Introduction
Recent retrospective studies suggest that patients with type 2 diabetes who use the insulin analogue insulin glargine (A21Gly,B31Arg,B32Arg human insulin) might have an increased risk of developing cancer. Hemkens et al. found a positive association between daily insulin dosage and the incidence of malignant neoplasms. Interestingly, this association was present for human insulin and insulin analogues such as glargine, lispro (B28Lys,B29Pro human insulin) and aspart (B28Asp human insulin). Moreover, after adjusting for insulin dose, they found a dose-dependently increased risk of cancer in patients treated with insulin glargine compared with patients treated with human insulin with an adjusted hazard ratio of 1.31 (95% CI 1.20–1.42) for a daily dose of 50 IU [
1]. In another report, Jonasson et al. mentioned that patients taking insulin glargine had an almost twofold increased risk of breast cancer compared with patients using other types of insulin [
2]. The SDRN Epidemiology Group observed a similarly increased risk, but attributed this to an allocation bias [
3]. A study by Currie et al. showed no association between insulin analogues and cancer progression as compared with human insulin, but did show that patients using any kind of insulin had a higher risk of developing adenocarcinomas than those on metformin [
4].
An important point to elucidate in this context is the pathophysiological mechanism through which insulin could be involved in tumour progression. In their editorial accompanying the above-mentioned articles, Smith and Gale [
5] noted that it is surprising that these studies were able to detect differences in cancer rates after only a few years of insulin exposure, because cancer takes many years to develop. Supported by existing literature, they suggest that, if insulin does indeed influence cancer incidence, this is possibly due to stimulatory effects on pre-existing malignant foci rather than to de-novo carcinogenesis. Insulin is known to augment mitogenesis and proliferation of distinctive cell types and its effect can be mediated through insulin receptors (IRs), insulin like growth factor-1 (IGF-1) receptors (IGF-1Rs) or hybrid IR-IGF-1Rs. IRs and IGF-1Rs are frequently present on pre-existing tumour cells and this could give insulin a molecular pathway to contribute to tumour expansion [
5]. However, we recently showed that IRs are also abundantly present on angiogenic structures in human atherosclerotic plaques suggesting a biological role of insulin on local angiogenesis [
6]. Furthermore, it has been shown that insulin is involved in angiogenesis during wound healing [
7]. We therefore hypothesised that such a mechanism might also be involved in tumour expansion and tumour growth.
We analysed whether IRs are expressed on angiogenic structures in adenocarcinomas. In addition, we evaluated the effect of commercially available human insulin and insulin analogues on capillary-like tube formation of human microvascular endothelial cells (hMVEC) in vitro.
Discussion
This study suggests that distinctive insulin compounds (exceeding a certain daily dose) could contribute to tumour growth by enhancing local angiogenesis, possibly parallel to the established direct mitogenic and proliferative effects of insulin on tumours [
5].
Our observations showed that common adenocarcinomas, which frequently afflict patients with type 2 diabetes, contain large numbers of neovessels and that these vessels abundantly express IRs, independent of tumour type. This indicates that local expression of IR on neovessels is a rather general phenomenon in the process of tumour angiogenesis and that high volumes of exogenous insulin could have side effects. Stimulation of these receptors with insulin could promote angiogenesis and subsequent tumour progression. It should be noted that tumours can have high expression of hybrid receptors and particularly of the IR-A isotype, which is not readily detectable with currently available antibodies. Therefore, the IR expression detected may be an underestimate.
The effect of insulin on angiogenesis has been observed before. Liu et al. [
7] recently reported that insulin stimulates hMVEC migration and two-dimensional Matrigel tube formation exclusively through the IR. Our in vitro experiments showed that not only the long-acting insulin analogue glargine enhances capillary-like tube formation, but also all other evaluated insulin compounds. Hence, insulin glargine is not an exclusive candidate for an assumed relationship between increased tumour growth and insulin use in patients with type 2 diabetes.
It is likely that the added insulin at high concentrations may act via both IRs and IGF-1Rs. This might be the case for insulin concentrations of 1 × 10
−6 and 1 × 10
−8, but not for 1 × 10
−9 mol/l. Moreover, differences in capillary-like tube formation between insulin detemir and insulin glargine were not found. It is known that insulin glargine has a much higher affinity for the IGF-1R compared with insulin detemir [
11]. Therefore, involvement of the IGF-1R becomes less probable. Furthermore, since we found increased IR expression and no increased IGF-1R expression on microvessels within adenocarcinomas, we assume that in vivo the insulin × IR interactions are likely to be of paramount importance for insulin-mediated angiogenic effects in adenocarcinomas.
We would have preferred to use human tumour-derived endothelial cells, instead of foreskin-derived hMVEC for our in vitro experiments. However, to our knowledge, no published reports describe the isolation and long-term culturing of endothelial cells from human tumour tissue. Since endothelial cells within tumour tissue are of benign origin and adenocarcinomas normally contain much fibrin, we consider our in vitro angiogenesis assay, which looks at tube formation of benign hMVEC into a three-dimensional fibrin matrix, to be appropriate.
In short, the current results suggest that insulin and insulin analogues play a biological role in tumour growth by stimulating local IRs with a subsequent increase in angiogenesis. Without any doubt, additional prospective studies will be required in order to confirm a possible relationship between insulin therapy and tumour progression either by direct effects of insulin on tumour growth or stimulation of local angiogenesis.
Acknowledgements
Th. B. Twickler received a grant from the Netherlands Organization of Science (NWO/ZonMW). J. H. von der Thüsen is an awardee of a Dutch Heart Foundation Dekker grant (NHS 2008 T050) and P. Koolwijk received a grant from the Dutch Program of Tissue Engineering (STW/DPTE, grant number BGT.6733).
Open AccessThis is an open access article distributed under the terms of the Creative Commons Attribution Noncommercial License (
https://creativecommons.org/licenses/by-nc/2.0), which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.