Lipids and carbohydrates represent the two principal cellular energy sources. Their catabolism and subsequent derivatives feed into the tricarboxylic acid (TCA) cycle and/or the electron transport chain (ETC) of the mitochondria. The ETC consists of four protein complexes embedded in the mitochondrial inner membrane. A series of redox reactions occurs in the ETC in which electrons are passed along to sequential acceptors. The transport of H+ across the inner membrane creates an electrochemical gradient (termed mitochondrial membrane potential or MMP), which drives protons through the ATP synthase, generating ATP for cellular use. Additional organic compounds produced by the TCA cycle and the ETC are also used in other cellular processes such as amino acid synthesis and nitrogen transportation.
A recent study showed that the MMP in phagocytes increases initially after ingestion of apoptotic cells, but decreases over time(Park et al., 2011
); however, ATP levels within the phagocyte were largely unchanged, suggesting that proton transport and ATP production were disengaged (Park et al, 2011
). One family of mitochondrial proteins that that can ‘uncouple’ the electrochemical gradient from ATP generation are the uncoupling proteins (Ucp), located in the inner mitochondrial membrane. Park et al show that Ucp2 levels increase in phagocytes after incubation with apoptotic cells, and that loss of Ucp2, resulted in an increase in mitochondrial membrane potential, and negatively affected phagocytic ability. Again, similar to the PPAR and LXR requirement, Ucp2 function was specific to apoptotic cell removal, and had little effect on phagocytosis of synthetic targets, zymosan particles, or E. coli
Autophagy (also referred to as macroautophagy), the process by which the cells may generate energy in nutrient-starved conditions through selective and regulated digestion of internal components, has received much attention recently (Mizushima and Komatsu, 2011
). Given that the mitochondria is a foci of energy production, the increase in mitochondrial membrane potential within the engulfing phagocyte may help generate ATP needed to facilitate plasma membrane remodeling and to support varied phosphorylation events as part of the engulfment process. Interestingly, components of the autophagic machinery, in particular, light chain 3 (LC3) have been found on phagosomes containing apoptotic cells (Florey et al., 2011
; Martinez et al., 2011
) (). This event, termed LC-3 associated phagocytosis (LAP), was initially observed on toll-like receptor (TLR) activated phagosomes of phagocytes that have engulfed particles containing TLR ligands (Sanjuan et al., 2007
; Lee et al., 2010
). LAP differs from classical autophagy in that the hallmark autophagic double-membrane structure is absent; instead, the phagosome containing the apoptotic cell is a single-membrane and decorated with LC-3. The LC-3 containing phagosomes mature and degrade the internalized cargo much more rapidly than non-LC-3 containing phagosomes (Martinez et al., 2011
; Sanjuan et al., 2007
). Some autophagic proteins, such as Atg7, Atg5, Beclin-1, and Vps34, but not the Ulk1-Atg13-FIP200 pre-initiation complex, are required for LAP (Martinez et al., 2011
; Florey et al., 2011
). Interestingly, Vps34 is also essential for proper phagosome maturation in the context of corpse removal. Whether or not the consequences of LAP- or non-LAP-associated apoptotic cell clearance resembles that of autophagy where the degraded products are used to fuel cellular functions remains to be determined. Nevertheless, given the energy demands of the engulfment process as well as the documented involvement of the mitochondria, the engulfment of apoptotic cells is likely a metabolic stress on the phagocyte, requiring the phagocyte to utilize either itself or the ingested apoptotic cell as energy sources for continued clearance. Indeed, phagocytes engulfing apoptotic cells, but not synthetic beads, have an increased rate of fatty acid oxidation (Park et al., 2011
), suggesting that this may be the case, although whether the products are used for generating additional energy is not clear. It will be interesting to determine whether LAP is always a component of apoptotic cell phagocytosis and if LAP plays a role in mediating the digestion and conversion of the engulfed cargo into usable energy.
The withdrawal of certain growth factors, such as insulin- growth factor like 1 (IGF-1) or nerve growth factor, induces autophagy. Interestingly, macrophages deficient in the growth factor progranulin, as well as C. elegans
mutants lacking the progranulin homologue display better engulfment capacity (Kao et al., 2011
), suggesting a connection between growth factor withdrawal and phagocytosis. More broadly, general nutrient sensing may be relevant in modulating engulfment of dying cells. Recently, thioglycollate-elicited peritoneal macrophages, following ingestion of apoptotic cells, were shown to increase AMP-activated protein kinase (AMPK) activity, a kinase involved in sensing cellular ATP levels (Bae et al., 2011
). Pharmacological activation of AMPK was shown to augment phagocytic ability, which was in this case proposed to be related to an AMPK-mediated increase in microtubule and actin dynamics. Typically, AMPK phosphorylation leads to changes in cellular lipid and glucose metabolism, and in mitochondrial biogenesis. Whereas the cellular metabolic consequences of AMPK activation were not explored by Bae et al (2011)
, the connection between this key energy sensor to enhanced phagocytosis is intriguing. Given the relationship between growth factors and autophagy, it will be of interest to determine whether and how these growth factors affect nutrient-sensing responses and potentially impact apoptotic cell clearance in a tissue or in an altered nutrient state (e.g. hypoxic tumor environment). AMPK activation was not seen in bone-marrow derived macrophages exposed to apoptotic cells (Park et al., 2011
), raising the possibility that there may be differential post-engulfment effects based on the activation status of certain phagocytes. This idea may even be extended to the question of whether professional phagocytes and non-professional phagocytes respond differently in digestion of ingested corpses.