Intracellular trafficking of the essential signaling metabolite heme in myeloid cells: impact on physiology and disease - The growth factors and transcriptional activators that give rise to diverse populations of functionally distinct circulating monocytes and tissue-resident macrophages (MFs) have largely been identified. In contrast, the influence of metabolic signals from the local tissue microenvironment on MF differentiation and function remains essentially unexplored. One endogenous signal that been associated to MF differentiation in homeostatic conditions is the essential metabolite heme, the body’s main purveyor of iron, which directs red pulp MF (RPM) differentiation. As RPMs degrade senescent red blood cells, they accrue large amounts of hemoglobin-derived heme. This buildup of intracellular heme triggers degradation of the heme-regulated transcriptional repressor BACH1, thus inducing expression of SPIC1, a transcription factor (TF) required for RPM development. Beyond its role in regulating RPM differentiation, our preliminary data show that BACH1 plays a previously unappreciated outsized role in myeloid cells. We find that BACH1 is part of the small set of core factors that regulate MF identity and function, and that it integrates the transcriptional response to signaling heme and pro-inflammatory stimuli. Supporting this notion, mice lacking BACH1 in myeloid cells have altered numbers of tissue-resident MFs, and cells derived from them show reduced fitness in bone marrow transplant repopulation studies. Moreover, myeloid BACH 1 KO mice have alterations in the inflammatory/repair process in the setting of acute muscle injury, and, unlike WT littermates, quickly perish when given with a sublethal dose of LPS. Given that BACH1 activity is regulated by heme, these unexpected findings stress the extent to which heme signaling regulates MF biology and highlight the need to gain insight into the pathways of intracellular heme delivery that control BACH1 function. Heme is a vital metabolite for life, but due to its highly oxidative iron content, free heme is very cytotoxic. As such, heme intracellular mobilization requires a protein network to enable its transfer throughout the cell, the components of which are largely unknown. We have discovered in adipocytes an intracellular heme trafficking pathway mediated by the poorly characterized proteins Progesterone Receptor Component 1 and 2 (PGRMC1, PGRMC2), that delivers heme to proteins in the ER and the nucleus, including heme-responsive TFs such as BACH1. We recently found that the PGRMC1/2 heme trafficking pathway is active in MFs, and that PGRMC1/2 DKO MFs have increased BACH1 levels and functional defects. Our hypothesis is that in monocytes/MFs, the PGRMC1/2 pathway is central for delivery of heme to the nucleus, and that BACH1 is a principal mediator of the effects of heme signaling on MF differentiation and function under normal and pathological conditions. In this project, we leverage unique mouse models with reduced nuclear heme, chemical tools that increase signaling heme flux to the nucleus, and new mouse mutants of the key mediator BACH1 to reveal a molecular pathway for metabolite control of MF differentiation/function.
