Insulin receptor-independent upregulation of cellular glucose uptake

Abstract

Background Cellular glucose uptake can be enhanced by upregulating Ras signaling in either insulin-dependent or - independent manner. In presence of insulin and intact insulin signaling, Ras has a negligible role in glucose uptake. Conversely, when insulin signaling is impaired in obesity or diabetes, the insulin-independent Ras pathway may be valuable for enhancing glucose disposal. We previously reported that Ad36, a human adenovirus, enhances cellular glucose uptake by upregulating the Ras/Glut4 pathway. Here, we investigated if Ad36-upregulated Ras via the insulin-independent pathway, to enhance glucose uptake. Furthermore, uncontrolled upregulation of Ras is linked with oncogenic cell transformation, if the tumor-suppressor gene p53 is also downregulated. Hence, we determined if upregulation of Ras by Ad36 would induce oncogenic cell transformation. Finally, we determined the relevance of Ad36 to insulin resistance in humans. Methods Insulin receptor (IR) was knocked down with small interfering RNA in 3T3-L1 adipocytes, to determine if Ad36 increases the Ras/Glut4 pathway and glucose uptake without IR-signaling. Next, the effects of Ad36 on cell transformation and p53 abundance were determined. Finally, overweight or obese women were screened for seropositivity to Ad36, as an indicator of natural Ad36 infection. Associations of Ad36 infection with adiposity and C-reactive proteins (CRPs)—two key markers of insulin resistance, and with glucose disposal, were determined. Results Unaffected by IR knock-down, Ad36 significantly increased the Ras pathway, Glut4 translocation and glucose uptake in 3T3-L1 adipocytes. Despite Ras upregulation, Ad36 did not transform 3T3-L1 cells. This may be because Ad36 significantly increased p53 protein in 3T3-L1 cells or mice adipose tissue. Ad36 seropositivity was associated with greater adiposity and CRP levels, yet a significantly higher systemic glucose disposal rate. Conclusions Overall, the study offers Ras/Glut4 pathway as an alternate to enhance glucose disposal when insulin signaling is impaired, and, importantly, provides Ad36 as a tool to understand the modulation of that pathway.

Long-Term Changes in Adiposity and Glycemic Control Are Associated With Past Adenovirus Infection

“…Although the results are consistent with our expectations based on previous reports by us and others (2,7,13,38,39), the seropositivity-based differences in adiposity and glycemic control are modest in magnitude. Moreover, we investigated the association of Ad36 seropositivity with adiposity and glycemic control in three BMI subgroups and both sexes.…”

E4orf1 induction in adipose tissue promotes insulin-independent signaling in the adipocyte

“…Recent in vitro studies have demonstrated E4orf1 to exert insulin-independent or ‘insulin-sparing’ actions that bypass insulin receptor signaling to promote cellular glucose uptake in adipocytes. Studies utilizing 3T3-L1 adipocytes revealed that E4orf1 directly inhibits IRS1/2 phosphorylation in the insulin signaling pathway [20,21] . Synergistically, and unaffected by an insulin receptor knockdown, E4orf1 enhances Ras activity to activate PI3K and subsequent phosphorylation of the PI3K downstream target PKB/Akt to promote membrane translocation of Glut4, in the absence of insulin [20–23] .…”

Harnessing the beneficial properties of adipogenic microbes for improving human health

“…Ad36 may maximize the adipose tissue capacity for storage and thus reduce ectopic fat accumulation and lipotoxicity. Ad36 appears to utilize the ‘Ras‐PI3K‐Akt‐Glut 4 axis’ pathway, independent of the proximal insulin signalling, to increase cellular glucose uptake . In addition to this signalling, roles of mammalian target of rapamycin 2 activation and glycogen synthase kinase‐3α/β inhibition by Ad36 E4orf1 expression also are suggested (unpublished).…”

“…In this regard, in vitro studies indicate that the adipogenic and glycaemic effects of Ad36 can potentially be uncoupled . Moreover, cell signalling studies indicate that Ad36 enhances cellular glucose uptake independent of proximal insulin signalling, via its E4orf1 gene . In fact, E4orf1 is likely to have insulin‐sparing action ; however, the in vivo efficacy and safety of E4orf1 in improving hyperglycaemia remains unknown.…”

“…Ad36 was shown to stimulate the Ras pathway, thereby leading to excess glucose transport into cells, particularly fat cells (60). Also, fatty acid synthase (FAS) is stimulated by the virus, and the combination of excess substrate (glucose) and FAS results in excess fat deposition in adipocytes and other human cell lines (12,16,23,24,(60)(61)(62)(63)(64)(65)(66)(67)(68)(69). Thus, Ad36 was thought to produce obesity by acting directly on adipocytes (inducing de novo lipogenesis).…”