Malignant mesothelioma (MM) has a poor prognosis and is largely resistant to standard treatments, so it is important to seek novel therapeutic strategies for this disease. Cancer-initiating cells (CICs) were previously identified in MM using stem cell-associated markers in combination with spheroid cultures. However, the mechanisms underlying the induction and maintenance of CICs in MM remain to be fully explored. Here we showed that the CICs, which had high aldehyde dehydrogenase levels (ALDH^bright) and stem cell-associated genes, were expanded in MM cells cultured under sphere-forming conditions. The MM spheroids also initiated tumors in immunodeficient mice more efficiently than did conventional adherent MM cells. In the MM spheroids, the expression of hyaluronan (HA) synthases was upregulated. Inhibiting the HA synthesis or CD44 functions by gene knockdown or neutralizing antibody abolished the formation of large-sized spheroids and the expansion of ALDH^bright CICs. The expression of activin-A was also increased in the spheroids, and type I activin-A receptor subunit (ALK4) was upregulated in the ALDH^bright CICs. The neutralization of activin-A or functional inactivation of ALK4 diminished the ALDH^bright CICs without affecting spheroid formation. The knockdown of CD44 or ALK4 strongly suppressed the tumor growth in immunodeficient mice. These results together suggest that the HA-CD44 and activin-A-ALK4 pathways differentially regulate the spheroid formation and maintenance of ALDH^bright CICs in MM cells, and that both pathways play critical roles in tumor growth in immunodeficient hosts. Our findings provide a novel therapeutic option for MM that targets signaling pathways that promote the CIC compartment through CD44 and ALK4.

中文翻译：

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通过CD44和ALK4信号通路对恶性间皮瘤的癌形成细胞进行球形成和维持的差异调节。

恶性间皮瘤（MM）的预后较差，并且对标准疗法有很大的抵抗力，因此寻找针对该疾病的新治疗策略非常重要。先前已在MM中使用干细胞相关标记物与球状培养物相结合来鉴定癌症起始细胞（CIC）。然而，在MM中诱导和维持CIC的潜在机制仍有待充分探索。在这里，我们显示了具有高醛脱氢酶水平的CIC（ALDH^亮）和干细胞相关基因在球形形成条件下培养的MM细胞中扩增。与常规贴壁MM细胞相比，MM球体还可以在免疫缺陷小鼠中更有效地引发肿瘤。在MM球体中，透明质酸（HA）合成酶的表达上调。通过基因敲低或中和抗体抑制HA合成或CD44功能，消除了大尺寸球体的形成和ALDH^亮CIC的扩展。激活素A的表达在球体中也增加，而I型激活素A受体亚基（ALK4）在ALDH^亮CIC中被上调。激活素A的中和作用或ALK4的功能失活减少了ALDH的^亮度CIC不影响球体的形成。击倒CD44或ALK4可以强烈抑制免疫缺陷小鼠的肿瘤生长。这些结果共同表明，HA-CD44和激活素-A-ALK4途径差异性调节MM细胞中ALDH^亮CIC的球状形成和维持，并且这两种途径在免疫缺陷宿主的肿瘤生长中均起关键作用。我们的发现为MM提供了一种新的治疗选择，其靶向通过CD44和ALK4促进CIC区域的信号通路。