Daeho Park

Metastasis is the primary cause of mortality in advanced prostate cancer, and the emergence of resistance to androgen receptor (AR)-targeted therapies highlights the urgent need for alternative therapeutic strategies. Metabolic reprogramming has increasingly been recognized as a key driver of metastatic progression. In this study, we uncover a novel tumor-suppressive role for cereblon (CRBN), a substrate receptor of the CRL4^CRBN E3 ubiquitin ligase complex, in modulating prostate cancer metastasis through regulation of 6-phosphogluconate dehydrogenase (6PGD), a critical enzyme in the oxidative pentose phosphate pathway (oxPPP). CRBN directly binds a conserved C-terminal α-helix in 6PGD, promoting its polyubiquitination and proteasomal degradation independently of immunomodulatory drugs (IMiDs). Genetic or pharmacological loss of CRBN via CRISPR/Cas9, RNA interference, or PROTAC-mediated degradation stabilized 6PGD and elevated the NADPH/NADP⁺ ratio. Conversely, re-expression of wild-type CRBN reduced 6PGD levels, restored NADPH/NADP⁺ ratio, and suppressed cell migration and invasion. Transcriptomic profiling revealed CRBN-induced upregulation of CDH1 and downregulation of the EMT marker MMP1, while CRBN degradation produced the opposite pattern—both effects were reversed by 6PGD inhibition. These regulatory effects were conserved across multiple cancer cell lines and observed in CRBN-deficient mouse tissues. Functional studies using intra-splenic xenograft models further demonstrated that CRBN suppresses metastatic dissemination. Collectively, our findings identify 6PGD as a novel endogenous substrate of CRBN and establish the CRBN–6PGD axis as a critical metabolic checkpoint in prostate cancer metastasis. Therapeutic targeting of this pathway may offer promising strategies for CRBN-deficient or 6PGD-driven cancers.

A pivotal step in phagocytosis of apoptotic cells, called efferocytosis, is phagocytic recognition of phosphatidylserine (PS) on dying cells, primarily via PS receptors. Among them, Tim-4 has been studied to elucidate signaling since its identification. However, how apoptotic cells secured by Tim-4 are internalized remains still unclear. Here, we reveal that Axl-mediated signal transduction is necessary for Tim-4 to ingest apoptotic cells. We found correlated expression patterns for Tim-4 and Axl across various tissues, and biochemical and cell biological analyses showed that Tim-4 interacted most strongly with Axl among the TAM receptor family. Mechanistically, the immunoglobulin variable (IgV) domain of Tim-4 interacted with two distinct domains within the extracellular region of Axl. Co-expression of Tim-4 and Axl synergistically promoted Tim-4-mediated efferocytosis. Crucially, disrupting the interaction or pharmacologically or genetically blocking Axl signaling abolished the synergistic effect of Axl on Tim-4-mediated efferocytosis. These findings collectively demonstrate that during Tim-4-mediated efferocytosis, Axl acts as a key signaling relay, biochemically interacting with Tim-4 to transmit apoptotic cell recognition signals into phagocytes, thereby enabling efficient apoptotic cell ingestion.

Profound cytoskeletal reorganization and plasma membrane deformation in phagocytes is indispensable for phagocytosis of massive apoptotic cells, but whether these dynamics accompany a mechanical signal modulating signaling during efferocytosis remains largely unexplored. Here, we report that tension between phosphatidylserine (PS) and PS receptors (PSR) generated at the phagocytic synapse serves as a signal to determine whether PS-exposing cells should be phagocytosed. Mechanistically, increased membrane tension of phagocytes via Rac1-dependent actin polymerization and membrane stiffness of apoptotic cells caused tension between PS and PSRs, leading to phosphoinositide 3-kinase recruitment to PSRs, which resulted in Rac1 inactivation and myosin II phosphorylation required for phagocytic cup closure. Our observations imply that tension between PS and PSRs acts as a mechanical fail-safe to prevent removal of all PS-exposing cells for the integrity of efferocytosis. One-Sentence Summary Tension between phosphatidylserine and its receptors acts as a final decision-maker determining phagocytosis of apoptotic cells.

Cells constituting a multicellular organism die in a variety of ways throughout life, and most of them die via apoptosis under normal conditions. The occurrence of apoptosis is especially prevalent during development and in tissues with a high cellular turnover rate, such as the thymus and bone marrow. Interestingly, although the number of apoptotic cells produced daily is known to be innumerable in a healthy adult human body, apoptotic cells are rarely observed. This absence is due to the existence of a cellular process called efferocytosis that efficiently clears apoptotic cells. Studies over the past decades have focused on how phagocytes are able to remove apoptotic cells specifically, swiftly, and continuously, resulting in defined molecular and cellular events. In this review, we will discuss the current understanding of the clearance of apoptotic cells at the molecular level.

Phagocytosis of apoptotic cells, called efferocytosis, requires calcium inside and outside of phagocytes. Due to its necessity, calcium flux is sophisticatedly modulated, and the level of intracellular calcium in phagocytes is ultimately elevated during efferocytosis. However, the role of elevated intracellular calcium in efferocytosis remains elusive. Here, we report that Mertk-mediated intracellular calcium elevation is necessary for internalization of apoptotic cells during efferocytosis. Drastic depletion of intracellular calcium abrogated the internalization step of efferocytosis by delaying phagocytic cup extension and closure. Especially, the defect of phagocytic cup closure for internalization of apoptotic cells was caused by impaired F-actin disassembly and the attenuated interaction of Calmodulin with myosin light chain kinase (MLCK), leading to diminished myosin light chain (MLC) phosphorylation. Genetic and pharmacological impairment of the Calmodulin-MLCK-MLC axis or Mertk-mediated calcium influx also resulted in inefficient efferocytosis due to a defect in internalization of the targets. Taken together, our observations imply that intracellular calcium elevation through Mertk-mediated calcium influx facilitates efferocytosis by inducing myosin II-mediated contraction and F-actin disassembly required for internalization of apoptotic cells.

Apoptotic cells generated during development and for tissue homeostasis are swiftly and continuously removed by phagocytes via a process called efferocytosis. Efficient efferocytosis can be achieved via transcriptional modulation in phagocytes that have engulfed apoptotic cells. However, such modulation and its effect on efferocytosis are not completely understood. Here, we report that phagocytes are recruited to apoptotic cells being cleared through the Mcp-1–Ccr2 axis, which facilitates clearance of apoptotic cells. We identified Mcp-1 as a modulated transcript using a microarray and found that Mcp-1 secretion was augmented in phagocytes engulfing apoptotic cells. This augmented Mcp-1 secretion was impaired by blocking phagolysosomal degradation of apoptotic cells. Conditioned medium from wild type (WT) phagocytes promoted cell migration, but that from Mcp-1^−/− phagocytes did not. In addition, blockade of Ccr2, the receptor for Mcp-1, abrogated cell migration to conditioned medium from phagocytes incubated with apoptotic cells. The intrinsic efferocytosis activity of Mcp-1^−/− and Ccr2^−/− phagocytes was unaltered, but clearance of apoptotic cells was less efficient in the peritoneum of Mcp-1^−/− and Ccr2^−/− mice than in that of WT mice because fewer Ccr2-positive phagocytes were recruited. Taken together, our findings demonstrate a mechanism by which not only apoptotic cells but also phagocytes induce chemoattraction to recruit phagocytes to sites where apoptotic cells are cleared for efficient efferocytosis.

The clearance of apoptotic cells is known to be a critical step in maintaining tissue and organism homeostasis. This process is rapidly/promptly mediated by recruited or resident phagocytes. Phagocytes that engulf apoptotic cells have been closely linked to the release of anti-inflammatory cytokines to eliminate inflammatory responses. Defective clearance of apoptotic cells can cause severe inflammation and autoimmune responses due to secondary necrosis of apoptotic cells. Recently accumulated evidence indicates that apoptotic cells and their clearance have important physiological roles in addition to immune-related functions. Herein, we review the current understanding of the mechanisms and fundamental roles of apoptotic cell clearance and the beneficial roles of apoptotic cells in physiological processes such as differentiation and development.

Swift and continuous phagocytosis of apoptotic cells can be achieved by modulation of calcium flux in phagocytes. However, the molecular mechanism by which apoptotic cells modulate calcium flux in phagocytes is incompletely understood. Here, using biophysical, biochemical, pharmaceutical, and genetic approaches, we show that apoptotic cells induced the Orai1-STIM1 interaction, leading to store-operated calcium entry (SOCE) in phagocytes through the Mertk-phospholipase C (PLC) γ1-inositol 1,4,5-triphosphate receptor (IP₃R) axis. Apoptotic cells induced calcium release from the endoplasmic reticulum, which led to the Orai1-STIM1 association and, consequently, SOCE in phagocytes. This association was attenuated by masking phosphatidylserine. In addition, the depletion of Mertk, which indirectly senses phosphatidylserine on apoptotic cells, reduced the phosphorylation levels of PLCγ1 and IP₃R, resulting in attenuation of the Orai1-STIM1 interaction and inefficient SOCE upon apoptotic cell stimulation. Taken together, our observations uncover the mechanism of how phagocytes engulfing apoptotic cells elevate the calcium level.

ExoIII tightly anchors itself at the AP site, cuts the backbone, and processively digests the downstream DNA without dissociation.

Tim-4 promotes the engulfment of apoptotic cells or exogenous particles by securing them on phagocytes. It is unable to transduce signals by itself but helps other engulfment receptors sense and internalize them. However, the identity of the engulfment receptors collaborating with Tim-4 is still incompletely understood. In this study, we searched for a candidate transmembrane protein with a FN3 domain, important for interaction with Tim-4, in silico and investigated whether it indeed interacts with Tim-4 and is involved in Tim-4-mediated phagocytosis. We found that EphA2 containing a FN3 domain in the extracellular region interacted with Tim-4, which was mediated by the IgV domain of Tim-4 and the FN3 domain of EphA2. Nevertheless, we found that EphA2 expression failed to alter Tim-4-mediated phagocytosis of apoptotic cells or polystyrene beads. Taken together, our findings suggest that EphA2, a new Tim-4 interacting protein, may intervene in a Tim-4-mediated cellular event even if it is not phagocytosis of endogenous or exogenous particles and vice versa.