Heme as a differentiation-regulatory transcriptional cofactor

Abstract

The hematopoietic transcription factor GATA1 induces heme accumulation during erythropoiesis by directly activating genes mediating heme biosynthesis. In addition to its canonical functions as a hemoglobin prosthetic group and enzyme cofactor, heme regulates gene expression in erythroid cells both transcriptionally and post-transcriptionally. Heme binding to the transcriptional repressor BACH1 triggers its proteolytic degradation. In heme-deficient cells, BACH1 accumulates and represses transcription of target genes, including α- and β-like globin genes, preventing the accumulation of cytotoxic free globin chains. A recently described BACH1-independent mechanism of heme-dependent transcriptional regulation is associated with a DNA motif termed heme-regulated motif (HERM), which resides at the majority of loci harboring heme-regulated chromatin accessibility sites. Progress on these problems has led to a paradigm in which cell type-specific transcriptional mechanisms determine the expression of enzymes mediating the synthesis of small molecules, which generate feedback loops, converging upon the transcription factor itself and the genome. This marriage between transcription factors and the small molecules that they control is predicted to be a canonical attribute of regulatory networks governing cell state transitions such as differentiation in the hematopoietic system and more broadly.

Keywords: BACH1; Differentiation; Erythroid; GATA1; Heme; Transcription.

Figure 1.. Heme post-translationally regulates BACH1 function.

A. Domain structure of murine BACH1 protein. BTB, bric-a-brac–tramtrack–broad complex domain; bZIP, basic leucine zipper domain; CP motif, cysteine-proline dipeptide that mediates heme binding. B. Heme regulates BACH1 DNA binding, nuclear export and proteasomal degradation. In heme-deficient cells, BACH1 dimerizes with a small Maf protein and inhibits target gene transcription via BACH1 DNA motifs. Upon heme binding to BACH1, BACH1 dissociates from DNA, exits the nucleus into the cytoplasm, and following ubiquitination, it is degraded by the proteasome.

Figure 2.. Context-dependent heme amplification of GATA1 activity: parallel BACH1-dependent and independent mechanisms.

A. Heme facilitates GATA1 activity via BACH1. GATA1 activates Bach1 transcription. In heme-deficient cells, BACH1 protein accumulates, binds BACH1 DNA motifs, and opposes GATA1 activity to activate target gene transcription. In a normal heme environment, BACH1 protein proteolysis dominates over mechanisms that promote BACH1 synthesis, thus negating the mechanism that antagonizes GATA1. B. Heme facilitates GATA1 activity via HERM. The presence of a HERM-binding transcriptional activator or the absence of a HERM-binding repressor in a physiological heme environment facilitates GATA1-mediated activation of target gene transcription.

Figure 3.. Context-dependent heme antagonism of GATA1.

A. Heme antagonism of GATA1 via HERM. Hypothetical mechanism of HERM-binding transcriptional activator that elevates GATA1 target gene transcription in heme-deficient cells, with heme opposing this mechanism. B. Heme accumulation inhibits GATA1 expression. Depletion of Flvcr1, the plasma membrane heme exporter, increases intracellular heme, which inhibits Gata1 RNA and GATA1 protein expression. C. HRI-dependent induction of ATF4 activates transcription in heme-deficient cells. In a low-heme environment, HRI is activated and phosphorylates the translational elongation factor, eIF2α, which increases ATF4 translation, enabling ATF4 to activate target genes transcription.

Figure 4.. Feedback loops involving GATA factor, genome, and small molecules control cellular differentiation.

GATA factor dependent genome regulation controls the expression of transporters (SLCs) and biosynthetic enzymes for small molecules. The small molecules then function in a feedback loop to control GATA factor and genome function via protein effectors, e.g. BACH1.