Gary J. Hausman

The biology of white adipocyte proliferation.

Expanded adipose tissue mass increases the risk for many clinical conditions including diabetes, hypertension, coronary atherosclerotic heart disease, and some forms of cancer. Therefore, it is imperative that we understand the mechanisms by which fat pads expand. The enlargement of fat cells during the development of obesity has been previously hypothesized to be a triggering factor for the proliferation of new fat cells. There is now a preponderance of evidence that adipose tissue is a source of growth factors such as IGF‐I, IGF binding proteins, TNFα, angiotensin II, and MCSF that are capable of stimulating proliferation. The relative importance of these autocrine/paracrine factors in the normal control of preadipocyte proliferation is unknown. In addition, the proliferative response of preadipocytes to the paracrine milieu is undoubtedly modulated by neural inputs to fat tissue and/or serum factors. Together, these multiple regulatory controls orchestrate overall and region‐specific adipose tissue cellularity responses associated with the development of hyperplastic obesity. Both in vivo and in vitro studies are needed to understand the complex, interacting physiological mechanisms by which growth of this important organ is regulated.

Response of Pericytes to Thermal Lesion in the Inguinal Fat Pad of 10-Day-Old Rats

The response of capillary pericytes to thermal lesion in the inguinal fat pad of 10-day-old rats was observed with the light and electron microscope. Samples were taken from the necrotic area and the immediately surrounding tissue at various time intervals from 1 h to 10 days post-lesion. The initial physical disruption and swelling of the capillary resulted in the liberation of some pericytes near the necrotic zone. Most pericytes remained, however, in the basal lamina and went through a period of activation that peaked at 48 h. Pericyte activation consisted of a noticeable increase in rough endoplasmic reticulum with the cisternae containing granular, electron-opaque material. There was evidence of migration of these activated cells from the capillary basal lamina between 24 and 72 h. By 72 h, fibroblast-like cells possessing different nuclear and cytoplasmic morphology were oriented along distinct bands of extracellular fibrin and necrotic tissue. These cells produced limited amounts of collagen. Differentiation into immature adipocytes occurred 5 days after the lesion. Small, unilocular adipocytes were located in the necrotic zone by 7 days. Complete restoration of the lesion area was accomplished by 10 days. It is proposed that pericytes act as a reserve cell in the recovery of the rat inguinal fat pad after thermal lesion.

Board-invited review: the biology and regulation of preadipocytes and adipocytes in meat animals.

The quality and value of the carcass in domestic meat animals are reflected in its protein and fat content. Preadipocytes and adipocytes are important in establishing the overall fatness of a carcass, as well as being the main contributors to the marbling component needed for consumer preference of meat products. Although some fat accumulation is essential, any excess fat that is deposited into adipose depots other than the marbling fraction is energetically unfavorable and reduces efficiency of production. Hence, this review is focused on current knowledge about the biology and regulation of the important cells of adipose tissue: preadipocytes and adipocytes.

Cell line models for differentiation: preadipocytes and adipocytes

In vitro models have been invaluable in determining the mechanisms involved in adipocyte proliferation, differentiation, adipokine secretion and gene/protein expression. The cells presently available for research purposes all have unique advantages and disadvantages that one should be aware of when selecting cells. Established cell lines, such as 3T3-L1 cells, are easier and less costly to use than freshly isolated cells, even though freshly isolated cells allow for various comparisons such as the in vitro evaluation of different in vivo conditions that may not be possible using cell lines. Moreover, stem cells, transdifferentiated cells or dedifferentiated cells are relatively new cell models being evaluated for the study of adipocyte regulation and physiology. The focus of this brief review is to highlight similarities and differences in adipocyte models to aid in appropriate model selection and data interpretation for successful advancement of our understanding of adipocyte biology.

The development and endocrine functions of adipose tissue

White adipose tissue is a mesenchymal tissue that begins developing in the fetus. Classically known for storing the bodys fuel reserves, adipose tissue is now recognized as an endocrine organ. As such, the secretions from adipose tissue are known to affect several systems such as the vascular and immune systems and play major roles in metabolism. Numerous studies have shown nutrient or hormonal manipulations can greatly influence adipose tissue development. In addition, the associations between various disease states, such as insulin resistance and cardiovascular disease, and disregulation of adipose tissue seen in epidemiological and intervention studies are great. Evaluation of known adipokines suggests these factors secreted from adipose tissue play roles in several pathologies. As the identification of more adipokines and determination of their role in biological systems, and the interactions between adipocytes and other cells types continues, there is little doubt that we will gain a greater appreciation for a tissue once thought to simply store excess energy.

Biology of leptin in the pig

The recently discovered protein, leptin, which is secreted by fat cells in response to changes in body weight or energy, has been implicated in regulation of feed intake, energy expenditure and the neuroendocrine axis in rodents and humans. Leptin was first identified as the gene product found deficient in the obese ob/ob mouse. Administration of leptin to ob/ob mice led to improved reproduction as well as reduced feed intake and weight loss. The porcine leptin receptor has been cloned and is a member of the class 1 cytokine family of receptors. Leptin has been implicated in the regulation of immune function and the anorexia associated with disease. The leptin receptor is localized in the brain and pituitary of the pig. The leptin response to acute inflammation is uncoupled from anorexia and is differentially regulated among swine genotypes. In vitro studies demonstrated that the leptin gene is expressed by porcine preadipocytes and leptin gene expression is highly dependent on dexamethasone induced preadipocyte differentiation. Hormonally driven preadipocyte recruitment and subsequent fat cell size may regulate leptin gene expression in the pig. Expression of CCAAT-enhancer binding proteinalpha (C/EBPalpha) mediates insulin dependent preadipocyte leptin gene expression during lipid accretion. In contrast, insulin independent leptin gene expression may be maintained by C/EBPalpha auto-activation and phosphorylation/dephosphorylation. Adipogenic hormones may increase adipose tissue leptin gene expression in the fetus indirectly by inducing preadipocyte recruitment and subsequent differentiation. Central administration of leptin to pigs suppressed feed intake and stimulated growth hormone (GH) secretion. Serum leptin concentrations increased with age and estradiol-induced leptin mRNA expression in fat was age and weight dependent in prepuberal gilts. This occurred at the time of expected puberty in intact contemporaries and was associated with greater LH secretion. Further work demonstrated that leptin acts directly on pituitary cells to enhance LH and GH secretion, and brain tissue to stimulate gonadotropin releasing hormone secretion. Thus, development of nutritional schemes and (or) gene therapy to manipulate leptin secretion will lead to practical methods of controlling appetite, growth and reproduction in farm animals, thereby increasing efficiency of lean meat production.

Animal growth regulation.

1 Placental Regulation of Fetal Growth.- 1. Introduction.- 2. Placental Anatomy and Pattern of Blood Flow.- 3. Placental Growth.- 4. Growth of Uteroplacental Blood Flow.- 5. Regulation of Blood Flow.- 6. Placental Transport.- 7. Hormone Secretion.- 8. Conclusions.- References.- 2 Endocrinology of Bone Formation.- 1. Introduction.- 1.1. Unique Aspects of Bone Physiology.- 1.2. Bone Structure.- 1.3. Methods of Investigating Bone Formation.- 2. Hormones.- 2.1. Somatomedins.- 2.2. PTH.- 2.3. Vitamin D.- 2.4. Calcitonin.- 2.5. Insulin.- 2.6. Thyroid Hormones.- 2.7. Sex Steroids.- 2.8 Glucocorticoids.- 3. Local Regulators of Bone Formation.- 3.1. Prostaglandins.- 3.2. Noncollagen Proteins.- 3.3. Growth Factors.- References.- 3 Endocrine Regulation of Adipogenesis.- 1. Introduction.- 2. Insulin.- 3. Growth Hormone.- 4. Insulinlike Growth Factors.- 5. Glucocorticoids.- 6. Indomethacin, Prostaglandins, and AMP.- 7. Thyroid Hormones.- 8. Cachectin.- 9. Autocrine Control.- 10. Perspectives on Integrated Endocrine Control.- References.- 4 Autocrine, Paracrine, and Endocrine Regulation of Myogenesis.- 1. Introduction.- 2. Muscle Structure.- 3. Myogenesis.- 4. Factors Affecting Myogenesis.- 4.1. Insulinlike Growth Factors (Somatomedins).- 4.2. Insulin.- 4.3. Transforming Growth Factor-? (TGF-?) (Differentiation Inhibitor).- 4.4. Transferrin.- 4.5. Fibroblast Growth Factor (FGF).- 4.6. Somatotropin.- 4.7. Glucocorticoids.- 4.8. Thyroid Hormone.- 5. Summary.- References.- 5 The Expression of Protooncogenes in Skeletal Muscle.- 1. Introduction.- 2. Categories of Oncogenes.- 2.1. Tyrosine Kinases.- 2.2. GTP-Binding Proteins.- 2.3. Nuclear Proteins.- 2.4. Peptide Hormones.- 3. Oncogene Expression in Skeletal Muscle.- References.- 6 Regulation of Myofibrillar Protein Gene Expression.- 1. Introduction.- 2. Multigene Families.- 3. Major Myofibrillar Proteins of the Sarcomere.- 3.1. Actin.- 3.2. Tropomyosin (TM).- 3.3. Troponin (TN).- 3.4. Myosin Light Chains (MLC).- 3.5. Myosin Heavy Chains (MHC).- 4. Isolation and Characterization of MHC Sequences.- 5. Effects of Various Stimuli on MHC Gene Expression.- 5.1. Thyroid Hormone.- 5.2. Exercise/Electrical Stimulation.- 6. Alternative Splicing and MHC Gene Structure.- 7. Conclusions.- References.- 7 Regulation of Growth by Negative Growth Regulators.- 1. Introduction.- 2. Transforming Growth Factor-?.- 3. Platelet-Derived Inhibitors.- 3.1. 37-kDa Protein.- 3.2. 27-kDa Protein.- 3.3. Proteins Greater than 200 kDa.- 4. Interferons.- 5. Liver-Derived Inhibitors.- 6. Mammary-Derived Inhibitor.- 7. Glycopeptide Inhibitors.- 8. Density-Dependent Inhibitors.- 9. Summary.- References.- 8 Skeletal Muscle Proteases and Protein Turnover.- 1. Introduction.- 2. General Features of Intracellular Protein Degradation.- 3. Intracellular Degradation of Muscle Proteins.- 4. Neutral and Alkaline Proteolytic Activities.- 4.1. Neutral and Alkaline Proteolytic Activities in Muscle Cells.- 4.2. The ATP-Stimulated and ATP-Dependent Proteinases.- 4.3. The Ca2 +-Dependent Proteinases.- 5. Protease Inhibitors.- 6. Summary.- References.- 9 Regulation of Protein Turnover.- 1. Introduction.- 2. Factors That Affect Tissue Growth and Protein Turnover.- 2.1. Protein Turnover in Different Tissues and the Effect of Developmental Age.- 2.2. Nutrient Intake.- 2.3. Functional Load.- 3. Factors That Control Protein Turnover.- 3.1. Substrates and Hormones and Their Relation to Nutritional Control.- 3.2. The Role of Initiation and Ribosomal Accretion in the Control of Translation.- 3.3. Comments on the Link between Receptor Activation and Protein Synthesis.- 4. Conclusion.- References.- 10 Energy Balance Regulation.- 1. Introduction.- 2. Evidence for Energy Balance Regulation in Farm Animals.- 2.1. Lactation.- 2.2. Growing Animals.- 2.3. Compensatory Growth.- 2.4. Egg Production.- 3. Mechanisms of Energy Balance Regulation.- 3.1. Lactation.- 3.2. Brown Adipose Tissue Metabolism.- 3.3. Compensatory Growth.- 4. CNS Control of Food Intake.- 4.1. Brain Areas Involved.- 4.2. Mechanisms of Action.- 5. CNS Control of Peripheral Metabolism.- 5.1. Brain Areas Involved.- 5.2. Direct Regulation.- 5.3. Indirect Regulation.- 6. Theories of Signals Regulating Food Intake.- 6.1. Short-Term Mechanisms.- 6.2. Long-Term Mechanisms.- 7. Role of Neurotransmitters and Neuropeptides.- 8. Summary.- References.- 11 Central Regulation of Growth Hormone Secretion.- 1. Introduction.- 2. Patterns of GH Secretion.- 3. Hypothalamic Peptides Involved in GH Regulation.- 3.1. Somatostatin.- 3.2. GH-Releasing Factor (GRF).- 3.3. Other Peptides That Increase GH Secretion.- 4. Hypothalamic Regions Involved in GH Regulation.- 5. Interaction of Somatostatin and GRF in Episodic GH Secretion.- 6. Neuropharmacological Regulation of GH Secretion.- 6.1. Norepinephrine (NE).- 6.2. Dopamine (DA).- 6.3. Serotonin (5-HT).- 6.4. Other Neurotransmitters.- 7. Gonadal Steroid Modulation of GH Secretion.- 8. Feedback Regulation of GH Secretion.- References.- 12 Mechanisms of Action for Somatotropin in Growth.- 1. Introduction.- 2. Chemical Nature of Somatotropin.- 2.1. Variants of Somatotropin.- 2.2. Fragments of Somatotropin.- 2.3. Receptor Mediation of Somatotropin Effects.- 3. Biological Response to Somatotropin.- 3.1. Nutrient Partitioning.- 3.2. Pattern of Administration.- 3.3. Dose-Response Relationships: Growth Model.- 4. Mechanisms of Action.- 4.1. General Considerations.- 4.2. Carbohydrate Metabolism.- 4.3. Bone and Mineral Metabolism.- 4.4. Adipose Tissue Lipid Metabolism.- 4.5. Muscle and Protein Metabolism.- 5. Summary and Perspectives.- References.- 13 Regulation of Somatomedin Production, Release, and Mechanism of Action.- 1. Origin of the Somatomedin Hypothesis.- 2. Methods of Analyses.- 3. Gene Expression and the Insulin Peptide Family.- 4. Site of Synthesis.- 5. Somatomedin Research in Domestic Animals.- References.- 14 Sexual Differentiation and the Growth Process.- 1. Introduction.- 2. Rats.- 3. Cattle and Sheep.- 4. Swine.- 5. Chickens.- 6. Sexual Differences in Growth-Related Endocrine Processes.- 7. Conclusions.- References.- 15 Potential Mechanisms for Repartitioning of Growth by ss-Adrenergic Agonists.- 1. Introduction.- 2. Biology of Adrenergic Hormones and Neurotransmitters.- 2.1. Endogenous Adrenergic Agents.- 2.2. Synthesis and Removal.- 2.3. Adrenergic Receptors.- 2.4. Coupling of Adrenergic Receptors to Intracellular Function.- 2.5. Physiological Effects of Adrenergic Agonists.- 3. Administration of ss-Adrenergic Agonists to Animals.- 3.1. Effects.- 3.2. Adipose Tissue.- 3.3. Muscle.- 3.4. Other Mechanisms.- References.- 16 Gene Transfer for Enhanced Growth of Livestock.- 1. Introduction.- 2. Identification of Genes for Transfer.- 3. Methods of Producing Transgenic Animals.- 3.1. Microinjection into Fertilized Ova.- 3.2. Retroviral Insertion of Genes.- 3.3. Insertion via Pluripotent Cells.- 3.4. Insertion by Transposons.- 4. Production of Transkaryotic Animals.- 5. Transfer of Growth-Related Genes into Livestock.- 5.1. Integration of Growth-Related Genes.- 5.2. Expression of Integrated Genes.- 5.3. Growth Performance of Transgenic Livestock.- 5.4. Germline Transmission of Fusion Genes.- 5.5. Expression in Animals with Transkaryotic Implants.- 6. Conclusions.- References.- 17 Status of Current Strategies for Growth Regulation.- 1. Introduction.- 2. Steroid Hormone and Xenobiotic Regulation of Animal Growth.- 3. Manipulation of Animal Growth with Exogenous Somatotropin.- 4. Use of Growth Hormone-Releasing Factor (GRF) to Alter Animal Growth.- 5. Use of ?-Adrenergic Agonists to Manipulate Animal Growth.- 5.1. Effects of Adrenergic Agonists in Sheep.- 5.2. Effects of Adrenergic Agonists in Cattle.- 5.3. Effects of Adrenergic Agonists in Swine.- 6. Use of Immunization to Manipulate Animal Growth.- 7. Summary and Perspectives.- References.

Leptin mRNA expression and serum leptin concentrations as influenced by age, weight, and estradiol in pigs.

Two experiments (EXP) were conducted to determine the roles of age, weight and estradiol (E) treatment on serum leptin concentrations and leptin gene expression. In EXP I, jugular blood samples were collected from gilts at 42 to 49 (n = 8), 105 to 112 (n = 8) and 140 to 154 (n = 8) d of age. Serum leptin concentrations increased (P < 0.05) with age and averaged 0.66, 2.7, and 3.0 ng/ml (pooled SE 0.21) for the 42- to 49-, 105- to 112-, and 140- to 154-d-old gilts, respectively. In EXP II, RNase protection assays were used to assess leptin mRNA in adipose tissue of ovariectomized gilts at 90 (n = 12), 150 (n = 11) or 210 (n = 12) d of age. Six pigs from each age group received estradiol (E) osmotic pump implants and the remaining animals received vehicle control implants (C; Day 0). On Day 7, back fat and blood samples were collected. Estradiol treatment resulted in greater (P < 0.05) serum E levels in E (9 +/- 1 pg/ml) than C (3 +/- 1 pg/ml) pigs. Serum leptin concentrations were not affected by age, nor E treatment. Leptin mRNA expression was not increased by age in C pigs nor by F in 90- and 150-d-old pigs. However, by 210 d of age, leptin mRNA expression was 2.5-fold greater (P < 0.01) in E-treated pigs compared to C animals. Serum insulin concentrations were similar between treatments for 210-d-old pigs. However, insulin concentrations were greater (P < 0.05) in E than C pigs at 90 d and greater in C than E animals at 150 d. Plasma glucose and serum insulin-like growth factor-I concentrations were not influenced by treatment. These results demonstrate that serum leptin concentrations increased with age and E-induced leptin mRNA expression is age- and weight-dependent.

What are subcutaneous adipocytes really good for...

Abstract:  Our acute awareness of the cosmetic, psychosocial and sexual importance of subcutaneous adipose tissue contrasts dramatically with how poorly we have understood the biology of this massive, enigmatic, often ignored and much‐abused skin compartment. Therefore, it is timely to recall the exciting, steadily growing, yet underappreciated body of evidence that subcutaneous adipocytes are so much more than just ‘fat guys’, hanging around passively to conspire, at most, against your desperate attempts to maintain ideal weight. Although the subcutis, quantitatively, tends to represent the dominant architectural component of human skin, conventional wisdom confines its biological key functions to those of energy storage, physical buffer, thermoregulation and thermoinsulation. However, already the distribution of human superficial adipose tissue, by itself, questions how justified the popular belief is that ‘skin fat’ (which actually may be more diverse than often assumed) serves primarily thermoinsulatory purposes. And although the metabolic complications of obesity are well appreciated, our understanding of how exactly subcutaneous adipocytes contribute to extracutaneous disease – and even influence important immune and brain functions! – is far from complete. The increasing insights recently won into subcutaneous adipose tissue as a cytokine depot that regulates innate immunity and cell growth exemplarily serve to illustrate the vast open research expanses that remain to be fully explored in the subcutis. The following public debate carries you from the evolutionary origins and the key functional purposes of adipose tissue, via adipose‐derived stem cells and adipokines straight to the neuroendocrine, immunomodulatory and central nervous effects of signals that originate in the subcutis – perhaps, the most underestimated tissue of the human body. The editors are confident that, at the end, you shall agree: No basic scientist and no doctor with a serious interest in skin, and hardly anyone else in the life sciences, can afford to ignore the subcutaneous adipocyte – beyond its ample impact on beauty, benessence and body mass.

MicroRNA regulation in mammalian adipogenesis.

Adipogenesis, the complex development from preadipocytes or mesenchymal stem cells to mature adipocytes, is essential for fat formation and metabolism of adipose tissues in mammals. It has been reported to be regulated by hormones and various adipogenic transcription factors which are expressed as a transcriptional cascade promoting adipocyte differentiation, leading to the mature adipocyte phenotype. Recent findings indicate that microRNAs (miRNAs), a family of small RNA molecules of approximately 22 nucleotides in length, are involved in the regulatory network of many biological processes, including cell differentiation, through post-transcriptional regulation of transcription factors and/or other genes. In this review, we focus on the recent understanding of the roles of miRNAs in adipogenesis, including the most recent and relevant findings that support the role of several miRNAs as pro- or antiadipogenic factors regulating adipogenesis in mice, human and cattle to propose the future role of miRNA in adipogenesis of farm animal models.