Hypoxia, or the condition of low oxygen, is a common phenomenon in solid neoplasms. It arises when tissue oxygen demands exceed oxygen supplies, due to aberrant blood vessel formation, fluctuations in blood flow and increasing oxygen demands from rapid tumor expansion.2
Since the recognition of tumoral hypoxia in 1955,3
it has been shown to limit tumor cells’ response to therapy and predispose them towards metastasis. Mechanistically, tumor hypoxia mediates tumor progression by selecting cells with diminished apoptotic potential and activating genes involved in angiogenesis, metastasis and metabolism.4–6
Presently, there are several methods for detecting tumor hypoxia but none represents a clear “gold standard”.7
The lack of an ideal hypoxia detection strategy is due to the complex nature of blood supplies and cellular oxygen consumption, giving rise to extreme spatial and temporal heterogeneities in tumor oxygen levels. None of the current methods can completely capture such heterogeneity. With regards to human lung cancers, the clinical data on hypoxia are quite meager. Although many approaches have been used to study hypoxia in superficially located tumors such as cervical or head and neck cancers, only three methods have been employed to assess hypoxia in lung cancers. These approaches are (1) measurement of partial oxygen pressure (pO2
) with needle electrodes, (2) detection of hypoxia-induced proteins in tumor or blood, (3) and imaging hypoxia and tumor vasculature.
Our group performed the only published study on in vivo
measurement in human non-small cell lung cancers (NSCLC).8
Since these tumors are deeply situated, such an approach can only be executed intraoperatively during surgical resection of the primary tumor. Twenty patients with resectable NSCLC were enrolled, and measurements of deflated normal lung and tumor pO2
were performed with the polarographic electrode (pO2
histograph, Eppendorf, Hamburg, Germany). We measured levels of plasma osteopontin (OPN), a secreted hypoxia-induced protein, and performed immunohistochemical (IHC) staining of tumor tissue for carbonic anhydrase-IX (CAIX), a hypoxia-induced membrane protein. We also performed gene expression profiling of fresh tumor tissues in 12 patients. We found that the tumor pO2
was lower than the lung pO2
in all but one patient. The ratio of tumor to normal lung (T/L) pO2
significantly correlated with plasma OPN levels (r = 0.53, p = 0.02) and CAIX expression (p = 0.006). Gene expression profiling showed that high CD44 expression, a known cell surface receptor for OPN, was a predictor for relapse, which was confirmed by tissue staining of CD44v6 protein. Other parameters associated with the risk of relapse were T-stage (p = 0.02), T/L pO2
(p =0.04) and OPN levels (p = 0.001). Overall, our study found that tumor hypoxia does exist in resectable NSCLC and correlated with poor prognosis. Such results, although intriguing, will need to be validated in larger studies.
In contrast to the small microelectrode study, there is a wealth of information on the relationship between treatment outcomes and the expression of certain hypoxia-regulated proteins, including the hypoxia inducible factor-1 (HIF-1), which regulates genes involved in metabolism, angiogenesis, invasion and metastasis 9
and some of its targets such as glucose transporter 1 (Glut-1) and CAIX. The results from representative large series (>40 patients) are summarized in . These data demonstrate that elevated expression of hypoxia markers, in general, portends poorer prognosis in patients treated with either surgical or non-surgical therapies. Interestingly, total protein expression of hypoxia markers may not tell the entire story. A recent study of 158 resected NSCLC found that CAIX staining in the stromal fibroblasts was more prognostic for survival than CAIX staining in adjacent tumor cells.10
These findings suggest the need to differentiate the contribution of stromal from tumor hypoxia.
Significance of HIF-1, CA IX & Glut-1 endogenous hypoxia markers for non-small cell lung cancers
Other hypoxia regulated proteins that have been studied in other solid cancers are VEGF, BNIP3 (Bcl-2/adenovirus E1B 19 kDA-interacting enzyme), Lysyl oxidase (LOX), Lactate Dehydrogenase isoenzyme-5 (LDH-5), Plasminogen activator inhibitor-1 (PAI-1) and Galectin-1.11–17
The clinical relevance of these proteins in lung cancer has not yet been explored.
Under hypoxic stress, tumor and surrounding stromal cells secrete proteins that can be detected in the circulation. Known circulating markers for hypoxia include VEGF and OPN. A systematic review of published studies indicated that VEGF overexpression was associated with a poor prognosis in both NSCLC and small cell lung cancers.18
Our group has previously identified OPN as a secreted hypoxia marker in head and neck cancer.19
We have also shown that circulating OPN levels correlated with tumor pO2
in 20 NSCLC patients (see above).8
More importantly, expression of OPN and its receptor, CD44v6, correlated with survival in these patients. We subsequently evaluated OPN levels in 172 patients with metastatic NSCLC treated with chemotherapy in a cooperative group study.20
Higher circulating OPN level was an independent prognostic factor for survival.