STAT5 acts as a repressor to regulate early embryonic erythropoiesis

Age-associated myeloid malignancies - the role of STAT3 and STAT5 in myelodysplastic syndrome and acute myeloid leukemia

In the last few decades, the increasing human life expectancy has led to the inflation of the elderly population and consequently the escalation of age-related disorders. Biological aging has been associated with the accumulation of somatic mutations in the Hematopoietic Stem Cell (HSC) compartment, providing a fitness advantage to the HSCs leading to clonal hematopoiesis, that includes non-malignant and malignant conditions (i.e. Clonal Hematopoiesis of Indeterminate Potential, Myelodysplastic Syndrome and Acute Myeloid Leukemia). The Janus Kinase-Signal Transducer and Activator of Transcription (JAK-STAT) pathway is a key player in both normal and malignant hematopoiesis. STATs, particularly STAT3 and STAT5, are greatly implicated in normal hematopoiesis, immunity, inflammation, leukemia, and aging. Here, the pleiotropic functions of JAK-STAT pathway in age-associated hematopoietic defects and of STAT3 and STAT5 in normal hematopoiesis, leukemia, and inflammaging are reviewed. Even though great progress has been made in deciphering the role of STATs, further research is required to provide a deeper understanding of the molecular mechanisms of leukemogenesis, as well as novel biomarkers and therapeutic targets for improved management of age-related disorders.

STAT proteins: a kaleidoscope of canonical and non-canonical functions in immunity and cancer

STAT proteins represent an important family of evolutionarily conserved transcription factors that play key roles in diverse biological processes, notably including blood and immune cell development and function. Classically, STAT proteins have been viewed as inducible activators of transcription that mediate cellular responses to extracellular signals, particularly cytokines. In this 'canonical' paradigm, latent STAT proteins become tyrosine phosphorylated following receptor activation, typically via downstream JAK proteins, facilitating their dimerization and translocation into the nucleus where they bind to specific sequences in the regulatory region of target genes to activate transcription. However, growing evidence has challenged this paradigm and identified alternate 'non-canonical' functions, such as transcriptional repression and roles outside the nucleus, with both phosphorylated and unphosphorylated STATs involved. This review provides a revised framework for understanding the diverse kaleidoscope of STAT protein functional modalities. It further discusses the implications of this framework for our understanding of STAT proteins in normal blood and immune cell biology and diseases such as cancer, and also provides an evolutionary context to place the origins of these alternative functional modalities.

Developmental transition to bilaterally symmetric cell divisions is regulated by Pax-mediated transcription in embryos of the leech Helobdella austinensis

The leech embryo develops by spiral cleavage, and establishes the symmetry properties of its adult body plan through the bilaterally symmetric divisions of mesodermal proteloblast DM″ and ectodermal proteloblast DNOPQ‴. We here show that transcriptional inhibitors α-amanitin and actinomycin D specifically disrupt the symmetry and orientation of these two proteloblast cell divisions while having no apparent effect on the timing or geometry of other divisions. Transcriptional inhibition had a similar effect on both proteloblasts, i.e. cytokinesis was highly asymmetric and the cleavage plane roughly orthogonal to that seen during normal development. These findings suggest that zygotic gene product(s) are required, either directly or indirectly, for the correct placement of the proteloblast cleavage furrow. The same phenotypes were also observed following in vivo expression of dominant-negative Pax gene constructs. These dominant-negative phenotypes depended on protein/DNA interaction, and could be rescued by coexpression of full length Pax proteins. However, symmetric cleavage of the mesodermal proteloblast was rescued by full length constructs of either Hau-Paxβ1 or Hau-Pax2/5/8, while only Hau-Paxβ1 rescued the symmetry of ectodermal cleavage. We conclude that both proteloblasts need Pax-mediated transcription to adopt their normally symmetric cleavage patterns, but differ in terms of the specific Pax proteins required. The implication of these findings for the evolution of spiral cleavage is discussed.

Protein phosphatase 2A catalytic subunit α (PP2Acα) maintains survival of committed erythroid cells in fetal liver erythropoiesis through the STAT5 pathway

Suppression of programmed cell death is critical for the final maturation of red blood cells and depends largely on the anti-apoptotic effects of EpoR-STAT5-Bcl-x(L) signaling. As the major eukaryotic serine/threonine phosphatase, protein phosphatase 2A (PP2A) regulates multiple cellular processes, including apoptosis. However, whether PP2A plays a role in preventing erythroid cells from undergoing apoptosis remains to be elucidated. We conditionally inactivated the catalytic subunit α of PP2A (PP2Acα), which is the predominant form of PP2Ac, during early embryonic hematopoiesis. Loss of PP2Acα in hematopoietic cells perturbed definitive erythropoiesis characterized by fetal liver atrophy, reduced Ter119(+) cell number, abnormal expression patterns of molecular markers, less colony formation, and a reduction in definitive globin expression. Levels of erythropoiesis-promoting cytokines and initial seeding with hematopoietic progenitors remained unchanged in PP2Acα(TKO) fetal livers. We noted impaired expansion of the fetal erythroid compartment, which was associated with increased apoptosis of committed erythroid cells. Mechanistically, PP2Acα depletion markedly reduced Tyr(694) phosphorylation of STAT5 and expression of Bcl-x(L). Unexpectedly, PP2Acα-deficient embryos did not manifest any early embryonic vascular defects. Collectively, these data provide direct loss-of-function evidence demonstrating the importance of PP2Acα for the survival of committed erythroid cells during fetal liver erythropoiesis.

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.

FGF signalling as a mediator of lineage transitions--evidence from embryonic stem cell differentiation

The fibroblast growth factor (FGF) signalling pathway is one of the most ubiquitous in biology. It has diverse roles in development, differentiation and cancer. Embryonic stem (ES) cells are in vitro cell lines capable of differentiating into all the lineages of the conceptus. As such they have the capacity to differentiate into derivatives of all three germ layers and to some extent the extra-embryonic lineages as well. Given the prominent role of FGF signalling in early embryonic development, we explore the role of this pathway in early ES cell differentiation towards the major lineages of the embryo. As early embryonic differentiation is intricately choreographed at the level of morphogenetic movement, adherent ES cell culture affords a unique opportunity to study the basic steps in early lineage specification in the absence of ever shifting complex in vivo microenvironments. Thus recent experiments in ES cell differentiation are able to pinpoint specific FGF dependent lineage transitions that are difficult to resolve in vivo. Here we review the role of FGF signalling in early development alongside the ES cell data and suggest that FGF dependent signalling via phospho-Erk activation maybe a major mediator of transitions in lineage specification.

EPOR signaling: 450 million years' history

Erythropoietin and the erythropoietin receptor have been cloned and studied in a lower vertebrate species. Key aspects of their structure and function have been evolutionarily conserved.