Sandahl Melissa, Hunter Debra M, Strunk Karen E, Earp H Shelton, Cook Rebecca S
UNC-Lineberger Comprehensive Cancer Center, 450 West Ave, Chapel Hill, North Carolina 27599, USA.
BMC Dev Biol. 2010 Dec 30;10:122. doi: 10.1186/1471-213X-10-122.
Mammary glands harbor a profound burden of apoptotic cells (ACs) during post-lactational involution, but little is known regarding mechanisms by which ACs are cleared from the mammary gland, or consequences if this process is interrupted. We investigated AC clearance, also termed efferocytosis, during post-lactational remodeling, using mice deficient for MerTK, Axl, and Tyro3, three related receptor tyrosine kinases (RTKs) regulating macrophage-mediated efferocytosis in monocytes. MerTK expression, apoptosis and the accumulation of apoptotic debris were examined in histological sections of MerTK-deficient, Axl/Tyro3-deficient, and wild-type mammary glands harvested at specific time points during lactation and synchronized involution. The ability of primary mammary epithelial cells (MECs) to engulf ACs was assessed in culture. Transplant of MerTK-deficient mammary epithelium into cleared WT mammary fat pads was used to assess the contribution of WT mammary macrophages to post-lactational efferocytosis.
ACs induced MerTK expression in MECs, resulting in elevated MerTK levels at the earliest stages of involution. Loss of MerTK resulted in AC accumulation in post-lactational MerTK-deficient mammary glands, but not in Axl and Tyro3-deficient mammary glands. Increased vascularization, fibrosis, and epithelial hyperproliferation were observed in MerTK-deficient mammary glands through at least 60 days post-weaning, due to failed efferocytosis after lactation, but did not manifest in nulliparous mice. WT host-derived macrophages failed to rescue efferocytosis in transplanted MerTK-deficient mammary epithelium.
Efferocytosis by MECs through MerTK is crucial for mammary gland homeostasis and function during the post-lactational period. Efferocytosis by MECs thus limits pathologic consequences associated with the apoptotic load following lactation.
在哺乳期后的乳腺退化过程中，乳腺中存在大量凋亡细胞（ACs），但对于ACs从乳腺中清除的机制，或者如果这个过程被中断会产生什么后果，我们知之甚少。我们利用缺乏MerTK、Axl和Tyro3的小鼠，研究了哺乳期后重塑过程中的AC清除，这三种相关的受体酪氨酸激酶（RTKs）在单核细胞中调节巨噬细胞介导的胞葬作用。在哺乳期和同步退化的特定时间点采集的MerTK缺陷型、Axl/Tyro3缺陷型和野生型乳腺的组织切片中，检测了MerTK表达、细胞凋亡和凋亡碎片的积累。在培养中评估了原代乳腺上皮细胞（MECs）吞噬ACs的能力。将MerTK缺陷型乳腺上皮移植到清除后的野生型乳腺脂肪垫中，以评估野生型乳腺巨噬细胞对哺乳期后胞葬作用的贡献。
ACs诱导MECs中MerTK表达，导致退化最早阶段MerTK水平升高。MerTK的缺失导致哺乳期后MerTK缺陷型乳腺中ACs积累，但在Axl和Tyro3缺陷型乳腺中未出现这种情况。在断奶后至少60天内，MerTK缺陷型乳腺中观察到血管生成增加、纤维化和上皮细胞过度增殖，这是由于哺乳期后胞葬作用失败所致，但在未生育的小鼠中未表现出来。野生型宿主来源的巨噬细胞未能挽救移植的MerTK缺陷型乳腺上皮中的胞葬作用。
MECs通过MerTK进行的胞葬作用对于哺乳期后乳腺的稳态和功能至关重要。因此，MECs的胞葬作用限制了与哺乳期后凋亡负荷相关的病理后果。
