Insulin Analogues and Cancer Risk: Cause for Concern or Cause Célèbre?

M. Pollak; D. Russell-Jones

Disclosures

Int J Clin Pract. 2010;65(5):628-636. 

In This Article

Investigating the Possible Mechanisms: Insulin-like Receptors, Insulin Analogues and Mitogenicity

The most plausible hypothesis concerning the mechanism underlying the potential link between insulin and related peptide hormones and cancer growth is that these act through the insulin and IGF-1 receptors to stimulate cell growth and inhibit apoptosis.[19] Studies in this area have been facilitated by the availability of a range of recombinant insulin analogues differing in receptor-binding characteristics.[7,39] Of course, the goals of developing modified insulins was to favourably alter pharmacokinetic profiles, rather than alter receptor binding. Slieker and co-workers, examining a series of analogues with modifications in the B10 and B26–30 regions of the insulin molecule, found that the ability to promote growth of human mammary epithelial cells in vitro was related to the affinity for the IGF-1 receptor.[39] Meanwhile, Hansen and colleagues found that occupancy time at the insulin receptor was also correlated with mitogenic potential: a marked increase in mitogenicity (up to sevenfold) was observed with analogues for which the dissociation constant from the insulin receptor was < 40% that of native human insulin.[8] Among the insulin analogues tested by Hansen and colleagues was X10, remarkable for a low insulin receptor off-rate (14% of that of human insulin) and an IGF-1 receptor affinity almost sixfold that of human insulin.[7,8] Insulin X10 has been shown to have a mitogenic potency in human osteosarcoma (Saos/B10) cells approaching 10 times that of human insulin (Table 1).[7] X10 was also more potent than insulin in promoting proliferation of the human breast cancer cell line MCF7[40] and preclinical studies with X10 indicated an increased tumorigenic potential in female Sprague–Dawley rats,[41] leading to discontinuation of its clinical development.

Together, these data provide evidence that mitogenic effects of insulin analogues may be mediated via inappropriate IGF-1R and/or insulin receptor signalling. Absolute proof of this mechanism would be provided by experiments that showed any cancer-promoting properties of an insulin analogue in animal models to be abolished or attenuated by IGF-1R blockade, and/or by competitive displacement from the insulin receptor. Such data do not yet exist; however, circumstantial support was provided by a study by Shukla and colleagues using human mammary cell lines sensitive to insulin.[42] In this study, benign (MCF10A) and malignant (MCF7) cell lines were exposed to a series of commercially available insulins. Compared with human insulin, insulin glargine showed a significant increase in proliferative potential of 1.6- to 3.1-fold in MCF7 cells, the mitogenicity of the other insulins being similar to or lower than human insulin. Insulin glargine was seen to strongly activate the MAPK pathway in the MCF7 cell line; inhibition of this pathway using a specific MAPK inhibitor completely suppressed this proliferative activity. Importantly, knockdown of the IGF-1 receptor, but not knockdown of the insulin receptor (by transfection with IGF-1R siRNA and IR siRNA respectively), also abolished proliferation of the cell line in response to insulin glargine, strongly implicating IGF-1 receptor mediated activation of the MAPK pathway as the driver of cell proliferation.[42] However, in other situations, it is conceivable that the degree of insulin receptor activation will be critical, and that peak insulin levels and/or the time course of insulin exposure will be important. There are important gaps in knowledge relating to differences between various insulins, with respect to activation of hybrid receptors, and also with respect to differences (if any) between the insulin receptor isoforms.

Given present knowledge, it is important that the receptor binding properties of insulin analogues are tested to ensure that the balance of IGF-IR:IR affinity is not disturbed, and that insulin receptor off-rate is not unduly reduced. It is of interest in the context of the findings of Shukla and colleagues,[42] and of the epidemiological findings discussed below that insulin glargine has an increased affinity for the IGF-1 compared with native human insulin in cells expressing this receptor.[7] Using the Saos B10 cell line, Kurtzhals and colleagues found a mitogenic potential for insulin glargine 7.83 times that of human insulin (Table 1), a finding consistent with data obtained by Kohn and colleagues, who reported a 4.1-fold increased proliferation of human mammary epithelial cells with insulin glargine vs. human insulin.[43]

Insulin glargine is widely prescribed and, as such, underwent rigorous testing in animal and clinical studies, and it should be noted that no increased cancer incidence was seen when it was tested in Sprague–Dawley rats,[44] in contrast to the findings with insulin X10; nor was any excess cancer risk reported during the insulin glargine clinical development programme. However, carcinogenicity studies in Sprague–Dawley rats using insulin glargine could not be performed at the doses where X10 manifested increased cancer incidence, because of the prolonged hypoglycaemic effect of insulin glargine.[45] Furthermore, as occult cancers are common in adults,[46,47] it can be argued that it is appropriate that safety studies of hormonal agents with respect to neoplastic disease should not be confined to carcinogenesis endpoints, but should also include assessment of effects on pre-existing tumours. This is in keeping with the highly plausible view that hormones may influence neoplastic disease by stimulating proliferation, rather than by acting as mutagens.

The difficulties of performing animal studies and the need to screen a large number of candidate molecules mean that in vitro studies remain an important tool for evaluating the relative mitogenic potential of insulin analogues. It is important that receptor binding is appropriately tested as there are two pitfalls that may give falsely reassuring data. Firstly, mitogenic properties are manifest in some cell lines and not in others. The effects tend to be greatest in cells that express a relatively high proportion of IGF-1 receptors compared with insulin receptors.[8,42] Therefore, mitogenicity must be tested in sensitive cell lines.

Secondly, cell lines responsive to insulin or IGF-1 have a maximum mitogenic response to these hormones that can be elicited by nearly all insulin-like molecules (including insulin itself and IGF-1) when given at sufficiently high concentrations, with the effective concentration differing between hormones (see Figure 1). It follows that the relative mitogenic potency of different insulins can only be assessed if full dose-response curves are constructed for parameters of metabolic and mitogenic activity. The dose required to elicit ED50 responses can then be calculated and the ratio of these for mitogenic vs. metabolic activity discerned. If dose-response curves are parallel, as is normal, this value will be a constant over the effective dose-response range of the insulins. Single-dose experiments will give very different results according to where the dose chosen relates to the dose-response curve (Figure 1). If the concentration is close to the maximum (or minimum) response level then no difference between insulins will be discerned, and this could again be falsely reassuring. These issues were illustrated by a study reported by Weinstein and colleagues, in which three cell lines were exposed to a single 100-nM level of four insulins (human insulin, insulin lispro, insulin detemir and insulin glargine); at this supraphysiological level, few differences were evident between insulins, whereas in a separate signalling experiment performed at a 10-fold lower concentration, activation of both insulin receptor and IGF-1 receptor was seen with insulin glargine, whereas insulin detemir activated the insulin receptor alone.[48]

Figure 1.

Schematic showing hypothetical insulin dose-response for two insulins that differ in their potency to evoke a response. This illustrates how single-dose assessments (vertical distances) will give varying results depending on the dose chosen, whereas differences based on ED50 (horizontal distance) will apply across the whole tissue-respondent portion of the dose-response curve

Comments

3090D553-9492-4563-8681-AD288FA52ACE
Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.
Post as:

processing....