A link between iatrogenic hypoglycemia and cardiovascular (CV) events has been repeatedly documented in diabetes, particularly in individuals with type 2 diabetes (1). However, the direct contribution of hypoglycemia to adverse vascular outcomes is under debate, as low glucose levels can be a marker of ill health (2–5), which is supported by the bidirectional relationship between hypoglycemia and CV risk (6,7). Moreover, CV events can occur late following significant hypoglycemia (8,9), and therefore understanding the mechanistic pathways linking low glucose to vascular pathology are key to characterize individuals at risk, consequently facilitating tailored treatment modalities.
In this issue of Diabetes, Verhulst et al. (10) have investigated the effects of a single episode of experimental hypoglycemia (plasma glucose at 50 mg/dL [2.8 mmol/L] for 60 min) on acute and longer term (up to day 7) inflammation in 15 people with type 2 diabetes and compared results with those for 16 healthy volunteers matched for age, sex, and BMI. Previous work examined monocyte function under hypoglycemic conditions in individuals with type 1 diabetes (11,12) and healthy control subjects (13), but studies in type 2 diabetes remain scarce. Therefore, the current study is a welcome addition to the literature attempting to understand hypoglycemia-driven inflammation in type 2 diabetes.
Using clamp studies, the authors show an acute inflammatory response to hypoglycemia, coinciding with peak adrenaline response and characterized by an increase in granulocytes, monocytes, and lymphocytes (10). The increase in lymphocytes and monocytes lasted up to day 3 post-hypoglycemia in individuals with type 2 diabetes and up to day 7 in control subjects. Despite the similar increase in the two groups, monocyte counts were higher in type 2 diabetes during the preceding euglycemic phase of the clamp (baseline conditions), suggesting an ongoing background inflammatory response in this group. While persistent lymphocytosis can be explained by the long cellular life span (weeks), monocytes have a short life span (∼1 day) (14). As suggested by the authors, selective mobilization of CD16+ monocytes from the marginal pool in response to hypoglycemia-induced adrenergic stress may explain the acute rise in monocytes (15). The persistent rise, however, may be due to adrenaline-mediated egress of progenitor cells from bone marrow niches, with cells seeding in the spleen and providing a sustained supply of monocytes. This phenomenon has been demonstrated in a murine model following sympathetic activation after myocardial infarction and was dampened with β-blockade (16).
In a logical extension to the work, the investigators phenotyped monocytes in subgroups of eight people with type 2 diabetes and six control subjects; proinflammatory and proatherogenic CD16+ monocyte subset mobilization was demonstrated during hypoglycemia, consistent with earlier findings in individuals with type 1 diabetes (12) and healthy control subjects (13). While these cells are known to mobilize in a catecholamine-dependent fashion (17), likely part of the fight-or-flight response, mobilization in hypoglycemia becomes maladaptive, thus potentially enhancing vascular inflammation.
Cell receptor changes on monocytes were also studied, and reduction in CCR2, CD11b, and CD36 was demonstrated following hypoglycemia. The authors speculate this is due to increased cell adherence to vascular endothelium, as part of in vivo patrolling behavior, or reduced cell surface expression in newly released CD16+ monocytes. An alternative explanation is the differential expression of receptors, as the classical monocyte subset (CD14++CD16−) richly expresses CCR2, in contrast to the nonclassical cells (CD14+CD16++), which mainly express CX3CR1. Thus, reduction in CCR2 may reflect a phenotypic shift to the nonclassical subset during hypoglycemia, and, while not performed here, a demonstration of increased CX3CR1 expression would have been insightful. Hypoglycemia-primed monocytes demonstrated a trend toward augmented proinflammatory cytokine production after in vitro simulation, in agreement with hypoglycemic priming of innate immune cells (11,13). Interestingly, the proinflammatory stance of isolated monocytes is not explained by changes in glucose metabolism (with the caveat of small numbers), and therefore studies probing underlying mechanisms are warranted. A temporal and brief rise in some circulating proinflammatory proteins (interleukin-6 and MCP-1) with sustained elevation of others (hsCRP) is noteworthy and may have future research implications.
The clinical implications of this study are subject to limitations of experimental design and general challenges in this field. Given some subtle differences in comparisons of individuals with type 2 diabetes to control subjects, it is possible that those with established CV disease, a group where hypoglycemia clamp studies are ethically questionable, behave differently, consequently making general applicability of the findings problematic. Future observational studies using continuous glucose monitoring may be able to elucidate the inflammatory response following hypoglycemia in those with type 2 diabetes and established CV disease. The clamp methodology used in this study is considered the gold standard for experimental hypoglycemia, but, given the anti-inflammatory properties of insulin, additional hyperinsulinemic-euglycemic clamp studies would have been useful (18). Moreover, while the authors report glucose infusion rates within each study group, data on corresponding insulin levels at the end of the clamp would have shed more light on small differences observed between control subjects and patients.
This study has certainly advanced our understanding by demonstrating a prolonged inflammatory response to hypoglycemia in type 2 diabetes, akin to the prothrombotic response in a similar group of patients (18). It should be noted that there are multiple putative mechanisms likely to contribute to increased atherothrombotic risk with hypoglycemia and other plausible pathways that require further research (Fig. 1).
However, several questions remain unanswered in relation to hypoglycemia and CV disease. First, what is the clinical phenotype of individuals where hypoglycemia is potentially directly driving short- and long-term CV risk as opposed to being a risk marker? Second, what is the role of low glucose in driving cellular CV stress independent of catecholamines? Third, what is the contribution of glucose variability, such as rebound hyperglycemia following treatment of hypoglycemia, in driving vascular risk? Finally, other than avoidance of low glucose levels, what could be a potential therapeutic target(s) to mitigate the deleterious vascular effects of hypoglycemia?
While waiting for further research to address these questions, individualization of glycemic goals and avoidance of hypoglycemia remain paramount to minimize CV morbidity and mortality in people with type 2 diabetes.
See accompanying article, p. 2716.
Duality of Interest. A.I. received speaker fees from AstraZeneca and educational grant support from Sanofi and also received research support from Dexcom Inc. R.F.S. received research grants from AstraZeneca and PlaqueTec and honoraria from AstraZeneca and was a consultant to or on an advisory board for AstraZeneca, Actelion, Avacta Life Sciences, Bayer, Bristol Myers Squibb/Pfizer, Idorsia, Novartis, and The Medicines Company. R.A.A. received institutional research grants from Abbott, Avacta Life Sciences, Bayer, Eli Lilly, Novo Nordisk, Roche, and Takeda and received honoraria or education support from or was a consultant to Abbott, AstraZeneca, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Eli Lilly, GlaxoSmithKline, Menarini Group, Merck Sharp & Dohme, Novo Nordisk, and Takeda. No other potential conflicts of interest relevant to this article were reported.