The inhibition of insulin action and glucose metabolism by porcine growth hormone in porcine adipocytes is not the result of any decrease in insulin binding or insulin receptor kinase activity

Abstract

The present study was undertaken to determine the effects of porcine growth hormone (pGH) on glucose transport, to establish which lipogenic enzymes were affected by pGH, and to determine if changes in insulin binding or insulin receptor kinase activity contributed to the diminished insulin responsiveness of adipocytes from pigs treated with pGH. Pigs were treated with pGH daily (70 micrograms/kg body wt.) for 7 days. pGH treatment reduced the basal (non-insulin-stimulated) glucose transport rate by 62% and the insulin-stimulated transport rate by 47%. The decline in glucose transport rate was paralleled by a 64% decrease in fatty acid synthesis. The reduction in the lipogenic rate was associated with a marked decline in the activity of several lipogenic enzymes: glucose-6-phosphate dehydrogenase (50% decrease), 6-phosphogluconate dehydrogenase (11% decrease), malic enzyme (62% decrease) and fatty acid synthase (activity not detectable after pGH treatment). The pGH-dependent decline in insulin responsiveness was not associated with any change in the binding of insulin to intact adipocytes or to plasma membrane preparations. The insulin-stimulated tyrosine kinase activity of the wheat-germ agglutinin-purified receptors from pGH-treated adipocytes was not different from that in control adipocytes, except when high concentrations of insulin were employed. These findings establish that pGH elicits a number of metabolic effects in porcine adipocytes which collectively diminish the rate of lipid synthesis, and thereby contribute to the decrease in lipid deposition observed in pGH-treated pigs. Furthermore, the pGH-dependent impairment in insulin action appears to be mediated at some location distal to the receptor kinase step or in other signal pathway(s) which mediate the biological effects of insulin that are not dependent on activation of insulin receptor tyrosine kinase activity.

Effect of growth hormone on fatty acid synthase gene expression in porcine adipose tissue cultures

“…This is consistent with the FAS concentration having a decisive role in the maximum capacity of adipose tissue to synthesize fatty acids de novo (Clarke, 1993). Along with the changes in FAS activity, the amount of acetyl-CoA carboxylase (ACC) is also known to be reduced in both ruminant and non-ruminant animals treated with growth hormone (Vernon and Finley 1988;Magri et al, 1990;Vernon et al, 1991;Harris et al, 1993;Lanna et al, 1995;Lanna and Bauman, 1999). Vernon & Finley (1988) and Vernon et al (1991) observed that growth hormone inhibited the increase in lipogenic rates and ACC activity caused by insulin and dexamethasone and that this effect was abolished by actinomycin D. It appears that growth hormone inhibits the synthesis of key proteins in this process, including factors involved in the transduction of the signals controlling lipogenesis.…”

GH and insulin affect fatty acid synthase activity in isolated porcine adipocytes in culture without any modifications of sterol regulatory element binding protein-1 expression

“…The effect of GH on the regulation of fatty acid synthase (FAS), the key lipogenic enzyme that catalyzes all the reactions implicated in the conversion of acetyl-CoA and malonyl-CoA to palmitic acid, is especially well documented in pigs. Both activity and mRNA levels of FAS (Magri et al 1990, Mildner & Clarke 1991, Harris et al 1993, Donkin et al 1996 are decreased in adipose tissue of GH-treated animals. It has also been shown that GH attenuates the stimulatory effect of insulin on FAS gene transcription in 3T3-F442A adipocytes (Yin et al 1998(Yin et al , 2001.…”

“…Nevertheless, the mechanism by which GH interferes with insulin action on FAS gene transcription has not been elucidated yet. The GH-dependent impairment of insulin action does not result from changes in insulin binding or insulin receptor kinase activity in porcine adipocytes (Magri et al 1990). It is mediated by neither protein kinase A, protein kinase C nor Janus kinase-2 in 3T3-F442A adipocytes (Yin et al 2001).…”

“…This leads to a marked decrease in insulin-regulated events such as glucose transport and lipogenic enzyme activities as assessed in vivo (Dunshea et al 1992) and in vitro (e.g. Magri et al 1990, Harris et al 1993, Wang et al 1999. The effect of GH on the regulation of fatty acid synthase (FAS), the key lipogenic enzyme that catalyzes all the reactions implicated in the conversion of acetyl-CoA and malonyl-CoA to palmitic acid, is especially well documented in pigs.…”

Facilitative glucose transporters in livestock species

“…The ability of insulin to stimulate in vitro glucose transport rate in isolated adipocytes is also lower in pigs (+80%) [65], and sheep (+120-170%) [81] than in humans (+300%) [37] or in rats (+ approximately 500%) [80]. However, the effects of insulin on glucose uptake is age/body weight related, making comparison among species difficult.…”