探討成對受體 Siglec-5 與 Siglec-14 在 巨噬細胞極化與小鼠大腸直腸癌模型中的角色

Abstract

腫瘤細胞常具有過度唾液酸化的特徵，並可透過與腫瘤微環境中免疫細胞表面的唾液酸受體Siglec結合，幫助其逃脫免疫監視並促進腫瘤進展。已知巨噬細胞表面的Siglec-5與Siglec-14成對受體可辨識相同的唾液酸配體，但分別傳遞抑制型與活化型訊號。由於大腸癌細胞表面常大量表現唾液酸，本研究旨在探討此對訊號相反的受體是否以不同方式調控巨噬細胞極化。我們發現，於標準第二型細胞激素mIL-4或小鼠大腸癌細胞MC38細胞培養液(CM)刺激下，表現 Siglec-14的骨髓源性巨噬細胞(S14/BMDM)更趨向於M2或腫瘤相關巨噬細胞(TAM)的表型，並且相較WT/BMDM表現較高的SYK與ERK磷酸化，但p38的磷酸化表現較低；相對地，表現Siglec-5的BMDM (S5/BMDM)則無顯著極化影響，S5/BMDM中SYK、ERK及p38磷酸化程度則低於WT/BMDM。在MC38皮下腫瘤模型當中，我們觀察到SIGLEC14Myeloid TG小鼠腫瘤生長快速且體積較大，伴隨NK細胞浸潤減少與巨噬細胞浸潤增加；而SIGLEC5Myeloid TG小鼠之免疫細胞組成和腫瘤生長則與 WT 小鼠相似。在AOM/DSS誘導的大腸直腸癌模型中，SIGLEC14Myeloid TG小鼠大腸腫瘤體積顯著增加，並呈現黏膜內腺癌特徵及嚴重黏膜損傷；而SIGLEC5Myeloid TG小鼠的腫瘤大小則與WT小鼠無顯著差異。綜合上述，我們發現Siglec-14 可促進巨噬細胞極化為 M2/TAM 表型，進而推動腫瘤生長與免疫抑制環境的建立。惟其與唾液酸配體結合後如何調控巨噬細胞訊息傳導與腫瘤進程，尚待進一步研究釐清。

Tumor cells often overexpress sialic acids, which engage Siglecs (sialic acid–binding immunoglobulin-like lectins) on immune cells to evade immune surveillance and promote tumor progression within the tumor microenvironment. Among Siglecs, Siglec-5 and Siglec-14 are paired receptors expressed on macrophages that recognize the same sialylated ligands but transmit opposing signals: Siglec-5 delivers inhibitory signals, while Siglec-14 activates immune responses. Given that colorectal cancer (CRC) cells are known to express high levels of sialic acids, we investigated the role of Siglec-5 and Siglec-14 in macrophage polarization and tumor development in murine CRC models. We found that bone marrow–derived macrophages (BMDMs) expressing Siglec-14 (S14/BMDMs) showed enhanced M2 or tumor-associated macrophage (TAM) polarization when stimulated with IL-4 or conditioned medium from murine CRC MC38 cells (MC38 CM). Under these conditions, S14/BMDMs exhibited increased phosphorylation of SYK and ERK and reduced phosphorylation of p38 compared to wild-type BMDMs. In contrast, BMDMs expressing Siglec-5 (S5/BMDMs) did not display enhanced polarization, and phosphorylation of SYK, ERK, and p38 was lower than in wild-type controls. In the AOM/DSS-induced spontaneous CRC model, SIGLEC14Myeloid TG mice developed larger tumors with intramucosal adenocarcinoma features and severe mucosal damage. Tumor sizes were comparable between SIGLEC5Myeloid TG and wild-type mice. Similarly, in the MC38 subcutaneous tumor model, SIGLEC14Myeloid TG mice exhibited larger and faster-growing tumors, accompanied by reduced NK cell infiltration and increased macrophage infiltration. Tumor growth and immune cell composition was comparable between SIGLEC5Myeloid TG and wild-type mice. Together, our findings suggest that Siglec-14 promotes macrophage polarization toward the M2/TAM phenotype, enhancing tumor growth and immune suppression in CRC models. However, the precise mechanism by which Siglec-14 interacts with its ligands to modulate macrophage signaling and influence tumor progression remains to be elucidated.