Efferocytosis: another function of uPAR

uPAR, the receptor for urokinase plasminogen activator, is a regulator of the uptake by macrophages of apoptotic neutrophils (efferocytosis). Its role and mechanism appear to be complex and possibly controversial.

The urokinase plasminogen activator was originally thought to function primarily by concentrating urokinase-dependent proteolytic activity on the surface of cells, hence, increasing the potential of cells to move and migrate through barriers. Over the years, however, it has been firmly established that uPAR is also a signaling receptor, albeit missing an intracellular domain, therefore needing to interact with other extracellular/transmembrane proteins to activate signaling pathways. The development of uPAR Ko mice did not move the field forward initially because the mice appeared normal. However, a subsequent series of phenotypes have been reported on closer study, showing that uPAR is required in vivo for the homeostasis of a wide variety of cells including hematopoietic stem cells, osteoblasts, osteoclasts, macrophages, and others. uPAR Ko mice are deficient in a series of important functions (inflammation, bone homeostasis, kidney and hematopoietic stem cells mobilization and homing).^1-4  Some of these have been linked to human pathology.

uPAR is an adhesion receptor. It directly binds with high affinity to the extracellular matrix component, vitronectin, and this appears to be essential for uPAR dimerization and signaling.^5-7  A direct interaction between uPAR and different integrins has been suggested by many publications; however, in our opinion, while there is no doubt of a functional interaction, there is no real evidence that the link is direct.

Clearance of apoptotic neutrophils by macrophages (efferocytosis) is an important mechanism regulating inflammation, host responses, and cancer. Clearance of dead cells is important to avoid unwanted inflammatory responses. In this issue, Park and colleagues show that uPAR modulates neutrophils efferocytosis exploiting macrophages and neutrophils isolated from uPAR Ko mice.⁸  Indeed, uPAR^−/− macrophages show increased engulfing activity of viable (uPAR^+/+) neutrophils both in vivo and in vitro. A similar increase in neutrophil uptake is observed when using uPAR^−/− neutrophils and uPAR^+/+ macrophages, but not when both cell types are uPAR-negative. The data suggest a mechanism different from the “eat me” or “don't eat me.”⁹  Interestingly, administration of exogenous suPAR (a soluble version of uPAR) reverses both phenotypes. Indeed, suPAR inhibits the increased uptake of uPAR^+/+ neutrophils by uPAR^−/− macrophages as well as the increased uptake of uPAR^−/− neutrophils by uPAR^+/+ macrophages. The authors' interpretation is that suPAR modulates the adhesion of neutrophils/macrophages through direct interactions with integrins both in cis and trans. Remarkably, the positive effect of unilateral uPAR deficiency on neutrophil uptake by macrophages seems to require the nonredundant functions of a large number of integrins including αM, αV, β1, β 2, and β 3 on both the neutrophil and the macrophage, as well as the LDL receptor-related protein, LRP. A direct effect of exogenous suPAR on the activity of Mac1 was previously described.¹⁰ 

The complexity of the uPAR/efferocytosis relationship is further underscored by another paper published almost at the same time.¹¹  D'Mello et al show that uPAR overexpression in nonprofessional phagocytes stimulates efferocytosis of apoptotic cells by cancer cells, specifically. While in this work the cells used are not professional phagocytes, the result is nevertheless opposite that of Park et al. Moreover, in this effect, a direct role of integrins seems to be excluded. It is important to realize that uPAR overexpression is a frequent event in cancer cells and that its presence may induce an environment more favorable for cancer invasion by regulating the infiltration and clearance of inflammatory cells.

Although the precise mechanism underlying uPAR function in efferocytosis still remains to be elucidated, the data from these papers clearly point to an unrecognized role in heterotypic cell-to-cell adhesion.

The role of the uPAR ligand, uPA, was not addressed in either of the 2 papers. However, contact between apoptotic cells and macrophages induces IL10, which is required for efferocytosis. In turn, IL10 induction requires the activity (and the tyrosine phosphorylation) of a transcription factor, Prep1.¹²  Prep1 (at that time called UEF3) was discovered years ago as a transcription factor binding to the enhancer region of the uPA gene in a region that serves to modulate the response to various proliferation (and other) stimuli.¹³  Is it possible that there is a connection between Prep1-dependent uPA expression, uPAR, and efferocytosis?