MerTK, a receptor tyrosine kinase (RTK) of the TYRO3/AXL/MerTK family, is expressed in myeloid lineage cells in which it acts to suppress proinflammatory cytokines following ingestion of apoptotic material. Using syngeneic mouse models of breast cancer, melanoma, and colon cancer, we found that tumors grew slowly and were poorly metastatic in MerTK -/-mice. Transplantation of MerTK -/-bone marrow, but not wild-type bone marrow, into lethally irradiated MMTV-PyVmT mice (a model of metastatic breast cancer) decreased tumor growth and altered cytokine production by tumor CD11b + cells. Although MerTK expression was not required for tumor infiltration by leukocytes, MerTK -/-leukocytes exhibited lower tumor cell-induced expression of wound healing cytokines, e.g., IL-10 and growth arrest-specific 6 (GAS6), and enhanced expression of acute inflammatory cytokines, e.g., IL-12 and IL-6. Intratumoral CD8 + T lymphocyte numbers were higher and lymphocyte proliferation was increased in tumor-bearing MerTK -/-mice compared with tumor-bearing wild-type mice. Antibody-mediated CD8 + T lymphocyte depletion restored tumor growth in MerTK -/-mice. These data demonstrate that MerTK signaling in tumor-associated CD11b + leukocytes promotes tumor growth by dampening acute inflammatory cytokines while inducing wound healing cytokines. These results suggest that inhibition of MerTK in the tumor microenvironment may have clinical benefit, stimulating antitumor immune responses or enhancing immunotherapeutic strategies.

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The ErbB receptor tyrosine kinase family has often been associated with increased growth of breast epithelial cells, as well as malignant transformation and progression. In contrast, ErbB4/HER4 exhibits unique attributes from a two step proteolytic cleavage which releases an 80 kilodalton, nuclear localizing, tyrosine kinase to a signal transduction mechanism that slows growth and stimulates differentiation of breast cells. This review provides an overview of ErbB4/HER4 in growth and differentiation of the mammary epithelium, including its physiologic role in development, the contrasting growth inhibition/tumor suppression and growth acceleration of distinct ErbB4/HER4 isoforms and a description of the unique cell cycle regulated pattern of nuclear HER4 ubiquitination and destruction.

BackgroundMammary 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.ResultsACs 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.ConclusionEfferocytosis 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.

Data concerning the prognostic value of ErbB4 in breast cancer and effects on cell growth have varied in published reports, perhaps due to the unknown signaling consequences of expression of the intracellular proteolytic ErbB4 s80 HER4 fragment or due to differing signaling capabilities of alternatively spliced ErbB4 isoforms. One isoform (Cyt1) contains a 16-residue intracellular sequence that is absent from the other (Cyt2). We expressed s80Cyt1 and s80 Cyt2 in HC11 mammary epithelial cells, finding diametrically opposed effects on the growth and organization of colonies in three-dimensional matrices. Whereas expression of s80Cyt1 decreased growth and increased the rate of three-dimensional lumen formation, that of s80Cyt2 increased proliferation without promoting lumen formation. These results were recapitulated in vivo, using doxycyclineinducible, mouse breast-transgenic expression of s80 Cyt1 amd s80 Cyt2 . Expression of s80 Cyt1 decreased growth of the mammary ductal epithelium, caused precocious STAT5a activation and lactogenic differentiation, and increased cell surface E-cadherin levels. Remarkably, ductal growth inhibition by s80Cyt1 occurred simultaneously with lobuloalveolar growth that was unimpeded by s80Cyt1 , suggesting that the response to ErbB4 may be influenced by the epithelial subtype. In contrast, expression of s80 Cyt2 caused epithelial hyperplasia, increased Wnt and nuclear ␤-catenin expression, and elevated expression of c-myc and cyclin D1 in the mammary epithelium. These results demonstrate that the Cyt1 and Cyt2 ErbB4 isoforms, differing by only 16 amino acids, exhibit markedly opposing effects on mammary epithelium growth and differentiation.Activation of the ErbB/HER/epidermal growth factor receptor (ErbB/HER/EGFR) family of receptor tyrosine kinases (RTKs) is often equated with proliferation and oncogenesis. The family consists of four members, HER1/ErbB1/EGFR, HER2/ErbB2/Neu, HER3/ErbB3, and HER4/ErbB4. Each protein has a large NH 2 -terminal extracellular domain, a transmembrane domain, and a large intracellular domain with a tyrosine-rich carboxy-terminal region and a tyrosine kinaselike sequence (12). HER1/ErbB1, HER2/ErbB2, and HER4/ ErbB4 exhibit ligand-inducible tyrosine kinase activity, whereas HER3/ErbB3 does not.ErbB receptor activation at the cell surface targets multiple cytoplasmic signaling cascades that transmit signals to the nucleus. In addition to this classic RTK signaling model, ErbB RTKs have exhibited nuclear localization (4,17,19,27,28,37,47,48). While ErbB1, -2, and -3 are transported to the nucleus as full-length receptors, one isoform of ErbB4 is cleaved upon ligand binding, liberating the entire soluble intracellular domain. This intracellular, kinase-active 80-kDa cleavage product (s80 HER4 ) exhibits nuclear-cytoplasmic shuttling (8,20,25,27 (14,21,44). Inhibition of ErbB4 cleavage or its nuclear localization impairs ErbB4-mediated STAT5a activation (25,46,48).Evidence suggests that, whereas ErbB1, -2, and -3 all contribute to proliferation and/or survi...