Effect of Adiponectin on the Steroidogenic Acute Regulatory Protein, P450 Side Chain Cleavage Enzyme and 3β-hydroxysteroid Dehydrogenase Gene Expression, Progesterone and Androstenedione Production by the Porcine Uterus during Early Pregnancy

Adiponectin is an adipocyte-derived hormone that acts as a major regulator of energy metabolism and insulin sensitivity (1, 2). In plasma, adiponectin occurs in different forms: the trimeric low molecular weight (LMW), the hexameric middle molecular weight (MMW) and the 12-18meric high molecular weight (HMW) complexes (3). Adiponectin can be found in a smaller globular form, which is produced through proteolytic cleavage (4). It circulates at high levels in the bloodstream, and its serum concentration is decreased in obesity and insulin resistance (5, 6). In an obese subject, reduction in body weight leads to an increase in plasma adiponectin levels (7). In diabetic mice, administration of recombinant adiponectin improves insulin resistance (1). Disturbed serum levels of adiponectin have also been demonstrated in patients with pancreatic cancer (8) and in rats with post-myocardial infarction heart failure (9). Adiponectin exerts its action by binding to specific seven-transmembrane adiponectin receptors 1 and 2 (AdipoR1, AdipoR2). AdipoR1 was identified as a high-affinity receptor for adiponectin in the form of a trimer and is predominantly expressed in skeletal muscles, whereas AdipoR2 has a higher affinity for MMW and HMW adiponectin and is found mainly in the liver (10). The effects of adiponectin are mediated through the activation of the adenosine monophosphate activated protein kinase (AMPK), p38 mitogen-activated protein kinase (MAPK) and the peroxisome proliferators-activated receptor-α (PPARα) (11). It is well known that reproduction can be affected by nutritional status. Adiponectin may be a part of a common endocrine system that controls metabolism and the reproductive system. It is believed to play a key role regarding reproduction by regulating the hypothalamic-pituitary-gonadal (HPG) axis (12, 13). The adiponectin system (adiponectin, AdipoR1 and AdipoR2) was recently found in porcine, rat and human hypothalami (14, 15), pituitary glands (16, 17) and ovaries (18-21). The secretion of gonadotropin-releasing hormone (GnRH) from GT1-7 hypothalamic cells was inhibited by adiponectin (22). Adiponectin decreased basal and GnRH-stimulated luteinizing hormone (LH) secretion by the isolated pituitary cells of rats (16). At the level of the mammalian ovaries, in vitro studies have shown that adiponectin is able to affect steroidogenic gene expression and steroidogenesis. This effect depends on differences among JOURNAL OF PHYSIOLOGY AND PHARMACOLOGY 2016, 67, 3, 443-456