnSO₄ and TPEN (1 μM) treatment (n = 3–5, **P < 0.01, ****P < 0.0001 by ANOVA). (I and J) Expression of SLC39A8 (n = 4–6, ****P < 0.0001) and SFTPC (n = 4–6, ***P < 0.001) in AEC2s with and without ZnSO₄ treatment by qPCR. Data are shown as mean ± SEM. B and E: unpaired 2-tailed Student’s t test; D, F, G, H, I, and J: 2-way ANOVA.

Figure 3. ZIP8-dependent zinc metabolism regulates AEC2 renewal through SIRT1.

(A) IPA pathway analysis of AEC2s from healthy and IPF lungs.(B) Sirtuin activation score of healthy and IPF AEC2s. (C and D) SIRT1 expression in freshly isolated AEC2s (C) and AEC2s derived from 3D organoids (D) by qPCR (n = 4–6, *P < 0.05). (E) SIRT1 expression of AEC2s derived from 3D-cultured organoids of ZIP8⁺ and ZIP8^– AEC2s (n = 5 each, *P < 0.05). (F) Intracellular SIRT1 levels in healthy and IPF AEC2s without and with ZnSO₄ treatment by flow cytometry (n = 3–4). (G) Intracellular SIRT1 levels in gated ZIP8⁺ and ZIP8^– AEC2s from healthy lungs (n = 4). (H and I) CFE with 3D organoid culture of AEC2s from healthy lungs treated with either 1 μM SRT1720 (n = 4–5, ***P < 0.001 by ANOVA) (H) or 125 and 200 μM splitomicin (n = 3, ***P < 0.001, ****P < 0.0001 by ANOVA) (I). (J) CFE of AEC2s from IPF lungs cultured with SRT1720 at the indicated concentrations (n = 3, **P < 0.01, ****P < 0.0001 by ANOVA). (K) CFE of AEC2s from IPF lungs cultured with 1 μM SRT1720, 100 μM ZnSO₄, or both (n = 3–4, ****P < 0.0001 by ANOVA). (L–N) A549 cells with SIRT1 knockout and control cells. SIRT1 ko 1, set 1 sgRNA; SIRT ko 2, set 2 sgRNA. (L) SIRT1 expression by qPCR. (M) SIRT1 expression by Western blot analysis; the same experiments were performed 3 times. (N) CFE with 3D organoid culture (n = 4, ***P < 0.001 by ANOVA). Data are shown as mean ± SEM. Unpaired 2-tailed Student’s t test.

Figure 4. Downregulated ZIP8/SIRT1 signaling and decreased renewal capacity of AEC2s from old mouse lungs.

(A) Flow cytometry analysis to gate out total AEC2s (R2), and ZIP8 expression AEC2s (R3) from lung homologies of young and old mice. (B) Percentage of ZIP8⁺ cells (R3) within total AEC2s (n = 5–6, **P < 0.01). (C) Number of ZIP8⁺ cells recovered from young and old mouse lung (n = 5–6, **P < 0.01). (D) CFE of mouse AEC2s isolated from young and old mouse lungs (n = 6–7, **P < 0.01). (E and F) Flow cytometry analysis of ZIP8 expression in gated AEC2s cultured with medium only or medium containing 100 μM ZnSO₄ (n = 6, ***P < 0.001). (G–J) 3D organoid culture of AEC2s isolated from lungs of 2.5-, 12-, 14-, and 18-month-old mice with and without 100 μM ZnSO₄ treatment. (G) CFE (n = 3–4, *P < 0.05, ***P < 0.001, ****P < 0.0001 by ANOVA). (H–J) Expression of Slc39a8 (H), Sftpc (I), and Pdpn (J) in AEC2s derived from 3D-cultured organoids with and without ZnSO₄ treatment by qPCR (n = 3–4, *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001 by ANOVA). (K) Violin plots of gene expression in AEC2s from lungs of bleomycin-treated young and old mice. (L) IPA pathway analysis of AEC2s from young and old mice on day 4 after bleomycin injury. (M and N) CFE of AEC2s from uninjured 10- to 12-week-old young mice treated with SRT1720 (n = 3–6, **P < 0.01, ****P < 0.0001 by ANOVA) (M) and splitomicin (n = 5–6, ****P < 0.0001 by ANOVA) (N) at the indicated doses and DMSO control. (O) CFE of AEC2s from uninjured 20- to 24-month-old mice treated with SRT1720 at the indicated doses and DMSO control (n = 4, **P < 0.01, ****P < 0.0001 by ANOVA). Data are shown as mean ± SEM. Unpaired 2-tailed Student’s t test.

Figure 5. Targeted deletion of Slc39a8 decreased AEC2 renewal.

(A) ZIP-expressing cells among gated AEC2s and (B) percentage of ZIP8⁺ cells within the total AEC2 population from uninjured Zip8^AEC2 and control mice by flow cytometry (n = 8, ****P < 0.001). (C and D) Intracellular zinc levels of AEC2s (C) and percentage of zinc⁺ AEC2s within the total AEC2 population (D) from Zip8^AEC2 (n = 4) and control mice (n = 8) by flow cytometry (**P < 0.01). (E) CFE of flow-sorted AEC2s from uninjured Zip8^AEC2 and control mice with 3D organoid culture (n = 6 each, *P < 0.05). (F and G) Expression of Sirt1 (n = 4, *P < 0.05) (F) and Pdpn (n = 5, **P < 0.01) (G) in AEC2s derived from 3D-cultured organoids. (H–J) AEC2s from day 4 bleomycin-injured Zip8^AEC2 and control mice. (H) Number of AEC2s recovered per lung (n = 5, ***P < 0.001). (I and J) CFE of AEC2s with 3D organoid culture (n = 5–7, **P < 0.01) (I) and colony size (n = 28–86, ****P < 0.0001) (J). (K and L) Ki-67 expression by flow cytometry (n = 6 each, *P < 0.05) (K) and Pdpn expression by qPCR (n = 5 each, ****P < 0.0001) (L) in AEC2s derived from 3D-cultured organoids. (M and N) Violin plots of gene expression in AEC2s with scRNA-Seq. (M) AEC2s from 2-month-old (Young) and 18- to 20-month-old (Old) C57BL/6 WT mice (n = 3). (N) AEC2s from 10- to 12-week-old (young) Zip8^AEC2 mice and littermate controls 2 weeks after 4 doses of tamoxifen injection. Data are shown as mean ± SEM. Unpaired 2-tailed Student’s t test.

Figure 6. Spontaneous and increased lung fibrosis after bleomycin injury in old Zip8^AEC2 mice and zinc metabolism–regulated lung fibrosis.

(A) Experimental layout for spontaneous lung fibrosis in 12-month-old Zip8^AEC2 and control mice. (B) Trichrome staining of lung sections showed that Zip8^AEC2 mice developed fibrosis in subpleural (arrow) and interstitial (arrowheads) regions. Scale bars: top: 1 mm; bottom: 200 μm. (C) Hydroxyproline content (μg per right lung) in the lungs of 12-month-old male Zip8^AEC2 (n = 4) and control (n = 5) mice (*P < 0.05). (D) Experimental layout for Zip8^AEC2 and control mice treated with bleomycin following tamoxifen injection. (E and F) Survival (E) and hydroxyproline levels (μg per whole lung) (F) of 12-month-old Zip8^AEC2 and control mice 21 days after 1.25 U/kg bleomycin treatment (E: n = 16–18, P = 0.33; F: n = 8–10, *P < 0.05). (G and H) Survival (G)and hydroxyproline content (μg per right lung) (H) of 7- to 10-month-old Zip8^AEC2 and control mice on day 21 after 2 U/kg bleomycin treatment (G: n = 32, *P < 0.05; H: n = 5–10, *P < 0.05). (I) Experimental layout for WT mice fed low-zinc and control diets and treated with bleomycin for lung fibrosis study. (J) Survival of mice fed low-zinc and control diets on day 14 after bleomycin injury (n = 16–18, P = 0.07). (K) Hydroxyproline content (μg per right lung) of lungs from mice fed low-zinc and control diets on day 21 after bleomycin injury (n = 8–10, *P < 0.05). (L) Experimental layout for WT mice treated with high-zinc and control diets and bleomycin for lung fibrosis study. (M) Hydroxyproline content (μg per right lung) of lungs from mice fed high-zinc and control diets on day 21 after bleomycin injury (n = 5–8, *P < 0.05). Data are shown as mean ± SEM. C, F, H, K, and M: unpaired 2-tailed Student’s t test; E, G, and J: log-rank test.

Figure 7. Summary of the role of the ZIP8/SIRT1 axis in regulating alveolar progenitor cell renewal.

In young and healthy AEC2s, sufficient ZIP8 ensures adequate levels of intracellular zinc, SIRT1 activity, and AEC2 renewal capacity. However, in old AEC2s and IPF AEC2s, severely downregulated ZIP8 results in intracellular zinc deficiency and defective SIRT1 activity, which impairs AEC2 renewal. In addition, enzymes regulating NAD⁺ synthesis were downregulated in IPF AEC2s, further exaggerating SIRT1 impairment. Therefore, the optimal combinations of zinc, NAD⁺, and SIRT1 activation may restore AEC2 integrity and mitigate fibrosis. MT, metallothionein.