Endocrine regulation of fetal adipose tissue metabolism in the pig: role of thyroxine

Endocrine Regulation of Fetal Adipose Tissue Metabolism in the Pig: Role of Hydrocortisone

Glucocorticoids have been shown to be essential for the excessive fat deposition and development of obesity in several animal models. This study was performed to characterize the role of glucocorticoids in the developmental regulation of adipose tissue metabolism. On day 70 of gestation, pig fetuses were hypophysectomized by micro-cauterization. Hypophysectomized fetuses were implanted subcutaneously with hydrocortisone pellets or received no hormone replacement. Fetuses were removed by laparotomy on day 90 of gestation. Additional fetuses were hypophysectomized on day 70, implanted with hydrocortisone pellets on day 90 and removed on day 105 of gestation. Several intact fetuses were also implanted subcutaneously with hydrocortisone pellets during this later gestational period. Serum cortisol concentrations were reduced in hypophysectomized pigs at both fetal ages and were restored to intact levels by hydrocortisone treatment. Hydrocortisone supplementation enhanced lipolytic response to isoproterenol in intact fetuses but failed to restore lipolytic response to isoproterenol in hypophysectomized animals at either fetal age. Hydrocortisone induced a slight increase in lipogenesis in hypophysectomized fetuses when administered from 70 to 90 days of gestation and a more dramatic increase when administered from days 90 to 105 of gestation. However, hydrocortisone had no effect on basal or insulin stimulated lipogenesis in intact fetuses when administered from days 90 to 105 of gestation. These results indicate that hydrocortisone may have a primary influence on adipose tissue metabolism during late fetal development only in the absence of inhibition from counterregulatory hormones of pituitary origin.

Subcutaneous fat in normal and diseased states

The quest for effective strategies to treat obesity has propelled fat research into an exploration of the molecular processes that drive adipocyte formation, and hence body fat mass. The development of obesity is dependent on the coordinated interplay of adipocyte hypertrophy (increased fat cell size), adipocyte hyperplasia (increased fat cell number), and angiogenesis. Evidence suggests that adipocyte hyperplasia, or adipogenesis, occurs throughout life, both in response to normal cell turnover as well as in response to the need for additional fat mass stores that arises when caloric intake exceeds nutritional requirements. Adipogenesis involves two major events-the recruitment and proliferation of adipocyte precursor cells, called preadipocytes, followed by the subsequent conversion of preadipocytes, or differentiation, into mature fat cells. In vitro studies using experimental and primary preadipocyte cell lines have uncovered the mechanisms that drive the adipogenic process, a tightly controlled sequence of events guided by the strict temporal regulation of multiple inhibitory and stimulatory signaling events involving regulators of cell-cycle functions and differentiation factors. This article reviews the current understanding of adipogenesis with emphasis on the various stages of adipocyte development; on key hormonal, nutritional, paracrine, and neuronal control signals; as well as on the components involved in cell-cell or cell-matrix interactions that are pivotal in regulating fat cell formation. Special consideration is given to clinical applications derived from adipogenesis research with impact on medical, surgical and cosmetic fields.

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, TNF alpha, 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.

Endocrine regulation of fetal adipose tissue metabolism in the pig: interaction of porcine growth hormone and thyroxine

OBJECTIVE

This study tested the hypothesis that combined treatment of thyroxine (T4) and growth hormone (GH) could normalize cellular and metabolic aspects of adipose tissue development of hypophysectomized fetal pigs.

RESEARCH METHODS AND PROCEDURES

On day 70 of gestation, pig fetuses were hypophysectomized by microcauterization or remained intact. Hypophysectomized fetuses remained untreated or were treated from day 90 to day 105 of gestation with T4, GH, or a combination of both hormones.

RESULTS

Body weights were unaffected by hypophysectomy or hormone treatment. De novo lipogenesis in subcutaneous adipose tissue was increased 10-fold by hypophysectomy, consistent with our previous results. This increase was abolished by GH treatment in the hypophysectomized fetuses. In contrast, T4 treatment of the hypophysectomized fetuses resulted in a 12-fold further increase in adipose tissue lipogenesis, an effect that was negated by concomitant administration of GH. Lipolytic response to isoproterenol was decreased by hypophysectomy, unaffected by GH treatment, and restored to intact values by T4 or by T4+GH treatment in the hypophysectomized fetuses.

DISCUSSION

In contrast to T4, GH does not influence serum insulin-like growth factor-I or adipose tissue lipolysis, but decreases lipogenesis in the fetal pig. However, replacing both T4 and GH normalized hypophysectomized fetuses to a greater extent than either GH or T4 alone. Thus, any influence of thyroid hormones on stimulating adipose tissue lipogenesis in the developing fetal pig may be normally counterregulated by pituitary-derived growth hormone.

The expression of insulin-like growth factor-1 during adipogenesis in vivo: effect of thyroxine

The expression of insulin-like growth factor-1 (IGF-1) was examined in subcutaneous (SQ) adipose tissue from 105-day pig fetuses by ribonuclease protection assays and in situ hybridization using a porcine IGF-1 riboprobe. Fetuses were hypophysectomized (hypox) at 70 days of gestation and at 90 days thyroxine (T4) pellets were implanted into some of the hypox fetuses. Fetuses were removed and SQ tissues collected on day 105 of gestation. RNase protection assays followed by scanning laser densitometry revealed that IGF-1 mRNA in SQ adipose tissue in hypox fetuses was significantly decreased to 19.8 +/- 1.2% of control values. T4 treatment increased the expression of IGF-1 by 174 +/- 26.6% of values for hypox fetuses. Using in situ hybridization we showed that fetal hypophysectomy reduced and T4 treatment increased expression of IGF-1 mRNA in the outer SQ adipose tissue layer (P < 0.05). However, T4 treatment after hypox did not influence IGF-1 expression in the inner SQ layer (P > 0.05). IGF-1 was expressed around capillaries, in small fat cells, and in fibroblasts in loose and dense connective tissue. Large fat cells, however, did not express IGF-1. Collectively, we concluded that (1) IGF-1 mRNA was decreased after hypox and increased by T4 treatment in SQ tissue of 105-day fetuses; (2) The expression of the IGF-1 gene was more sensitive to T4 treatment after hypox in outer SQ tissue than in inner SQ tissue; (3) Most stromal cells produced IGF-1 mRNA, and as a result they may influence adipogenesis in the outer layer of SQ adipose tissue; and (4) Once fat cells enlarged, expression of IGF-1 was not detected. Therefore, these studies provide evidence that IGF-1 may mediate the influence of T4 on fetal adipogenesis.