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Based on studies performed with a selected mouse monoclonal anti-transketolase- like (TKTL)-1 antibody (clone JFC12T10), overexpression of TKTL-1 has been shown to be correlated with poor survival and increased metastatic spread in several human tumor entities. Since the clinical aggressiveness mediated by TKTL-1 has been partially related to resistance to hypoxia,we originally aimed to explore the influence of hypoxia on the expression of TKTL-1. Unexpectedly, results of our experiments indicated that the antibody clone JFC12T10 lacks target specificity. Since the majority of data on the role of TKTL-1 in human cancer is based upon studies performed with this antibody clone, we subsequently re-evaluated the expression of TKTL-1 in six different cancer cell lines (HeLa, MCF-7, A549, HT-1080, M21 and TF-1). Using RT-PCR and consecutive sequence analysis, we show that transketolase (TKT), not TKTL-1, is the dominant isoform of transketolases in the cell lines analyzed. Our data argue against a major role of TKTL-1 for the metabolism of cancer cells.

In malignant urogenital tumors, tissue oxygenation is compromised and very heterogeneous,with steep and fluctuating spatio-temporal oxygen gradients signaling a complex instability in tumor oxygenation (complex “4D-heterogeneity”). Tumor hypoxia is highly dynamic, and rapidly changing pO2 gradients may be key factors driving hypoxia-dependent adaptive processes leading to malignant progression. The grand median oxygen tension in malignant urogenital tumors is 7–11 mmHg. In contrast, benign leiomyomas of the uterus are severely, but uniformly, hypoxicwith only shallow oxygen gradients (“static hypoxia”). In these benign tumors, the median pO2 is 1 mmHg and signs of hypoxia-driven processes are missing.

Results from our laboratory have put a question mark on the existence of a direct quantitative relationship between tumor hypoxia and HIF-mediated protein expression in cancers of the uterine cervix. In the present study, this subject has been further explored by the analysis of HIF-related marker expression in a benign tumor entity - uterine leiomyomas - using immunohistochemistry, western blotting and RT-PCR. The oxygenation status of 17 leiomyomas was assessed by means of intraoperative polarographic needle electrode measurements. Results show that these tumors are severely and uniformly hypoxic, but do not induce HIF-1alpha, HIF-2alpha, glucose transporter (GLUT)-1 or carbonic anhydrase (CA) IX. Furthermore, this downregulation of the HIF-system was not caused by an overexpression of the hypoxia-inducible prolyl hydroxylase domain proteins (PHDs) 2 and 3. Compared with normal myometrium, leiomyomas also show a poorer vascularization. Conversely, leiomyosarcomas show abundant expression of HIF-related markers despite a similar microvascular density. In conclusion, these results indicate that the strong activation of the HIF system observed in solid malignant tumors may be mechanistically linked to their transformed phenotype, rather than being a physiological reaction activated in a pathological context.

The pivotal role of hypoxia within the pathophysiological framework of solid malignant tumors is now considered to be indisputable. The fact that hypoxia can cause resistance to various cancer therapies and promote malignant progression is reflected in its adverse impact on prognosis which is repeatedly shown for various tumor entities. Knowledge in this area is based on direct assessment of the oxygenation status using O2-sensitive microsensors. However, weaknesses of this standard method are its invasiveness and limitation to accessible tumor entities. Hypoxia-inducible factor (HIF)-1α, the master transcriptional regulator of the hypoxic response, as well as certain downstream genes, e.g., glucose transporter (GLUT)-1 and carbonic anhydrase (CA) IX, have been considered to be suitable as surrogate biomarkers of hypoxia due to their tight regulation by O2 levels under certain, well-defined in vitro conditions. The fact that statistical correlations between the expression of these proteins and direct pO2 measurements in the clinic have been sporadically reported seemed to support their role as “endogenous hypoxia markers”. Remaining disparities were mainly attributed to the influence of tumor heterogeneity. In a series of studies, we have addressed this question by examining the expression of HIF-1α, GLUT-1 and CA IX in tissue microareas where direct O2 measurements had previously been carried out, so that the influence of tumor heterogeneity could be reduced to a minimum. Using this methodology, no correlation between the expression of “endogenous hypoxia markers” and direct pO2 measurements could be found. In conclusion, while there may be a stringent association between these markers and the oxygenation status under standardized in vitro conditions, this is not transferable to the clinical assessment of oxygenation status in patients. The term “endogenous hypoxia markers” should therefore be avoided, at least in the clinical oncology setting.