Biological correlates of FDG uptake in non-small cell lung cancer

Summary

Purpose

Each pathological stage of non-small cell lung cancer (NSCLC) consists of a heterogeneous population containing patients at much higher risk than others. Noninvasive functional imaging modalities, such as ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET), could play a role in further characterization of NSCLCs. As many factors can influence the extent of FDG uptake, the underlying mechanisms for FDG accumulation in tumors, are still a matter of debate. The aim of the present study was to investigate these possible mechanisms in the primary site of early stage preoperatively untreated NSCLC.

Methods

19 patients with early stage NSCLC, who had undergone both preoperative FDG-PET imaging and curative surgery, were enrolled in this study. Standardized uptake values (SUVs) were used for evaluation of primary tumor FDG uptake. Final diagnosis, tumor type, tumor cell differentiation and size of the primary tumors were confirmed histopathologically in resected specimens. Histologic sections were analyzed for amount of inflammation and necrosis. Expression of the glucose membrane transporters (GLUT-1 and GLUT-3); the isoforms of the glycolytic enzyme hexokinase (HK-I, HK-II and HK-III); and the cysteine protease caspase-3, was evaluated immunohistochemically.

Results

FDG uptake was significantly higher in squamous cell carcinomas (mean SUV 13.4 ± 4.9, n = 8) compared to adenocarcinomas (7.1 ± 3.3, n = 8, p = 0.007), or large cell carcinomas (5.9 ± 1.9, n = 3, p = 0.02). The degree of FDG accumulation seemed to depend especially on GLUT-1, GLUT-3 and tumor cell differentiation. The summed standardized values of these three parameters correlated significantly with the SUV (r = 0.47, p = 0.05).

Conclusion

The present study supports the hypothesis that tumor cell differentiation in combination with overexpression of GLUT-1 and GLUT-3 determine the extent of FDG accumulation and that squamous cell carcinomas accumulate more FDG than adenocarcinomas or large cell carcinomas.

Introduction

Lung cancer is the leading cause of cancer related death in both men and women. About 3 million new cases a year are estimated to arise worldwide, of which more than 200 000 are in the European Union. An increase in incidence is to be expected until the first decades of the 21st century [1]. For the average patient with a diagnosis of lung cancer the overall 5-year survival rates have increased from 12% in the early 70s to 15% in 2001 [2]. Survival ranges from 75% for patients with pT1N0 disease to virtually nil for patients with stage IV non-small cell lung cancer (NSCLC) [3]. Surgery with curative intent represents the best chance for cure, but is only an option in patients with stages I, II, and selected cases of stage IIIA (T₃N₁M₀) NSCLC. However, only 30% of NSCLC cases present at these early stages. Even if a complete curative resection can be performed, the majority of patients will relapse. The majority of these relapses occur at distant sites, indicating micrometastatic disease at presentation [4]. Progress is being made due to the introduction of novel treatment programs, like induction chemotherapy [5], concurrent chemo-radiation for stage III disease [6], [7], and more recently, adjuvant chemotherapy for earlier stages [8], [9]. It is of major importance to be able to predict the relapses and to prevent them with these active intensified treatment regimens. The tumor-node-metastasis (TNM) staging system, to date considered the most important tool in estimation of prognosis and guidance of treatment decisions [10], however, provides an incomplete biologic profile of NSCLC, does not always provide a satisfactory explanation for differences in relapse and survival and is thus far from perfect as a prognostic indicator [11], [12]. Each pathological stage consists of a heterogeneous population containing individuals at much higher risk of recurrence and death than others [11], [12], [13]. Therefore, there is a need for noninvasive quantitative measures of biological aggressiveness that could play a role in further characterization of NSCLCs. A better understanding of biological mechanisms in lung tumor cells could be helpful and finally might lead to a better selection of patients who may benefit from (neo)adjuvant therapy. Such noninvasive quantitative measures of biological aggressiveness may be of particular value in stratifying patients for clinical trials.

The introduction of the combined use of fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET) and computed tomography (CT) has had a great impact on the diagnosis and staging of lung cancer. FDG-PET provides noninvasive mediastinal staging and reduces the number of futile thoracotomies and mediastinoscopies [14], [15], [16]. Furthermore, FDG-PET detects unsuspected extrathoracic metastases in 14–16.9% of patients otherwise deemed potentially resectable [17]. The number of clinical applications for FDG-PET in NSCLC continues to increase. Recently, FDG-PET has also demonstrated its value in radiation treatment planning, detection of recurrent disease, in identifying tumor response to chemotherapy at an early phase of treatment and in identifying subsets of patients with poor outcome [18]. One of the great advantages of this technique is that it cannot only visualize but can also quantify FDG uptake to distinguish metabolically highly active from less active tumor tissues and therefore offers an opportunity for noninvasive, in vivo tissue characterization.

The biological basis of FDG-PET is the increased glucose metabolism of malignant cells as compared to noncancerous tissues. After administration of the glucose analog FDG, it will be transported into the tumor cell and will be phosphorylated by hexokinase. The intracellular FDG-6-phosphate is trapped in the malignant cells, as it will not be processed in the glycolytic pathway and can thus be visualized using PET. A variety of mechanisms have been proposed for accelerated glucose use in growing tumors and in transformed and malignant cells: passive diffusion, Na⁺-dependent glucose transport and via facilitative glucose transporters (GLUT). The latter is considered to be the most important mechanism for enhancing glucose influx into cells [19]. The glucose transporters GLUT-1 and GLUT-3, subtypes with a relatively high affinity for glucose, belong to the sugar transporter family, which currently includes 133 individual members [20]. Increased concentrations of the glucose phosphorylation enzyme, hexokinase, with decreased rates of glucose-6-phophatase are considered to accelerate glucose phosphorylation, which results in enhanced FDG intracellular trapping. Upregulation of hexokinase and glucose transporters, especially GLUT-1, and downregulation of glucose-6-phosphatase are frequently associated with malignant transformation. Glucose transport activity can be regulated by alterations in the expression of GLUT transporters and by post-translational mechanisms, including transporter translocation to plasma membranes [21]. Akhurst et al. recently reported that the use of chemotherapy could alter FDG uptake in tumors by altering the activity of hexokinase [22]. Moreover, the rate of FDG uptake in the primary site of NSCLC has been correlated with tumor doubling time [23] and proliferation rates [24] which, in turn, are known to correlate with tumor aggressiveness [25], [26], [27]. Furthermore, apoptosis plays a central role in the elimination of (the precursors of) tumor cells. Therapy resistance can be attributed, at least in part, to a disabled apoptotic program [28]. Sasaki et al. demonstrated that primary tumors showing high FDG uptake have the potential to be resistant to therapy and to metastasize [29]. It was recently reported that strong expression of the cysteine protease caspase-3, which is a key enzyme in apoptotic cell death, was a significant factor to predict poor prognosis [30]. Better understanding of a possible relationship between FDG uptake and apoptosis may provide insights into sensitivity or resistance of tumor cells. Furthermore, tumors that grow too rapidly or have a deficient vascular system are characterized by the formation of necrosis. Necrosis reflects cell death caused by hypoxia. Hypoxia results in enhanced anaerobic glycolysis and hence in increased FDG uptake [31]. Finally, the presence of inflammatory cells, might be confounding, since inflammatory cells may have a major impact on FDG uptake [32].

At present it is still not fully elucidated which of these factors contribute to the variable levels of FDG uptake in NSCLC. Results from studies on other tumor types cannot be extrapolated to NSCLC, as different tumors have different glucose-regulating mechanisms and enzyme expression patterns in association with various oncogenic alterations [33]. The aim of the present study was to investigate the mechanisms that drive FDG in the primary site of early stage untreated NSCLC. The relationship among FDG uptake and the immunohistochemical expressions of the key glucose membrane transporters, GLUT-1 and GLUT-3; the isoforms of the glycolytic enzyme hexokinase, HK-I, HK-II and HK-III; the cysteine protease, caspase-3, and several histological parameters was evaluated.