Diabetes Mellitus is a disease of absolute or relative insulin insufficiency, i.e., DM1 (absolute) and DM2 (relative). The US now has a very costly epidemic of obesity with enormous negative impact on public health. This obesity very frequently leads to DM2. These patients are usually obese, diabetic, hypertensive, hyperlipidemic and at high risk of coronary artery disease or cerebral vascular accident. This complex of diseases is referred to as “metabolic syndrome”. The high insulin reflects an increased amount of insulin secreted by the pancreatic beta cells to try and overcome resistance in tissues such as skeletal muscle, adipose tissue, and liver.
There are a number of different mechanisms that have already been shown to be involved in induction of insulin resistance (IR). Of the cytokines proposed, the pro-inflammatory cytokine Tumor Necrosis Factor –alpha (TNF-a,-a) is produced under conditions of oxidative stress and inflammation in adipose tissue depots. TNF-a can be secreted into the circulation from central depots or it can be generated and act locally (autocrine effect) in fatty deposits in skeletal muscle, adipose tissue itself, or liver.
Today, despite continued investigative work, the important contribution of the cytokine TNF-a to IR is understood only superficially. We know that TNF-a promotes phosphorylation of serine on IRS-1, and that this inhibits insulin stimulated tyrosine phosphorylation of IRS-1 and results in failure of insulin signaling or IR.
For the last 15 years, we have been exploring the role of TNF-a in inducing IR in minimal deviant rat hepatoma (H411E -LC) cells, in tissue culture. These experiments have led us to define both the proteome and the genome for TNF-a induced IR in our LC model. In a recent publication, we emphasized the role of the phosphatase PTEN(phosphatase and tensin homolog deleted on chromosome 10) in both insulin signaling , IR and Apoptosis .
In contrast to these basic science experiments, conclusive proof of TNF-a involvement in IR in obese DM 2 patients has been minimal. On the clinical side, to examine the role of TNF alpha inducing IR in humans, in a retrospective chart study, we selected patients who were getting large doses of anti-TNF-a therapy for their Rheumatoid Arthritis (RA) or Crohn’s Disease (CR), who also had significant DM 2. These patients had been treated with anti-TNF-a therapy for 8-10 years. We found that after this time frame they had benefited substantially in terms of control of their DM, with an improvement in FBS of 21% after 10 years of anti-TNF-a RX .
Spurred on by this clinical study we again returned to our incubation studies in which we have obtained preliminary data demonstrating a critical role for PTEN, in TNF-a induced IR. PTEN is a phosphatase which terminates insulin signaling, thus producing IR. At the same time , PTEN is a positive effector in the Apoptosis Pathway. Similarly, TNF-a inhibits insulin action ( produces IR ) and facilitates Apoptosis.
In the current study, we analyzed the effects of various treatments including insulin, TNFa, 3- hydroxypicolinate vanadium (IV)complex (VO-OHpic), and siRNA PTEN on the levels of PTEN and p-Akt. While most phosphatase inhibitors have very general and nonspecific inhibitory effects, VO-OHpic is currently the most specific chemical inhibitor of PTEN due to its non-competitive mechanism of inhibition. This experiment used siRNA targeting PTEN mRNA to see the effects of a specific knockdown of PTEN on insulin signaling and IR. Our results show a central role for PTEN in TNFa induced IR in H-411E rat hepatoma cells with this form of IR to some extent mimicking IR as seen in obese human DM 2. Hence, we believe that PTEN is a “new”potential therapeutic target for DM2.