APOPTOSIS IN THE PANCREATIC-ISLET CELLS OF THE NEONATAL RAT IS ASSOCIATED WITH A REDUCED EXPRESSION OF INSULIN-LIKE-GROWTH-FACTOR-II THAT MAY ACT AS A SURVIVAL FACTOR

Islet cell ontogeny will define adult beta-cell mass and will consist of a balance of islet cell birth and death. We have investigated the o ntogeny of factors that may be related to developmental apoptosis in t he islets, insulin-like growth factor II (IGF-II) and inducible nitric oxide synthase (iNOS), in pancreata of young Wistar rats. Pancreata w ere collected from rats of 21 days gestation to 29 days postnatal age. In situ. hybridization and immunohistochemistry showed that IGF-II wa s expressed and present in fetal and neonatal islet cells, but decline d rapidly 2 weeks after birth. Little IGF-I was associated with fetal or postnatal islets. Apoptosis in islet cells was visualized by molecu lar histochemistry for DNA breakage in tissue sections. Apoptosis was low in the fetus, but increased in incidence postnatally so that 13% o f islet cells were undergoing apoptosis on postnatal day 14, with the incidence declining thereafter. Immunohistochemistry for iNOS showed t hat it was expressed within beta-cells and was most abundant 12 days a fter birth. When islets were isolated from rat pancreata 20-22 days af ter birth, islet cell viability, DNA synthetic rate, and insulin relea se were reduced after incubation with interleukin-1 beta, tumor necros is factor, or interferon-gamma. An increased rate of islet cell surviv al was found after simultaneous incubation with IGF-I or -II. Cytokine -mediated islet cell death involved the induction of apoptosis. Islets isolated from neonatal rats were not killed after exposure to these c ytokines at the same concentrations, but cytokine-induced cell death w as seen when neonatal islets were incubated with a neutralizing antibo dy against IGF-II. These experiments show that a peak of islet cell ap optosis that is maximal in the rat pancreas 14 days after birth is tem porally associated with a fall in the islet cell expression of IGF-II. IGF-II was shown to function as an islet survival factor in vitro. Th e induction of islet cell apoptosis in vivo may involve an increased e xpression of iNOS within beta-cells.