シロクマからヒトへの警告

―北極圏におけるビタミンA貯蔵細胞の研究から―

抄録

  Nobu Shirase (born in Konoura in Akita Prefecture) was a pioneer of polar expedition. He reached 80° 5' south latitude in 1912 after Amundsen and Scott reached the South Pole. White bear (polar bear) is a unique animal in the Arctic, and stores a large amount of vitamin A in the liver. Hepatic stellate cells (vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, Ito cells) exist in the space between parenchymal cells and sinusoidal endothelial cells of the hepatic lobule, and store 80 % of vitamin A in the whole body as retinyl palmitate in lipid droplets in the cytoplasm in mammals such as human or rats. In physiological conditions, these cells play pivotal roles in the regulation of vitamin A homeostasis; they express specific receptors for retinolbinding protein (RBP), a binding protein specific for retinol, on their cell surface, and take up the complex of retinol and RBP by receptormediated endocytosis. Hepatic stellate cells in top predators in arctic animals such as polar bears and arctic foxes store 20-100 times the levels of vitamin A found in human or rat. Nuclear deviation in hepatic parenchymal cells, degeneration of Glisson's sheath, inflammation of the intestine, and a shift of vitamin A-storing site from the liver to the kidney were found in the arctic top predators. These findings were not reported in the wild animals and alarming to the human beings.