Somatotropin antagonism of insulin‐stimulated glucose utilization in 3T3‐L1 adipocytes

Abstract

It is well established that somatotropin (GH) antagonizes insulin action in vivo and that supraphysiologic concentrations of GH frequently result in insulin resistance and glucose intolerance. However, the demonstration of an anti-insulin activity by GH in vitro has been difficult. This study, therefore, set out to determine whether cultures of 3T3-L1 adipocytes could be used to examine the anti-insulin activity of GH. The ability of insulin to stimulate glucose utilization by 3T3-L1 adipocytes increases approximately five-fold during the first 4 days following treatment of the cells with a differentiation medium. It was found that glucose utilization in 3T3-L1 adipocytes is regulated in a reciprocal fashion by insulin and GH. Bovine or human GH directly inhibit up to 50% of insulin-stimulated [14C]-glucose incorporation into lipids in a concentration-dependent manner. The 3T3-L1 sensitivity to GH appears to be at the maximum (50% inhibition of an insulin response) immediately following removal of the cells from the differentiation medium and remains essentially constant during the subsequent 4 days. The GH inhibition of insulin action does not appear to be due GH enhancement of cellular degradation of insulin, competitive binding of GH to the insulin receptor, or GH-induced decrease in cell number. The 3T3-L1 adipocyte system appears to be a sensitive and reliable in vitro model with which to study the molecular mechanisms involved in both GH antagonism of insulin action and development of hormone responsiveness during cellular differentiation into adipocytes.

Selective Modification of Recombinant Bovine Placental Lactogen by Site-directed Mutagenesis at Its C Terminus

“…However, in a homologous lactogenic receptormediated bioassay using bovine mammary gland explants, we found that the native bPL is also as potent as hGH or bPRL (7). Binding experiments to various microsomal fractions revealed that bPL binds with high affinity to prolactin receptors and to somatogenic receptors (2,4,5). In addition, we have documented that bPL binds also to unique receptors in bovine endometrium (8).…”

“…Deglycosylation of native bPL had no effect on PRL-like mitogenic activity in an Nb 2 lymphoma cell proliferation assay in which bPL was equally potent to human (h)GH, bPRL, and ovine (o)PRL, and exhibited only slightly reduced binding to bGH receptors (4). Recombinant bPL was also equally potent to hGH or oGH in somatogenic receptor-mediated 3T3-L1 or 3T3-F442A preadipocyte bioassays (5,6). However, in a homologous lactogenic receptormediated bioassay using bovine mammary gland explants, we found that the native bPL is also as potent as hGH or bPRL (7).…”

The Pattern of Growth Hormone Delivery to Peripheral Tissues Determines Insulin-Like Growth Factor-1 and Lipolytic Responses in Obese Subjects

“…In contrast, continuous GH component appears to be predominant in inducing hepatic IGF-I mRNA (6), CYP 3A4 enzymes (10,11), and GH receptors (12). In humans, GH is known to have multiple effects on peripheral tissues, such as promoting somatic growth through generation of IGF-I (6), increasing lipolysis (13), and having antiinsulin effects (14). However, whether these actions were mediated through global GH output or through a pattern-specific exposure of GH pulsatility is largely unknown.…”

The effects of protease inhibitors on basal and insulin-stimulated lipid metabolism, insulin binding, and signaling

“…The cells were then incubated with insulin in MEM for 6 h. d -[ 14 C(U)]glucose (2.2 dpm/pmol) was then added for 18 h at 37 Њ C in a humidified incubator with 5% C0 2 . The cells were then placed on ice and the medium removed after which the monolayers were washed 3 ϫ with PBS at 4 Њ C. Triglycerides were then extracted for 30 min 2 ϫ with 1 ml isopropanol-heptane (2:3; v/v) and collected after extraction with alum (14,15). The incorporation of d -[ 14 C]glucose into triglycerides was determined by scintillation counting.…”