Insulin resistance and β-cell dysfunction are largely regarded as two distinct processes participating in the pathogenesis of T2D. Recent data demonstrate that insulin itself augments GSIS in isolated human islets (2
), rodent models (8
), and healthy humans in vivo (20
), suggesting an important physiologic role of insulin to modulate β-cell function in humans in vivo. Here we demonstrate that insulin’s affects to augment GSIS are diminished in insulin-resistant persons with IGT and T2D. Thus, insulin resistance at the level of the β-cell could represent a novel mechanism coupling insulin resistance and β-cell dysfunction.
Our findings in humans are consistent with the multiple genetic rodent studies demonstrating insulin’s positive physiologic affects on β-cell mass and function. Mice with βIRKO manifest diminished GSIS and progressive glucose intolerance progressing to overt diabetes in some animals (8
). Likewise, deletion of insulin signaling proteins IRS-1, IRS-2, phosphoinositide 3-kinase (PI3K), or the serine/threonine protein kinase Akt2 alters glucose sensing and β-cell growth (1
). Insulin and the insulin-mimetic compound L-783281 enhance insulin synthesis in isolated rodent islets (33
). Exogenous insulin also leads to increased intracellular Ca2+
, suggesting insulin may promote its own secretion by mobilizing Ca2+
from the endoplasmic reticulum (35
). Higher insulin concentrations may induce β-cell glucokinase expression, potentiating glucose-stimulated insulin release. Moreover, glucose affects on β-cell growth and survival require activation of insulin signaling proteins (36
), and hyperglycemia-induced reduction in expression of insulin receptor and activation of the proapoptotic cascade is physiologically antagonized by insulin signaling through the insulin signaling (IRS-PI3K-AKT-Bad) cascade (37
). However, while multiple studies demonstrate an important role for the insulin signaling pathway in β-cell development and function, the physiologic role of insulin signaling on β-cell function has not previously been clearly demonstrated in humans in vivo.
Recently, we developed a novel methodology for direct measurement of endogenous insulin secretion in the presence of exogenous insulin by infusing an insulin analog that is biologically equivalent to—but can be discriminated immunologically from—endogenous insulin. In this way we can use a selective immunoassay to accurately distinguish endogenous insulin from the infused product (20
). We have previously reported an effect of insulin to augment GSIS in healthy humans (20
). To our knowledge, our dynamic study is the first to demonstrate that the effect of insulin to augment secretion following glucose stimulation is diminished in insulin-resistant persons. Our data are consistent with the recent findings of Anderwald et al. (21
), which demonstrated at euglycemia, in the absence of additional glucose stimulation, insulin infusion leads to increased insulin secretion in insulin-sensitive—but not insulin-resistant—subjects. However, previous studies have not yielded consistent findings. Our findings were most apparent during the highest rate of glucose infusion, which could be due to either glucose dose or duration of β-cell exposure. Several factors could account for the largely negative findings of older studies (11
). First, most prior studies were performed at euglycemia and therefore did not evaluate the effect of glucose-stimulated insulin secretion. Second, the presence or magnitude of insulin resistance in subjects was not accounted for in many prior studies, which may have influenced the results given our findings that the effect is altered in insulin resistance. Third, most studies relied on C-peptide measures to estimate β-cell function as a substantial portion of endogenously secreted insulin is sequestered and/or cleared by insulin receptor–mediated mechanism(s) in the liver (40
), but C-peptide clearance measured using stable isotopic infusion methods is increased during hyperinsulinemia (20
), and intracellular insulin processing may lead to altered insulin to C-peptide secretion rates (41
), which could bias conclusions toward an underestimation of change in β-cell response. Finally, most but not all prior studies (42
) were unable to distinguish exogenous from endogenous insulin that may have masked the accurate interpretation of dynamic secretion.
We find that both insulin-resistant cohorts have reduced insulin potentiation of GSIS compared with healthy control subjects, but it is worth distinguishing that the potentiation effect was diminished in the T2D group and altogether absent in the IGT cohort. This was unanticipated as the T2D group, with a more advanced stage of disease, had been predicted to have the greatest effect reduction. The IGT group was more insulin resistant and tended to be more overweight than the T2D group. The T2D group also had relatively mild disease as evidenced by treatment by lifestyle without pharmacologic intervention. Consequently, insulin augmentation of the β-cell response to glucose in our study was in fact maximally attenuated in the most insulin-resistant group (IGT). We speculate additional lifestyle or other environmental factors could contribute to between-group differences. Multiple physiologic and pathophysiologic processes regulate β-cell function and mass. For example, it is possible that insulin signaling pathways cross-talk with incretin signaling, and additional studies will reveal whether this cross-talk is differentially regulated in patients with IGT versus those with T2D. It is also possible that T2D could have reduced β-cell mass or greater magnitude insulin resistance in peripheral tissues relative to that of the β-cell and, thus, manifest more advanced disease (dysglycemia) with lesser magnitude of β-cell insulin resistance. However, we demonstrate in two independent insulin-resistant cohorts that the effect of insulin to potentiate glucose-stimulated insulin secretion is reduced.
Since the T2D group was more dysglycemic, our findings further suggest that the attenuation of insulin potentiation of glucose-stimulated insulin secretion is not due to early glucose dysregulation. Furthermore, if the effect of insulin to augment β-cell function was mediated indirectly by providing β-cell rest, one might have anticipated the effect to be similar in the IGT and T2D cohorts, which was not seen.
Moreover, the IGT cohort demonstrated significantly higher insulin levels at each glucose concentration achieved during the clamp, suggesting that this cohort had relatively preserved β-cell function with retained capacity to compensate for insulin resistance by augmenting insulin secretion. In contrast, the T2D cohort exhibited more profound β-cell dysfunction despite better peripheral insulin sensitivity. We hypothesize that the increased insulin concentrations manifest in the IGT cohort already provided a maximum stimulatory effect on the β-cell such that no further augmentation could be demonstrated during a hyperinsulinemic clamp. The preserved, though diminished, effect observed in the T2D cohort may suggest that the effect of insulin to stimulate GSIS is decreased but not entirely lost as β-cell dysfunction progresses, and there is loss of glucose sensitivity.
Glucose could play an important role in modulating the effect of insulin to potentiate β-cell function. Lower magnitude of glycemic stimulation could result in reduced β-cell responsiveness to insulin. However, this does not appear to have been the case in this study as glucose concentrations achieved during graded glucose infusions were highest in the insulin-resistant cohorts. In addition, glucotoxicity could diminish the effects of insulin to potentiate GSIS. As expected, our insulin-resistant cohorts had higher fasting and clamp glucoses than healthy control subjects. However, the IGT cohort had a greater reduction in insulin potentiation of GSIS but lower glycemia compared with T2D, so glucotoxicity does not accurately explain our study findings.
Multiple other metabolic factors could be significant in mediating a diminished effect of insulin on β-cell function. We have studied the effect of FFAs, and insulin augmentation of GSIS cannot be attributed to insulin suppression of FFA in healthy persons (43
). Glucagon is another potent nonglucose secretagogue that promotes insulin secretion independent of the effects on glucose (44
) and could have altered β-cell response. However, glucagon concentrations were reduced during hyperinsulinemic clamps, suggesting physiologic insulin action at the level of the α-cell (28
). Likewise, low serum potassium has been associated with attenuated insulin secretion (46
), but it is unlikely that the small differences in serum potassium seen in our cohorts explain the observed results because concentrations were similarly modestly reduced in the control subjects. Based on our findings, we cannot exclude the possibility that insulin potentiation of GSIS could be secondary to short β-cell rest with insulin administration during the period of isoglycemic-hyperinsulinemic pre-exposure rather than directly modulated by insulin signaling, and that the interval of rest was insufficient to promote potentiation of GSIS in the insulin-resistant cohorts. Additional important factors that were not assessed in our studies could also contribute to β-cell dysfunction, including inflammation (47
) and islet amyloid deposition (48
We recognize our human in vivo experiments are limited because we cannot demonstrate dynamic changes in insulin signaling in β-cells directly. The differences in weight and degree of insulin resistance in the IGT versus T2D cohorts are additional study limitations. Finally, the two insulin-resistant cohorts were older than the healthy control subjects, and we cannot exclude confounding effects of age on insulin dynamics. Further studies are warranted to better understand how insulin modulates β-cell function in insulin-sensitive and -resistant populations.
In conclusion, we have demonstrated that while the β-cell is insulin responsive and pre-exposure to insulin enhances glucose-stimulated insulin secretion in healthy humans, this effect is reduced in states of insulin resistance. Thus, we demonstrate physiologically that the β-cell is an insulin-responsive tissue in humans in vivo. Our findings are consistent with a growing body of literature that suggests that pancreatic β-cell dysfunction could be caused by a defect in insulin signaling within β-cells, and this β-cell insulin resistance may lead to a loss of β-cell function and/or mass, contributing to relative hypoinsulinemia in response to glucose stimulation and subsequent hyperglycemia. Our findings also suggest that therapies for type 2 diabetes, particularly insulin-sensitizing agents, may exert their effect at least in part by restoring β-cell insulin sensitivity, therefore enhancing the capacity of the β-cell to secrete insulin as a response to glucose stimulation.