CD47 differentially regulates white and brown fat function

Cell autonomous functions of CD47 in regulating cellular plasticity and metabolic plasticity

CD47 is a ubiquitously expressed cell surface receptor, which is widely known for preventing macrophage-mediated phagocytosis by interacting with signal regulatory protein α (SIRPα) on the surface of macrophages. In addition to its role in phagocytosis, emerging studies have reported numerous noncanonical functions of CD47 that include regulation of various cellular processes such as proliferation, migration, apoptosis, differentiation, stress responses, and metabolism. Despite lacking an extensive cytoplasmic signaling domain, CD47 binds to several cytoplasmic proteins, particularly upon engaging with its secreted matricellular ligand, thrombospondin 1. Indeed, the regulatory functions of CD47 are greatly influenced by its interacting partners. These interactions are often cell- and context-specific, adding a further level of complexity. This review addresses the downstream cell-intrinsic signaling pathways regulated by CD47 in various cell types and environments. Some of the key pathways modulated by this receptor include the PI3K/AKT, MAPK/ERK, and nitric oxide signaling pathways, as well as those implicated in glucose, lipid, and mitochondrial metabolism. These pathways play vital roles in maintaining tissue homeostasis, highlighting the importance of understanding the phagocytosis-independent functions of CD47. Given that CD47 expression is dysregulated in a variety of cancers, improving our understanding of the cell-intrinsic signals regulated by this molecule will help advance the development of CD47-targeted therapies.

Differential regulation by CD47 and thrombospondin-1 of extramedullary erythropoiesis in mouse spleen

Extramedullary erythropoiesis is not expected in healthy adult mice, but erythropoietic gene expression was elevated in lineage-depleted spleen cells from Cd47-/- mice. Expression of several genes associated with early stages of erythropoiesis was elevated in mice lacking CD47 or its signaling ligand thrombospondin-1, consistent with previous evidence that this signaling pathway inhibits expression of multipotent stem cell transcription factors in spleen. In contrast, cells expressing markers of committed erythroid progenitors were more abundant in Cd47-/- spleens but significantly depleted in Thbs1-/- spleens. Single-cell transcriptome and flow cytometry analyses indicated that loss of CD47 is associated with accumulation and increased proliferation in spleen of Ter119-CD34+ progenitors and Ter119+CD34- committed erythroid progenitors with elevated mRNA expression of Kit, Ermap, and Tfrc. Induction of committed erythroid precursors is consistent with the known function of CD47 to limit the phagocytic removal of aged erythrocytes. Conversely, loss of thrombospondin-1 delays the turnover of aged red blood cells, which may account for the suppression of committed erythroid precursors in Thbs1-/- spleens relative to basal levels in wild-type mice. In addition to defining a role for CD47 to limit extramedullary erythropoiesis, these studies reveal a thrombospondin-1-dependent basal level of extramedullary erythropoiesis in adult mouse spleen.

Sex differences in the effects of brown adipocyte CD47 deficiency on age-related weight change and glucose homeostasis

Our previous studies demonstrated that mice with global CD47 deficiency are lean and resistant to diet or aging-associated obesity and metabolic complications. This protective effect is partially through modulating brown fat function. To definitively determine the role of brown fat CD47 in age-related metabolic homeostasis, inducible brown adipocyte-specific cd47 deficient mice were generated by crossbreeding cd47 floxed mice with UCP1-CreERT2 mice and characterized in this study. Efficient knockdown of CD47 in brown fat was achieved in both male and female mice through tamoxifen administration. Intriguingly, our findings indicated that male mice lacking CD47 in brown fat displayed a notable reduction in body weight starting at 23 weeks of age when housed at a temperature of 22 °C, in comparison to control mice. This reduction in weight was accompanied by improved glucose tolerance. Remarkably, this phenotype persisted even when the male mice were housed under thermoneutral conditions (30 °C). Conversely, female knockout mice did not exhibit significant changes in weight throughout the study. In addition to the enhanced glucose homeostasis, brown fat CD47 deficiency in male mice also prevented age-related hypertriglyceridemia and non-alcoholic fatty liver disease. Furthermore, the brown fat tissue of male knockout mice exhibited reduced whitening, while maintaining comparable levels of thermogenic markers. This suggests the involvement of a thermogenesis-independent mechanism. Altogether, these findings highlight a sex difference in the impact of brown adipocyte CD47 deficiency on age-related weight changes and glucose homeostasis.

Physiological Approaches Targeting Cellular and Mitochondrial Pathways Underlying Adipose Organ Senescence

The adipose organ is involved in many metabolic functions, ranging from the production of endocrine factors to the regulation of thermogenic processes. Aging is a natural process that affects the physiology of the adipose organ, leading to metabolic disorders, thus strongly impacting healthy aging. Cellular senescence modifies many functional aspects of adipose tissue, leading to metabolic alterations through defective adipogenesis, inflammation, and aberrant adipocytokine production, and in turn, it triggers systemic inflammation and senescence, as well as insulin resistance in metabolically active tissues, leading to premature declined physiological features. In the various aging fat depots, senescence involves a multiplicity of cell types, including mature adipocytes and immune, endothelial, and progenitor cells that are aging, highlighting their involvement in the loss of metabolic flexibility, one of the common features of aging-related metabolic disorders. Since mitochondrial stress represents a key trigger of cellular senescence, and senescence leads to the accumulation of abnormal mitochondria with impaired dynamics and hindered homeostasis, this review focuses on the beneficial potential of targeting mitochondria, so that strategies can be developed to manage adipose tissue senescence for the treatment of age-related metabolic disorders.

CD47 antisense oligonucleotide treatment attenuates obesity and its-associated metabolic dysfunction

Previous study from our lab has revealed a new role of CD47 in regulating adipose tissue function, energy homeostasis and the development of obesity and metabolic disease in CD47 deficient mice. In this study, the therapeutic potential of an antisense oligonucleotide (ASO) targeting to CD47 in obesity and its-associated complications was determined in two obese mouse models (diet induced and genetic models). In diet induced obesity, male C57BL6 mice were fed with high fat (HF) diet to induce obesity and then treated with CD47ASO or control ASO for 8 weeks. In genetic obese mouse model, male six-week old ob/ob mice were treated with ASOs for 9 weeks. We found that CD47ASO treatment reduced HF diet-induced weight gain, decreased fat mass, prevented dyslipidemia, and improved glucose tolerance. These changes were accompanied by reduced inflammation in white adipose tissue and decreased hepatic steatosis. This protection was also seen in CD47ASO treated ob/ob mice. Mechanistically, CD47ASO treatment increased mice physical activity and energy expenditure, contributing to weight loss and improved metabolic outcomes in obese mice. Collectively, these findings suggest that CD47ASO might serve as a new treatment option for obesity and its-associated metabolic complications.

Advances in Anti-metabolic Disease Treatments Targeting CD47

Metabolic disorders include a cluster of conditions that result from hyperglycemia, hyperlipidemia, insulin resistance, obesity, and hepatic steatosis, which cause the dysfunction of immune cells and innate cells, such as macrophages, natural killer cells, vascular endothelial cells, hepatocytes, and human kidney tubular epithelial cells. Besides targeting the derangements in lipid metabolism, therapeutic modulations to regulate abnormal responses in the immune system and innate cell dysfunctions may prove to be promising strategies in the management of metabolic diseases. In recent years, several targets have been explored for the CD47 molecule (CD47), a glycosylated protein, which was originally reported to transmit an anti-phagocytic signal known as "don't eat me" in the atherosclerotic environment, hindering the efferocytosis of immune cells and promoting arterial plaque accumulation. Subsequently, the role of CD47 has been explored in obesity, fatty liver, and lipotoxic nephropathy, and its utility as a therapeutic target has been investigated using anti-CD47 antibodies or inhibitors of the THBS1/CD47 axis and the CD47/SIRPα signaling pathway. This review summarizes the mechanisms of action of CD47 in different cell types during metabolic diseases and the clinical research progress to date, providing a reference for the comprehensive targeting of CD47 to treat metabolic diseases and the devising of potential improvements to possible side effects.

Mitochondrial regulation and white adipose tissue homeostasis

The important role of mitochondria in the regulation of white adipose tissue (WAT) remodeling and energy balance is increasingly appreciated. The remarkable heterogeneity of the adipose tissue stroma provides a cellular basis to enable adipose tissue plasticity in response to various metabolic stimuli. Regulating mitochondrial function at the cellular level in adipocytes, in adipose progenitor cells (APCs), and in adipose tissue macrophages (ATMs) has a profound impact on adipose homeostasis. Moreover, mitochondria facilitate the cell-to-cell communication within WAT, as well as the crosstalk with other organs, such as the liver, the heart, and the pancreas. A better understanding of mitochondrial regulation in the diverse adipose tissue cell types allows us to develop more specific and efficient approaches to improve adipose function and achieve improvements in overall metabolic health.

Absence of CD47 maintains brown fat thermogenic capacity and protects mice from aging-related obesity and metabolic disorder

Brown and beige adipocytes burn energy to produce heat and could serve as a therapeutic target to counteract metabolic diseases including obesity and type 2 diabetes. Aging is associated with reduced brown fat mass and thermogenic capacity and a risk factor for metabolic diseases. Our previous studies implicated a role for CD47 in regulating brown fat function and energy balance in young adult animals. In this study, we further determined its role in natural aging related metabolic disorders. The results demonstrated that aged CD47 deficient mice (under normal chow diet) had reduced body weight and fat mass, and improved glucose tolerance as compared to aged wild type (WT) mice. Indirect calorimetry result showed that food intake and total activity were comparable between two genotypes. However, CD47 deficient mice had increased energy expenditure and better cold tolerance, accompanied by increased white adipose tissue browning and well-maintained juvenile morphology of brown adipose tissue (BAT). Moreover, transcriptome (RNA-seq) and pathway enrichment analysis revealed that BAT from aged CD47 deficient mice had upregulated genes involving in mitochondria oxidative phosphorylation, thermogenesis, fatty acid metabolism, and valine, leucine and isoleucine (BCAA) degradation, indicating the activated BAT status in aged CD47 deficient mice. Collectively, these data suggest that blocking CD47 signaling protects mice from natural aging-associated obesity and glucose intolerance, partially though activation and expansion of the thermogenic machinery, further supporting that CD47 maybe a potential target for aging related metabolic disorder.

A Potential Role of the CD47/SIRPalpha Axis in COVID-19 Pathogenesis

The coronavirus SARS-CoV-2 is the cause of the ongoing COVID-19 pandemic. Most SARS-CoV-2 infections are mild or even asymptomatic. However, a small fraction of infected individuals develops severe, life-threatening disease, which is caused by an uncontrolled immune response resulting in hyperinflammation. However, the factors predisposing individuals to severe disease remain poorly understood. Here, we show that levels of CD47, which is known to mediate immune escape in cancer and virus-infected cells, are elevated in SARS-CoV-2-infected Caco-2 cells, Calu-3 cells, and air-liquid interface cultures of primary human bronchial epithelial cells. Moreover, SARS-CoV-2 infection increases SIRPalpha levels, the binding partner of CD47, on primary human monocytes. Systematic literature searches further indicated that known risk factors such as older age and diabetes are associated with increased CD47 levels. High CD47 levels contribute to vascular disease, vasoconstriction, and hypertension, conditions that may predispose SARS-CoV-2-infected individuals to COVID-19-related complications such as pulmonary hypertension, lung fibrosis, myocardial injury, stroke, and acute kidney injury. Hence, age-related and virus-induced CD47 expression is a candidate mechanism potentially contributing to severe COVID-19, as well as a therapeutic target, which may be addressed by antibodies and small molecules. Further research will be needed to investigate the potential involvement of CD47 and SIRPalpha in COVID-19 pathology. Our data should encourage other research groups to consider the potential relevance of the CD47/ SIRPalpha axis in their COVID-19 research.

CD47 and thrombospondin-1 regulation of mitochondria, metabolism, and diabetes

Thrombospondin-1 (TSP1) is the prototypical member of a family of secreted proteins that modulate cell behavior by engaging with molecules in the extracellular matrix and with receptors on the cell surface. CD47 is widely displayed on many, if not all, cell types and is a high-affinity TSP1 receptor. CD47 is a marker of self that limits innate immune cell activities, a feature recently exploited to enhance cancer immunotherapy. Another major role for CD47 in health and disease is to mediate TSP1 signaling. TSP1 acting through CD47 contributes to mitochondrial, metabolic, and endocrine dysfunction. Studies in animal models found that elevated TSP1 expression, acting in part through CD47, causes mitochondrial and metabolic dysfunction. Clinical studies established that abnormal TSP1 expression positively correlates with obesity, fatty liver disease, and diabetes. The unabated increase in these conditions worldwide and the availability of CD47 targeting drugs justify a closer look into how TSP1 and CD47 disrupt metabolic balance and the potential for therapeutic intervention.

Thrombospondin 1 in Metabolic Diseases

The thrombospondin family comprises of five multifunctional glycoproteins, whose best-studied member is thrombospondin 1 (TSP1). This matricellular protein is a potent antiangiogenic agent that inhibits endothelial migration and proliferation, and induces endothelial apoptosis. Studies have demonstrated a regulatory role of TSP1 in cell migration and in activation of the latent transforming growth factor beta 1 (TGFβ1). These functions of TSP1 translate into its broad modulation of immune processes. Further, imbalances in immune regulation have been increasingly linked to pathological conditions such as obesity and diabetes mellitus. While most studies in the past have focused on the role of TSP1 in cancer and inflammation, recently published data have revealed new insights about the role of TSP1 in physiological and metabolic disorders. Here, we highlight recent findings that associate TSP1 and its receptors to obesity, diabetes, and cardiovascular diseases. TSP1 regulates nitric oxide, activates latent TGFβ1, and interacts with receptors CD36 and CD47, to play an important role in cell metabolism. Thus, TSP1 and its major receptors may be considered a potential therapeutic target for metabolic diseases.