Statins impair glucose uptake in tumor cells

Abstract

Statins play a pivotal role in lowering the blood cholesterol level, which is critical for patients with hypercholesterolemia. In addition to its benefits in cardiovascular protection, statins have been found to be useful in several other clinical conditions, including cancer. In a recent report that appeared in Neoplasia, Malenda et al., have demonstrated that statins inhibit glucose uptake in cancer cells. Using multiple statins and glucose analogs (¹⁸FDG and 6-NBDG) they showed that inhibition of cholesterol synthesis underlies the blockade of glucose uptake in several cancer cell lines. Further, based on an exploratory clinical study, they also showed that diagnostic PET-CT imaging in patients treated for hypercholesterolemia was affected due to statin-mediated inhibition of glucose uptake. As the finding is based on the data from a single patient (out of four), it seems that (1) the need for a large cohort study and (2) the detailed characterization of the molecular mechanisms underlying such biological effects would be justified.

Keywords: :

• statins
• glucose uptake
• FDG
• PET imaging
• cholesterol
• GLUT1
• cancer

Mounting evidence demonstrates that targeting tumor metabolism is a potential therapeutic strategy in cancer.¹ Cancer cells exhibit remarkable sensitivity to metabolic perturbations hence energy metabolism is aptly referred as cancer’s “Achilles’ heel”². Several preclinical reports have demonstrated that metabolism of glucose, glutamine, choline and cholesterol provide a window of opportunity in treating cancer.^3-6 These metabolic pathways have been known to be interconnected and critical for the survival and growth of cancer cells, particularly in a constantly changing microenvironment.⁷ Since specific-inhibitors of energy metabolism have shown promising results in preclinical models, intense investigations are underway for their clinical translation. Surprisingly, inhibitors of the enzyme 3-hydroxy-3-methylglutaryl (HMG) coenzyme A (Co A) reductase, a rate-limiting enzyme of cholesterol synthesis, have shown effects beyond lipid metabolism. The recent report by Malenda et al.⁸ reveals that statins’ (HMG CoA inhibitors) effect on glucose-metabolism is via the blockade of glucose transport indicating a new role for statins as glucose-transport inhibitors in these cancer cells.

Metabolic adaptation is one of the prominent features of cancer cells that enable them to achieve uninterrupted growth and proliferation even under fluctuating microenvironments (e.g., nutrients, oxygen, pH). Like glucose, deprivation of cholesterol or low-cholesterol diet has long been known to affect tumor growth.⁹ The emergence of statins as effective cholesterol-lowering agents during hypercholesterolemia also brings with it anticancer benefits beyond cardiovascular protection. The findings of Malenda et al.,⁸ reported in Neoplasia reveal the effect of statins on glucose metabolism of tumor cells. Although earlier reports have indicated that statins can promote apoptosis or, arrest proliferation in cancer cells; in this report the investigators demonstrate using sound methodology that pre-incubation of tumor cells with statins impair glucose uptake. Although there are reports on the glucose metabolism of Raji cells, the Neoplasia paper demonstrates for the first time the impact of statins on glucose transport or uptake and underscores its significance in image-guided clinical diagnosis (e.g., PET imaging).

Interestingly, despite a substantial decrease in glucose uptake, statin-treatment was not cytotoxic. It is plausible that a transient but not chronic decline in glucose metabolism could halt biosynthetic processes just to sustain the basal metabolism enabling the cancer cells to remain viable but non-proliferative. This is supported by the data from cell cycle analysis where lovastatin dependent increase G₁ phase and a corresponding decrease in S phase and G₂/M phase has been observed. Despite the authors’ inference that statins did not affect cell cycle in Raji cells as the increase in G₁ phase cells was marginal, it is worth investigating (at various doses and treatment periods) if the data are statistically significant, and consistent in multiple cell lines. Such an in-depth characterization would enable us to understand the effect of statins on cell cycle, i.e., its cytostatic effect. Moreover, though previous reports indicate lovastatin as cytotoxic in multiple leukemic cell lines, nevertheless differential sensitivity have been observed at various treatment periods.¹⁰ In case of Raji cells, perhaps a longer incubation with lovastatin or other statins would have provided additional data on whether these cells are resistant to statin-mediated apoptosis.

Compelling data from expression analysis of the major glucose transporters such as Glut1, Glut2, Glut3 and Glut4 revealed that the diminished glucose intake was not due to altered expression of glucose transporters. Hence it is intriguing to understand the mechanism underlying the blockade of glucose uptake during statin treatment. The authors indicate possible mechanisms that could contribute for the diminished glucose uptake. For example, the binding of cholesterol with GLUT1 may be vital for the functioning of GLUTs, if not many membrane-bound receptors and transporters. Data from bioinformatics also confirmed the presence of cholesterol binding motifs in GLUT1 suggesting a role for cholesterol on the membrane-bound transporters. Further, the role of membrane fluidity as indicated by authors is critical for the normal functioning of any membrane transporters or receptors. The role of pleiotropic effects of inhibition of HMG-CoA has also been indicated as a potential mechanism involved in the abrogation of glucose-uptake. It is noteworthy, that among possible mechanisms, the inhibition of cholesterol synthesis resulting in the accumulation of precursors or substrate sources such as Acetyl CoA or glycerol could block glucose uptake via feedback inhibition. A similar feedback inhibition of glycolysis or glucose metabolism has been reported.^11,12 It is highly likely that a lowered-cholesterol level in cells can trigger multiple mechanisms that either collectively or synergistically can affect the rate of glucose uptake.

Inhibition of glucose transport eventually would deplete the level of intracellular ATP. However, a transient and minimal decrease in cellular ATP may not be expected to have severe impact on energy consuming processes such as translation, synthesis or secretory activities. Conversely, a prolonged blockade of glucose uptake would necessitate the cells to curtail or temporarily arrest all major energy-dependent synthetic and secretory activities, ultimately leading to cell death. Since Malenda et al.,⁸ haven’t observed any cell death it is likely the cells are under transient metabolic arrest. Such an arrest in protein synthesis or translation could be assessed by several approaches, such as ³⁵S-methionine incorporation. Further, analysis of intracellular stress markers especially the endoplasmic reticulum stress during the blockade of glucose uptake would provide in-depth insights into the molecular consequences of statins mediated glucose-block.

The authors have performed an exploratory clinical investigation where they found that statin administration affected ¹⁸FDG-uptake in mantle lymphoma of a single patient. Although the concern that statins may interfere with diagnostic PET imaging could be argued in terms of the sample size (only one patient out of four), it certainly urges further investigations. First, based on the in vitro data, if glucose uptake is inhibited by statins it is intriguing to note that in the clinical study only one out of four patients showed statin mediated inhibition of glucose uptake. Probably, the variability in tumor phenotype among patients may be attributed to the difference in the inhibition of glucose uptake. Second, the lack of glucose uptake in the patient that showed inhibition of glucose uptake may be a consequence of diminished metabolic activity of tumor cells owing to statin’s anti-proliferative effects as seen in other cancer cells.¹³

The data reported by Malenda et al. in Neoplasia stimulate multiple lines of investigations especially at biochemical and molecular levels. The article convincingly demonstrates that statins markedly affect glucose uptake. While the consequence of it will be evident in the subsequent glucose metabolic pathway, it is intriguing to know if other major metabolic processes involving fatty acids and glutamine are also affected by statins. This is particularly relevant in the context of cancer metabolism where emerging reports have emphasized the critical role of both glutamine and fatty acid metabolism. Similarly, although the experimental data indicate that there is no significant alteration in the expression of GLUTs at protein and mRNA levels, it is worth investigating if there are statin-dependent posttranslational modifications (PTMs) of GLUT proteins (e.g., glycosylation). This is appropriate especially when PTMs are known to greatly impact the functional properties of several proteins irrespective of their subcellular localization.

Nonetheless, the report stimulates further research by raising at least two prominent questions. First, the data indicate that lovastatin affects glucose uptake even in non-malignant cells such as HEK293T, thus cautioning the possibility of unwanted, side-effects on normal cells where glucose-deprivation might be toxic. Second, it should be noted that among all the statins used in this study, the atorvastatin is the weakest in blocking glucose uptake as evidenced by the MFI in 6-NBDG treatment. If so, could other statins be more effective in blocking cancer-glucose metabolism. This provides an opportunity for considering other statins either as a single agent or in combination for targeting highly proliferative cancer cells. In summary, while statins block glucose uptake in cancer cells indicating their anticancer potential, the finding that statins may affect diagnostic PET imaging raises a concern. Further research focusing on the characterization of statin’s impact on PET imaging would contribute to understanding the therapeutic potential of statins in cancer.