AMPK Regulates the Expression of the Fe-S Cluster Assembly Enzyme (ISCU) and ALAS2, Modulating Cellular Iron Metabolism and Increasing Hemoglobin Synthesis

Iron-sulfur (Fe-S) clusters are iron-containing prosthetic groups and enzymatic cofactors. They are strong oxidants when unbound yet essential in many processes like facilitating ATP production in mitochondria, promoting DNA, RNA and protein syntheses during cell proliferation and enhancing DNA repair in antioxidant defense. In particular, Fe-S clusters are indispensable in erythropoiesis, where the majority of physiological iron is utilized and where Fe-S clusters are required for the heme synthesis. Deficient Fe-S cluster synthesis predisposes individual to various diseases, such as cancer, metabolic and neurodegeneration diseases and blood disorders. However, it is unclear how Fe-S cluster synthesis is regulated and coordinates with environmental and developmental needs to prevent oxidative damage.

The 5' AMP-activated protein kinase (AMPK) is a kinase activated by oxidative stress and energy starvation and critical for maintaining redox and energy homeostasis. In this study, we investigated the role of AMPK on Fe-S clusters synthesis and function and extended our findings in normal and thalassemic erythroid cells.

Through bioinformatic analysis, we found that the Fe-S cluster assembly enzyme (ISCU), a scaffold protein indispensable for Fe-S cluster biogenesis, contains putative AMPK phosphorylation motifs at serine (S) residues 14 and 29 (human numbering). Using the human cell line 293T, we confirmed that AMPK phosphorylates ISCU, while point mutations in these residues prevented this activity. Moreover, AMPK-mediated phosphorylation promoted ISCU binding to 14-3-3s, a family of proteins that, once associate with phosphorylated residues, modulates the stability and function of targeted proteins. Indeed, increased association with 14-3-3s stabilized ISCU proteins, corroborating the observation that AMPK promotes the activity of ISCU proteins.

We extended our studies using A549 cells that do not have AMPK activity since they harbor a mutant LKB1 kinase, which is responsible for activating AMPK. By overexpression of wild type (WT)-LKB1 and LKB1 kinase-dead mutant (KDM), we found that only WT-LKB1 restored AMPK activity, binding of ISCU to 14-3-3s and stability of ISCU. Moreover, under hydrogen peroxide incubation and glucose starvation, ISCU protein levels and Fe-S cluster synthesis were both increased only in the presence of LKB1-WT, but not in cells harboring KMD. LKB1-WT overexpressed cells also survived hydrogen peroxide incubation and glucose starvation better than those with KMD. Together, these data suggest that AMPK activation stabilizes ISCU protein and preserves Fe-S cluster synthesis to maintain a healthy redox and energy homeostasis.

We then explored the effect of AMPK on Fe-S cluster synthesis in erythropoiesis by using the drug AICAR, an AMPK activator, in murine erythroleukemia (MEL) cells. We found that in MEL cells, AICAR treatment stabilized ISCU, increased Fe-S cluster levels and promoted the synthesis of the aminolevulinic acid synthase 2 (ALAS2) protein, which represents the rate-limiting enzyme in erythroid heme synthesis. Furthermore, this was associated with increased heme and globin chain synthesis, with a trend in increasing β-globin mRNA and proteins more than α-globin. We further confirmed these observations in Human Umbilical Cord Blood-Derived Erythroid Progenitor (HUDEP-2) and CD34⁺ cells derived from peripheral blood isolated from both healthy individuals and ß-thalassemic patients. In these cells, we found that AMPK upregulation by AICAR administration not only increased ALAS2 expression and erythroid heme levels, but also enhanced the synthesis of both a- and ß-globin chains, though with a preference for increasing β-globin levels. Analysis using specimens from thalassemic mice is in progress.

In conclusion, our work demonstrates that under redox and energetic stress, activated AMPK phosphorylates and stabilizes ISCU protein, thereby enhancing Fe-S cluster synthesis and maintaining their function. Moreover, AMPK activation with AICAR treatment increases erythroid heme synthesis and hemoglobin expression. Given that AMPK is the major kinase that responds to oxidative and energetic cues, our work provides a mechanistic explanation for how erythropoiesis responds to energy starvation and redox stress as well as a potential novel therapeutic target to treat blood and metabolic disorders.