The function of the epidermal growth factor receptor (EGFR) family member HER4 remains unclear because its activating ligand, heregulin, results in either proliferation or differentiation. This variable response may stem from the range of signals generated by HER4 homodimers versus heterodimeric complexes with other EGFR family members. The ratio of homo- and heterodimeric complexes may be influenced both by a cell9s EGFR family member expression profile and by the ligand or even ligand isoform used. To define the role of HER4 in mediating antiproliferative and differentiation responses, human breast cancer cell lines were screened for responses to heregulin. Only cells that expressed HER4 exhibited heregulin-dependent antiproliferative responses. In-depth studies of one line, SUM44, demonstrated that the antiproliferative and differentiation responses correlated with HER4 activation and were abolished by stable expression of a kinase-inactive HER4. HB-EGF, a HER4-specific ligand in this EGFR-negative cell line, also induced an antiproliferative response. Moreover, introduction and stable expression of HER4 in HER4-negative SUM102 cells resulted in the acquisition of a heregulin-dependent antiproliferative response, associated with increases in markers of differentiation. The role of HER2 in these heregulin-dependent responses was examined through elimination of cell surface HER2 signaling by stable expression of a single-chain anti-HER2 antibody that sequestered HER2 in the endoplasmic reticulum. In the cell lines with either endogenously (SUM44) or exogenously (SUM102) expressed HER4, elimination of HER2 did not alter HER4-dependent decreases in cell growth. These results suggest that HER4 is both necessary and sufficient to trigger an antiproliferative response in human breast cancer cells.

Mer is a member of theAxl/Mer/Tyro3 receptor tyrosine kinase family, a family whose physiological function is not well defined. We constructed a Mer chimera using the epidermal growth factor receptor (EGFR) extracellular and transmembrane domains and the Mer cytoplasmic domain. Stable transfection of the Mer chimera into interleukin 3 (IL-3)-dependent murine 32D cells resulted in ligand-activable surface receptor that tyrosine autophosphorylated, stimulated intracellular signaling, and dramatically reduced apoptosis initiated by IL-3 withdrawal. However, unlike multiple other ectopically expressed receptor tyrosine kinases including full-length EGFR or an EGFR/Axl chimera, the Mer chimera did not stimulate proliferation. Moreover, and in contrast to EGFR,Mer chimera activation induced adherence and cell flattening in the normally suspension-growing 32D cells. TheMer chimera signal also blocked IL-3-dependent proliferation leading to G₁/S arrest, dephosphorylation of retinoblastoma protein, and elongation of cellular processes. Unlike other agonists that lead to a slow (4–8 days) ligand-dependent differentiation of 32D cells, the combinedMer and IL-3 signal resulted in differentiated morphology and growth cessation in the first 24 h. Thus the Merchimera blocks apoptosis without stimulating growth and produces cytoskeletal alterations; this outcome is clearly separable from the proliferative signal produced by most receptor tyrosine kinases.

The ERH gene encodes a highly conserved small nuclear protein with a unique amino acid sequence and three-dimensional structure but unknown function. The gene is present in animals, plants, and protists but to date has only been found in few fungi. Here we report that ERH homologs are also present in all four species from the genus Schizosaccharomyces, S. pombe, S. octosporus, S. cryophilus, and S. japonicus, which, however, are an exception in this respect among Ascomycota and Basidiomycota. The ERH protein sequence is moderately conserved within the genus (58% identity between S. pombe and S. japonicus), but the intron-rich genes have almost identical intron-exon organizations in all four species. In S. pombe, erh1⁺ is expressed at a roughly constant level during vegetative growth and adaptation to unfavorable conditions such as nutrient limitation and hyperosmotic stress caused by sorbitol. Erh1p localizes preferentially to the nucleus with the exception of the nucleolus, but is also present in the cytoplasm. Cells lacking erh1⁺ have an aberrant cell morphology and a comma-like shape when cultured to the stationary phase, and exhibit a delayed recovery from this phase followed by slower growth. Loss of erh1⁺ in an auxotrophic background results in enhanced arrest in the G1 phase following nutritional stress, and also leads to hypersensitivity to agents inducing hyperosmotic stress (sorbitol), inhibiting DNA replication (hydroxyurea), and destabilizing the plasma membrane (SDS); this hypersensitivity can be abolished by expression of S. pombe erh1⁺ and, to a lesser extent, S. japonicus erh1⁺ or human ERH. Erh1p fails to interact with the human Ciz1 and PDIP46/SKAR proteins, known molecular partners of human ERH. Our data suggest that in Schizosaccharomyces sp. erh1⁺ is non-essential for normal growth and Erh1p could play a role in response to adverse environmental conditions and in cell cycle regulation.

Chronic and acute myeloid leukemia evade immune system surveillance and induce immunosuppression by expanding proleukemic Foxp3+ regulatory T cells (Tregs). High levels of immunosuppressive Tregs predict inferior response to chemotherapy, leukemia relapse, and shorter survival. However, mechanisms that promote Tregs in myeloid leukemias remain largely unexplored. Here, we identify leukemic extracellular vesicles (EVs) as drivers of effector proleukemic Tregs. Using mouse model of leukemia-like disease, we found that Rab27a-dependent secretion of leukemic EVs promoted leukemia engraftment, which was associated with higher abundance of activated, immunosuppressive Tregs. Leukemic EVs attenuated mTOR-S6 and activated STAT5 signaling, as well as evoked significant transcriptomic changes in Tregs. We further identified specific effector signature of Tregs promoted by leukemic EVs. Leukemic EVs-driven Tregs were characterized by elevated expression of effector/tumor Treg markers CD39, CCR8, CD30, TNFR2, CCR4, TIGIT, and IL21R and included 2 distinct effector Treg (eTreg) subsets: CD30+CCR8hiTNFR2hi eTreg1 and CD39+TIGIThi eTreg2. Finally, we showed that costimulatory ligand 4-1BBL/CD137L, shuttled by leukemic EVs, promoted suppressive activity and effector phenotype of Tregs by regulating expression of receptors such as CD30 and TNFR2. Collectively, our work highlights the role of leukemic extracellular vesicles in stimulation of immunosuppressive Tregs and leukemia growth. We postulate that targeting of Rab27a-dependent secretion of leukemic EVs may be a viable therapeutic approach in myeloid neoplasms.

Both chronic myeloid leukemia and acute myeloid leukemia evade the immune response during their development and disease progression. As myeloid leukemia cells modify their bone marrow microenvironment, they lead to dysfunction of cytotoxic cells, such as CD8+ T cells or NK cells, simultaneously promoting development of immunosuppressive regulatory T cells and suppressive myeloid cells. This facilitates disease progression, spreading of leukemic blasts outside the bone marrow niche and therapy resistance. The following review focuses on main immunosuppressive features of myeloid leukemias. Firstly, factors derived directly from leukemic cells – inhibitory receptors, soluble factors and extracellular vesicles, are described. Further, we outline function, properties and origin of main immunosuppressive cells - regulatory T cells, myeloid derived suppressor cells and macrophages. Finally, we analyze interplay between recovery of effector immunity and therapeutic modalities, such as tyrosine kinase inhibitors and chemotherapy.