Nitric Oxide Trickle Drives Heme into Hemoglobin and Muscle Myoglobin

Visualizing mitochondrial heme flow through GAPDH in living cells and its regulation by NO

Iron protoporphyrin IX (heme) is a redox-active cofactor that is bound in mammalian cells by GAPDH and allocated by a process influenced by physiologic levels of NO. This impacts the activity of many heme proteins including indoleamine dioxygenase-1 (IDO1), a redox enzyme involved in immune response and tumor growth. To gain further understanding we created a tetra-Cys human GAPDH reporter construct (TC-hGAPDH) which after labeling could indicate its heme binding by fluorescence quenching. When purified or expressed in a human cell line, TC-hGAPDH had properties like native GAPDH and heme binding quenched its fluorescence by 45-65%, allowing it to report on GAPDH binding of mitochondrially-generated heme in live cells in real time. In cells with active mitochondrial heme synthesis, low-level NO exposure increased heme allocation to IDO1 while keeping the TC-hGAPDH heme level constant due to replenishment by mitochondria. When mitochondrial heme synthesis was blocked, low NO caused a near complete transfer of the existing heme in TC-hGAPDH to IDO1 in a process that required IDO1 be able to bind the heme and have an active hsp90 present. Higher NO exposure had the opposite effect and caused IDO1 heme to transfer back to TC-hGAPDH. This demonstrated: (i) flow of mitochondrial heme through GAPDH is tightly coupled to target delivery, (ii) NO up- or down-regulates IDO1 activity by promoting a conserved heme exchange with GAPDH that goes in either direction according to the NO exposure level. The ability to drive a concentration-dependent, reversible protein heme exchange is unprecedented and reveals a new role for NO in biology.

Visualizing Mitochondrial Heme Flow through GAPDH to Targets in Living Cells and its Regulation by NO

Iron protoporphyrin IX (heme) is an essential cofactor that is chaperoned in mammalian cells by GAPDH in a process regulated by NO. To gain further understanding we generated a tetra-Cys human GAPDH reporter construct (TC-hGAPDH) which after being expressed and labeled with fluorescent FlAsH reagent could indicate heme binding by fluorescence quenching. When purified or expressed in HEK293T mammalian cells, FlAsH-labeled TC-hGAPDH displayed physical, catalytic, and heme binding properties like native GAPDH and its heme binding (2 mol per tetramer) quenched its fluorescence by 45-65%. In live HEK293T cells we could visualize TC-hGAPDH binding mitochondrially-generated heme and releasing it to the hemeprotein target IDO1 by monitoring cell fluorescence in real time. In cells with active mitochondrial heme synthesis, a low-level NO exposure increased heme allocation into IDO1 while keeping steady the level of heme-bound TC-hGAPDH. When mitochondrial heme synthesis was blocked at the time of NO exposure, low NO caused cells to reallocate existing heme from TC-hGAPDH to IDO1 by a mechanism requiring IDO1 be present and able to bind heme. Higher NO exposure had an opposite effect and caused cells to reallocate existing heme from IDO1 to TC-hGAPDH. Thus, with TC-hGAPDH we could follow mitochondrial heme as it travelled onto and through GAPDH to a downstream target (IDO1) in living cells, and to learn that NO acted at or downstream from the GAPDH heme complex to promote a heme reallocation in either direction depending on the level of NO exposure.

A natural heme deficiency exists in biology that allows nitric oxide to control heme protein functions by regulating cellular heme distribution

A natural heme deficiency that exists in cells outside of the circulation broadly compromises the heme contents and functions of heme proteins in cells and tissues. Recently, we found that the signaling molecule, nitric oxide (NO), can trigger or repress the deployment of intracellular heme in a concentration-dependent hormetic manner. This uncovers a new role for NO and sets the stage for it to shape numerous biological processes by controlling heme deployment and consequent heme protein functions in biology.

Hemoglobin resident in the lung epithelium is protective for smooth muscle soluble guanylate cyclase function

Hemoglobin (Hb) present in the lung epithelium is of unknown significance. However Hb being an nitric oxide (NO) scavenger can bind to NO and reduce its deleterious effects. Hence we postulated an NO scavenging role for this lung Hb. Doing transwell co-culture with bronchial epithelial cells, A549/16-HBE (apical) and human airway smooth muscle cells (HASMCs as basal), we found that Hb can protect the smooth muscle soluble guanylyl cyclase (sGC) from excess NO. Inducing the apical A549/16-HBE cells with cytokines to trigger iNOS expression and NO generation caused a time dependent increase in SNO-sGC and this was accompanied with a concomitant drop in sGC-α1β1 heterodimerization. Silencing Hbαβ in the apical cells further increased the SNO on sGC with a faster drop in the sGC heterodimer and these effects were additive along with further silencing of thioredoxin 1 (Trx1). Since heme of Hb is critical for NO scavenging we determined the Hb heme in a mouse model of allergic asthma (OVA) and found that Hb in the inflammed OVA lungs was low in heme or heme-free relative to those of naïve lungs. Further we established a direct correlation between the status of the sGC heterodimer and the Hb heme from lung samples of human asthma, iPAH, COPD and cystic fibrosis. These findings present a new mechanism of protection of lung sGC by the epithelial Hb, and suggests that this protection maybe lost in asthma or COPD where lung Hb is unable to scavenge the NO due to it being heme-deprived.