Qun Lin

A role of miR‐27 in the regulation of adipogenesis

MicroRNAs (miRNAs) are involved in a plethora of important biological processes, from embryonic development to homeostasis in adult tissues. Recently, miRNAs have emerged as a class of epigenetic regulators of metabolism and energy homeostasis. We have investigated the role of miRNAs in the regulation of adipogenic differentiation. In this article, we demonstrate that the miR‐27 gene family is downregulated during adipogenic differentiation. Overexpression of miR‐27 specifically inhibited adipocyte formation, without affecting myogenic differentiation. We also found that expression of miR‐27 resulted in blockade of expression of PPARγ and C/EBPα, the two master regulators of adipogenesis. Importantly, expression of miR‐27 was increased in fat tissue of obese mice and was regulated by hypoxia, an important extracellular stress associated with obesity. Our data strongly suggest that miR‐27 represents a new class of adipogenic inhibitors and may play a role in the pathological development of obesity.

Differentiation Arrest by Hypoxia

The stem cell niche is a unique tissue microenvironment that regulates the self-renewal and differentiation of stem cells. Although several stromal cells and molecular pathways have been identified, the microenvironment of the stem cell niche remains largely unclear. Recent evidence suggests that stem cells are localized in areas with low oxygen. We have hypothesized that hypoxia maintains the undifferentiated phenotype of stem/precursor cells. In this report, we demonstrate that hypoxia reversibly arrests preadipocytes in an undifferentiated state. Consistent with this observation, hypoxia maintains the expression of pref-1, a key stem/precursor cell gene that negatively regulates adipogenic differentiation. We further demonstrate that the hypoxia-inducible factor-1 (HIF-1) constitutes an important mechanism for the inhibition of adipogenic differentiation by hypoxia. Our findings suggest that hypoxia in the stem cell niche is critical for the maintenance of the undifferentiated stem or precursor cell phenotype.

Adaptive Myogenesis under Hypoxia

ABSTRACT Previous studies have indicated that myoblasts can differentiate and repair muscle injury after an ischemic insult. However, it is unclear how hypoxia or glucose deprivation in the ischemic microenvironment affects myoblast differentiation. We have found that myogenesis can adapt to hypoxic conditions. This adaptive mechanism is accompanied by initial inhibition of the myoD, E2A, and myogenin genes followed by resumption of their expression in an oxygen-dependent manner. The regulation of myoD transcription by hypoxia is correlated with transient deacetylation of histones associated with the myoD promoter. It is noteworthy that, unlike the differentiation of other cell types such as preadipocytes or chondroblasts, the effect of hypoxia on myogenesis is independent of HIF-1, a ubiquitous regulator of transcription under hypoxia. While myogenesis can also adapt to glucose deprivation, the combination of severe hypoxia and glucose deprivation found in an ischemic environment results in pronounced loss of myoblasts. Our studies indicate that the ischemic muscle can be repaired via the adaptive differentiation of myogenic precursors, which depends on the levels of oxygen and glucose in the ischemic microenvironment.

Hypoxic Tumor Microenvironment and Cancer Cell Differentiation

Hypoxia or oxygen deficiency is a salient feature of solid tumors. Hypoxic tumors are often resistant to conventional cancer therapies, and tumor hypoxia correlates with advanced stages of malignancy. Hypoxic tumors appear to be poorly differentiated. Increasing evidence suggests that hypoxia has the potential to inhibit tumor cell differentiation and thus plays a direct role in the maintenance of cancer stem cells. Studies have also shown that hypoxia blocks differentiation of mesenchymal stem/progenitor cells, a potential source of tumor-associated stromal cells. It is therefore likely that hypoxia may have a profound impact on the evolution of the tumor stromal microenvironment. These observations have led to the emergence of a novel paradigm for a role of hypoxia in facilitating tumor progression. Hypoxia may help create a microenvironment enriched in poorly differentiated tumor cells and undifferentiated stromal cells. Such an undifferentiated hypoxic microenvironment may provide essential cellular interactions and environmental signals for the preferential maintenance of cancer stem cells. This hypothesis suggests that effectively targeting hypoxic cancer stem cells is a key to successful tumor control.

Hypoxia-Regulated Delta-like 1 Homologue Enhances Cancer Cell Stemness and Tumorigenicity

Reduced oxygenation, or hypoxia, inhibits differentiation and facilitates stem cell maintenance. Hypoxia commonly occurs in solid tumors and promotes malignant progression. Hypoxic tumors are aggressive and exhibit stem cell-like characteristics. It remains unclear, however, whether and how hypoxia regulates cancer cell differentiation and maintains cancer cell stemness. Here, we show that hypoxia increases the expression of the stem cell gene DLK1, or delta-like 1 homologue (Drosophila), in neuronal tumor cells. Inhibition of DLK1 enhances spontaneous differentiation, decreases clonogenicity, and reduces in vivo tumor growth. Overexpression of DLK1 inhibits differentiation and enhances tumorigenic potentials. We further show that the DLK1 cytoplasmic domain, especially Tyrosine339 and Serine355, is required for maintaining both clonogenicity and tumorigenicity. Because elevated DLK1 expression is found in many tumor types, our observations suggest that hypoxia and DLK1 may constitute an important stem cell pathway for the regulation of cancer stem cell-like functionality and tumorigenicity.

Impact of the hypoxic tumor microenvironment on the regulation of cancer stem cell characteristics.

Solid tumors often contain regions with insufficient oxygen delivery, a condition called hypoxia. Tumor hypoxia is an independent prognostic factor significantly correlated with advanced stages of malignancy, increased resistance to conventional therapy, and reduced disease-free survival. Hypoxic tumor cells exhibit poorly differentiated phenotypes resembling stem or progenitor cells. Studies have shown that hypoxia can inhibit tumor cell differentiation and promote maintenance of cancer stem cells. In addition, hypoxia also blocks the differentiation of mesenchymal stem/progenitor cells, a potential source of tumor-associated stromal cells. Therefore, hypoxia may play a critical role during the evolution of the tumor stromal microenvironment and formation of the putative cancer stem cell niches. Conceptually, hypoxia may help create a microenvironment enriched both in poorly differentiated tumor cells and in undifferentiated stromal cells. Such an undifferentiated hypoxic microenvironment may provide essential cellular interactions and environmental signals for the preferential maintenance of cancer stem cells. This review will discuss the hypoxia-regulated stem cell pathways and their roles in the maintenance of cancer stem cell functions.

Differential Hypoxic Regulation of Hypoxia-Inducible Factors 1α and 2α

The hypoxia-inducible transcription factors (HIF) 1α and HIF-2α play a critical role in cellular response to hypoxia. Elevated HIF-α expression correlates with poor patient survival in a large number of cancers. Recent evidence suggests that HIF-2α appears to be preferentially expressed in neuronal tumor cells that exhibit cancer stem cell characteristics. These observations suggest that expression of HIF-1α and HIF-2α is differentially regulated in the hypoxic tumor microenvironment. However, the underlying mechanisms remain to be fully investigated. In this study, we investigated the transcriptional regulation of HIF-1α and HIF-2α under different physiologically relevant hypoxic conditions. We found that transcription of HIF-2α was consistently increased by hypoxia, whereas transcription of HIF-1α showed variable levels of repression. Mechanistically, differential regulation of HIF-α transcription involved hypoxia-induced changes in acetylation of core histones H3 and H4 associated with the proximal promoters of the HIF-1α or HIF-2α gene. We also found that, although highly stable under acute hypoxia, HIF-1α and HIF-2α proteins become destabilized under chronic hypoxia. Our results have thus provided new mechanistic insights into the differential regulation of HIF-1α and HIF-2α by the hypoxic tumor microenvironment. These findings also suggest an important role of HIF-2α in the regulation of tumor progression under chronic hypoxia. Mol Cancer Res; 9(6); 757–65. ©2011 AACR.

Oxygen and Cell Fate Decisions.

Molecular oxygen has been known to play a critical role in a wide range of biological processes including glycolysis, mitochondrial respiration, angiogenesis, pulmonary functions, and cardiovascular activities. An emerging theme has developed in recent years that oxygen has significant impact on embryonic development, maintenance of stem cells, and cellular differentiation or cell fate decisions. Among the notable observations, early embryonic development takes place in a hypoxic microenvironment. Hematopoietic stem cells appear to be located in hypoxic regions within the bone marrow. Majority of the current observations have shown that hypoxia seems to prevent cellular differentiation and to maintain pluripotency of stem/progenitor cells. Genetic studies have demonstrated a critical role of hypoxia-inducible factors 1α and 2α in embryonic development. These intriguing observations demonstrate an important role of molecular oxygen in such fundamental biological processes as stem cell maintenance and regulation of cell fate decisions. Herein, we describe some of the latest advances in the biology of molecular oxygen and provide our perspectives on the potential impact of these interesting findings.

DLK1, delta-like 1 homolog (Drosophila), regulates tumor cell differentiation in vivo

The stem cell-like characteristics of tumor cells are not only essential for tumor development and malignant progression, but also significantly contribute to therapy resistance. However, it remains poorly understood how cancer cell differentiation or stemness is regulated in vivo. We investigated the role of the stem cell gene DLK1, or delta-like 1 homolog (Drosophila), in the regulation of cancer cell differentiation in vivo using neuroblastoma (NB) xenografts as a model. We found that loss-of-function mutants of DLK1 significantly enhanced NB cell differentiation in vivo likely by increasing the basal phosphorylation of MEK and ERK kinases, a mechanism that has been shown to facilitate neuronal differentiation. We also found that DLK1(+) cells are preferentially located in hypoxic regions. These results clearly demonstrate that DLK1 plays an important role in the maintenance of undifferentiated, stem cell-like phenotypes of NB cells in vivo.

Cellular and Molecular Mechanisms Underlying Oxygen-Dependent Radiosensitivity

Molecular oxygen has long been recognized as a powerful radiosensitizer that enhances the cell-killing efficiency of ionizing radiation. Radiosensitization by oxygen occurs at very low concentrations with the half-maximum radiosensitization at approximately 3 mmHg. However, robust hypoxia-induced signal transduction can be induced at <15 mmHg and can elicit a wide range of cellular responses that will affect therapy response as well as malignant progression. Great strides have been made, especially since the 1990s, toward identification and characterization of the oxygen-regulated molecular pathways that affect tumor response to ionizing radiation. In this review, we will discuss the current advances in our understanding of oxygen-dependent molecular modification and cellular signal transduction and their impact on tumor response to therapy. We will specifically address mechanistic distinctions between radiobiological hypoxia (0–3 mmHg) and pathological hypoxia (3–15 mmHg). We also propose a paradigm that hypoxia increases radioresistance by maintaining the cancer stem cell phenotype.