Role of hematopoietic stem cells in angiogenesis

RUN(X) out of blood: emerging RUNX1 functions beyond hematopoiesis and links to Down syndrome

BACKGROUND

RUNX1 is a transcription factor and a master regulator for the specification of the hematopoietic lineage during embryogenesis and postnatal megakaryopoiesis. Mutations and rearrangements on RUNX1 are key drivers of hematological malignancies. In humans, this gene is localized to the 'Down syndrome critical region' of chromosome 21, triplication of which is necessary and sufficient for most phenotypes that characterize Trisomy 21.

MAIN BODY

Individuals with Down syndrome show a higher predisposition to leukemias. Hence, RUNX1 overexpression was initially proposed as a critical player on Down syndrome-associated leukemogenesis. Less is known about the functions of RUNX1 in other tissues and organs, although growing reports show important implications in development or homeostasis of neural tissues, muscle, heart, bone, ovary, or the endothelium, among others. Even less is understood about the consequences on these tissues of RUNX1 gene dosage alterations in the context of Down syndrome. In this review, we summarize the current knowledge on RUNX1 activities outside blood/leukemia, while suggesting for the first time their potential relation to specific Trisomy 21 co-occurring conditions.

CONCLUSION

Our concise review on the emerging RUNX1 roles in different tissues outside the hematopoietic context provides a number of well-funded hypotheses that will open new research avenues toward a better understanding of RUNX1-mediated transcription in health and disease, contributing to novel potential diagnostic and therapeutic strategies for Down syndrome-associated conditions.

RUNX Family in Hypoxic Microenvironment and Angiogenesis in Cancers

The tumor microenvironment (TME) is broadly implicated in tumorigenesis, as tumor cells interact with surrounding cells to influence the development and progression of the tumor. Blood vessels are a major component of the TME and are attributed to the creation of a hypoxic microenvironment, which is a common feature of advanced cancers and inflamed premalignant tissues. Runt-related transcription factor (RUNX) proteins, a transcription factor family of developmental master regulators, are involved in vital cellular processes such as differentiation, proliferation, cell lineage specification, and apoptosis. Furthermore, the RUNX family is involved in the regulation of various oncogenic processes and signaling pathways as well as tumor suppressive functions, suggesting that the RUNX family plays a strategic role in tumorigenesis. In this review, we have discussed the relevant findings that describe the crosstalk of the RUNX family with the hypoxic TME and tumor angiogenesis or with their signaling molecules in cancer development and progression.

Roles of RUNX in Hypoxia-Induced Responses and Angiogenesis

During the past two decades, Runt domain transcription factors (RUNX1, 2, and 3) have been investigated in regard to their function, structural elements, genetic variants, and roles in normal development and pathological conditions. The Runt family proteins are evolutionarily conserved from Drosophila to mammals, emphasizing their physiological importance. A hypoxic microenvironment caused by insufficient blood supply is frequently observed in developing organs, growing tumors, and tissues that become ischemic due to impairment or blockage of blood vessels. During embryonic development and tumor growth, hypoxia triggers a stress response that overcomes low-oxygen conditions by increasing erythropoiesis and angiogenesis and triggering metabolic changes. This review briefly introduces hypoxic conditions and cellular responses, as well as angiogenesis and its related signaling pathways, and then describes our current knowledge on the functions and molecular mechanisms of Runx family proteins in hypoxic responses, especially in angiogenesis.

Inflammatory phase of bone healing initiates the regenerative healing cascade

Bone healing commences with an inflammatory reaction which initiates the regenerative healing process leading in the end to reconstitution of bone. An unbalanced immune reaction during this early bone healing phase is hypothesized to disturb the healing cascade in a way that delays bone healing and jeopardizes the successful healing outcome. The immune cell composition and expression pattern of angiogenic factors were investigated in a sheep bone osteotomy model and compared to a mechanically-induced impaired/delayed bone healing group. In the impaired/delayed healing group, significantly higher T cell percentages were present in the bone hematoma and the bone marrow adjacent to the osteotomy gap when compared to the normal healing group. This was mirrored in the higher cytotoxic T cell percentage detected under delayed bone healing conditions indicating longer pro-inflammatory processes. The highly activated periosteum adjourning the osteotomy gap showed lower expression of hematopoietic stem cell markers and angiogenic factors such as heme oxygenase and vascular endothelial growth factor. This indicates a deferred revascularization of the injured area due to ongoing pro-inflammatory processes in the delayed healing group. Results from this study suggest that there are unfavorable immune cells and factors participating in the initial healing phase. In conclusion, identifying beneficial aspects may lead to promising therapeutical approaches that might benefit further by eliminating the unfavorable factors.

A 5' untranslated region containing the IRES element in the Runx1 gene is required for angiogenesis, hematopoiesis and leukemogenesis in a knock-in mouse model

Although internal ribosome entry site (IRES)-mediated translation is considered important for proper cellular function, its precise biological role is not fully understood. Runx1 gene, which encodes a transcription factor implicated in hematopoiesis, angiogenesis, and leukemogenesis, contains IRES sequences in the 5' untranslated region. To clarify the roles of the IRES element in Runx1 function, we generated knock-in mice for either wild-type Runx1 or Runx1/Evi1, a Runx1 fusion protein identified in human leukemia. In both cases, native promoter-dependent transcription was retained, whereas IRES-mediated translation was eliminated. Interestingly, homozygotes expressing wild-type Runx1 deleted for the IRES element (Runx1(Delta IRES/Delta IRES)) died in utero with prominent dilatation of peripheral blood vessels due to impaired pericyte development. In addition, hematopoietic cells in the Runx1(Delta IRES/Delta IRES) fetal liver were significantly decreased, and exhibited an altered differentiation pattern, a reduced proliferative activity, and an impaired reconstitution ability. On the other hand, heterozygotes expressing Runx1/Evi1 deleted for the IRES element (Runx1(+/RE Delta IRES)) were born normally and did not show any hematological abnormalities, in contrast that conventional Runx1/Evi1 heterozygotes die in utero with central nervous system hemorrhage and Runx1/Evi1 chimeric mice develop acute leukemia. The findings reported here demonstrate the essential roles of the IRES element in Runx1 function under physiological and pathological conditions.

Genetic risk factors for hepatopulmonary syndrome in patients with advanced liver disease

BACKGROUND & AIMS

Hepatopulmonary syndrome (HPS) affects 10%-30% of patients with cirrhosis and portal hypertension and significantly increases mortality. Studies in experimental models indicate that pulmonary angiogenesis contributes to the development of HPS, but pathogenesis in humans is poorly understood. We investigated genetic risk factors for HPS in patients with advanced liver disease.

METHODS

We performed a multicenter case-control study of patients with cirrhosis being evaluated for liver transplantation. Cases had an alveolar-arterial oxygen gradient > or = 15 mm Hg (or > or =20 mm Hg if age > 64 years) and contrast echocardiography with late appearance of microbubbles after venous injection of agitated saline (intrapulmonary vasodilatation); controls did not meet both criteria for case status. The study sample included 59 cases and 126 controls. We genotyped 1086 common single nucleotide polymorphisms (SNPs) in 94 candidate genes.

RESULTS

Forty-two SNPs in 21 genes were significantly associated with HPS after adjustments for race and smoking. Eight genes had at least 2 SNPs associated with disease: CAV3, ENG, NOX4, ESR2, VWF, RUNX1, COL18A1, and TIE1. For example, rs237872 in CAV3 showed an odds ratio of 2.75 (95% confidence interval: 1.65-4.60, P = .0001) and rs4837192 in ENG showed an odds ratio of 0.35 (95% confidence interval: 0.14-0.89, P = .027). Furthermore, variation in CAV3 and RUNX1 was associated with HPS in gene-based analyses.

CONCLUSIONS

Polymorphisms in genes involved in the regulation of angiogenesis are associated with the risk of HPS. Further investigation of these biologic pathways might elucidate the mechanisms that mediate the development of HPS in certain patients with severe liver disease.

Regulation of lymphatic-blood vessel separation by endothelial Rac1

Sprouting angiogenesis and lymphatic-blood vessel segregation both involve the migration of endothelial cells, but the precise migratory molecules that govern the decision of blood vascular endothelial cells to segregate into lymphatic vasculature are unknown. Here, we deleted endothelial Rac1 in mice (Tie1-Cre(+);Rac1(fl/fl)) and revealed, unexpectedly, that whereas blood vessel morphology appeared normal, lymphatic-blood vessel separation was impaired, with corresponding edema, haemorrhage and embryonic lethality. Importantly, normal levels of Rac1 were essential for directed endothelial cell migratory responses to lymphatic-inductive signals. Our studies identify Rac1 as a crucial part of the migratory machinery required for endothelial cells to separate and form lymphatic vasculature.

Long-term hematopoietic reconstitution by transplantation of kidney hematopoietic stem cells in lethally irradiated clonal ginbuna crucian carp (Carassius auratus langsdorfii)

The study of hematopoietic stem cells (HSCs) has been facilitated by the transplantation of bone marrow cell populations into lethally irradiated mice. It is widely known that HSCs have the capacity for long-term and multilineage hematopoietic reconstitution in lethally irradiated hosts. Here, we developed a transplantation model system using clonal ginbuna crucian carp (Carassius auratus langsdorfii) that were exposed to a lethal dose of X-rays. The minimum lethal dose (MLD) of ginbuna was approximately 25Gy, which is lethal due to hematopoietic failure. The transplantation of kidney hematopoietic cells into lethally irradiated ginbuna resulted in the rescue of recipient fish for more than 180 days. We examined the reconstitution activity of head kidney (HK), body kidney (BK), and spleen cells. Transplantation experiments showed that only HK and BK cells had the long-term and multilineage reconstitution activity. These results indicate that teleost HSCs have the ability to fully reconstitute the hematopoietic system in lethally irradiated hosts, and that they are present in HK and BK, but not in spleen. This transplantation model system using clonal ginbuna is useful for studies of in vivo kinetics and functions of HSCs in teleosts.

Not just angiogenesis--wider roles for the angiopoietins

Since the discovery of the angiopoietins, much interest has been focused on their biological actions and their potential use as therapeutic targets. It is generally accepted that the angiopoietins play an important role in angiogenesis and hence are described as angiogenic factors. However, it is becoming increasingly clear that this is not their only role and it is likely that the angiopoietins have important roles in a wider range of biological and pathological functions.