J. Lee Franklin

Anti-Inflammatory Action of Insulin via Induction of Gadd45-β Transcription by the mTOR Signaling Pathway.

Insulin regulates a large number of genes in a tissue-specific manner. We have previously identified genes modulated by insulin in the liver and in liver-derived cells that have not yet been characterized as insulin regulated, and results of these previous studies indicated that numerous genes are induced by insulin via the MEK-ERK pathway. We now describe new studies indicating that Gadd45-β can be induced by acute insulin treatment. Although other regulators of Gadd45-β expression may utilize the MEK-ERK pathway, the data indicate that insulin utilizes signaling pathways separate from either MEK-ERK, PI3-K, or p38 signaling pathways in the regulation of Gadd45-β transcription. Our findings show that activation of a downstream effector of multiple signaling pathways, mTOR, was required for insulin-induction of Gadd45-β gene transcription. Increased expression of Gadd45-β can inhibit c-Jun N-terminal kinase (JNK) activity. Since TNFα is increased during inflammation, and acts, at least in part, via the JNK signaling pathway, insulin induction of Gadd45-β suggests a mechanism for the anti-inflammatory actions of insulin.

Polytrauma-induced hepatic stress response and the development of liver insulin resistance

Insulin resistance and metabolic dysfunction are common following injury. Polytrauma is defined as combined injuries to more than one body part or organ system, and is common in modern warfare, as well as automobile and industrial accidents. Polytrauma can include any combination of burn injury, fracture, hemorrhage, trauma to the extremities, and blunt or penetrating trauma. Multiple minor injuries are often more deleterious than a more severe single injury. To investigate the mechanisms of development of insulin resistance following injury, we have developed a rat model of polytrauma which combined soft tissue trauma with burn injury and penetrating gastrointestinal (GI) trauma. Male Sprague-Dawley rats were subjected to a laparotomy plus either a 15-18% total body surface area scald burn or a single puncture of the cecum (CLP) with a G30 needle, or the combination of both burn and CLP injuries (polytrauma). We examined the effects of polytrauma which increased markers of hepatic endoplasmic reticulum (ER) stress, and increased hepatic Trib3 mRNA levels coincident with reduced insulin-inducible insulin signaling. Phosphorylation/activation of the insulin receptor (IR) and AKT were decreased at 24, but not 6h following polytrauma. These results demonstrate a complex, time-dependent development of hepatic ER-stress and a diminished response to insulin, which were among the pathological sequelae following polytrauma.

Insulin attenuates TNFα-induced hemopexin mRNA: An anti-inflammatory action of insulin in rat H4IIE hepatoma cells

Proinflammatory cytokines, including TNF-α and IL-6, can contribute to insulin resistance. Conversely, insulin has some actions that can be considered anti-inflammatory. Hemopexin is a Class 2 acute phase reactant and control of its transcription is predominantly regulated by IL-6, with TNF-α and IL-1β also inducing hemopexin gene expression. Thus, we asked whether insulin could inhibit the ability of TNF-α to stimulate hemopexin mRNA expression. In cultured rat hepatoma (H4IIE) cells, TNF-α significantly increased hemopexin mRNA accumulation. The TNF-α-induced increase of hemopexin mRNA was dramatically attenuated by insulin, even though TNF-α reduced peak insulin activation of ERK. Thus, even though TNF-α can contribute to insulin resistance, the residual insulin response was still able to counteract TNF-α actions.

Prenylation differentially inhibits insulin-dependent immediate early gene mRNA expression.

Increased activity of prenyl transferases is observed in pathological states of insulin resistance, diabetes, and obesity. Thus, functional inhibitors of farnesyl transferase (FTase) and geranylgeranyl transferase (GGTase) may be promising therapeutic treatments. We previously identified insulin responsive genes from a rat H4IIE hepatoma cell cDNA library, including β-actin, EGR1, Pip92, c-fos, and Hsp60. In the present study, we investigated whether acute treatment with FTase and GGTase inhibitors would alter insulin responsive gene initiation and/or elongation rates. We observed differential regulation of insulin responsive gene expression, suggesting a differential sensitivity of these genes to one or both of the specific protein prenylation inhibitors.