Detection of different hypoxic cell subpopulations in human melanoma xenografts by pimonidazole immunohistochemistry

Rationale for Combining Radiotherapy and Immune Checkpoint Inhibition for Patients With Hypoxic Tumors

In order to compensate for the increased oxygen consumption in growing tumors, tumors need angiogenesis and vasculogenesis to increase the supply. Insufficiency in this process or in the microcirculation leads to hypoxic tumor areas with a significantly reduced pO2, which in turn leads to alterations in the biology of cancer cells as well as in the tumor microenvironment. Cancer cells develop more aggressive phenotypes, stem cell features and are more prone to metastasis formation and migration. In addition, intratumoral hypoxia confers therapy resistance, specifically radioresistance. Reactive oxygen species are crucial in fixing DNA breaks after ionizing radiation. Thus, hypoxic tumor cells show a two- to threefold increase in radioresistance. The microenvironment is enriched with chemokines (e.g., SDF-1) and growth factors (e.g., TGFβ) additionally reducing radiosensitivity. During recent years hypoxia has also been identified as a major factor for immune suppression in the tumor microenvironment. Hypoxic tumors show increased numbers of myeloid derived suppressor cells (MDSCs) as well as regulatory T cells (Tregs) and decreased infiltration and activation of cytotoxic T cells. The combination of radiotherapy with immune checkpoint inhibition is on the rise in the treatment of metastatic cancer patients, but is also tested in multiple curative treatment settings. There is a strong rationale for synergistic effects, such as increased T cell infiltration in irradiated tumors and mitigation of radiation-induced immunosuppressive mechanisms such as PD-L1 upregulation by immune checkpoint inhibition. Given the worse prognosis of patients with hypoxic tumors due to local therapy resistance but also increased rate of distant metastases and the strong immune suppression induced by hypoxia, we hypothesize that the subgroup of patients with hypoxic tumors might be of special interest for combining immune checkpoint inhibition with radiotherapy.

A Microdevice Platform Recapitulating Hypoxic Tumor Microenvironments

Hypoxia plays a central role in cancer progression and resistance to therapy. We have engineered a microdevice platform to recapitulate the intratumor oxygen gradients that drive the heterogeneous hypoxic landscapes in solid tumors. Our design features a "tumor section"-like culture by incorporating a cell layer between two diffusion barriers, where an oxygen gradient is established by cellular metabolism and physical constraints. We confirmed the oxygen gradient by numerical simulation and imaging-based oxygen sensor measurement. We also demonstrated spatially-resolved hypoxic signaling in cancer cells through immunostaining, gene expression assay, and hypoxia-targeted drug treatment. Our platform can accurately generate and control oxygen gradients, eliminates complex microfluidic handling, allows for incorporation of additional tumor components, and is compatible with high-content imaging and high-throughput applications. It is well suited for understanding hypoxia-mediated mechanisms in cancer disease and other biological processes, and discovery of new therapeutics.

Characterization of the Tumor Microenvironment and Tumor-Stroma Interaction by Non-invasive Preclinical Imaging

Tumors are often characterized by hypoxia, vascular abnormalities, low extracellular pH, increased interstitial fluid pressure, altered choline-phospholipid metabolism, and aerobic glycolysis (Warburg effect). The impact of these tumor characteristics has been investigated extensively in the context of tumor development, progression, and treatment response, resulting in a number of non-invasive imaging biomarkers. More recent evidence suggests that cancer cells undergo metabolic reprograming, beyond aerobic glycolysis, in the course of tumor development and progression. The resulting altered metabolic content in tumors has the ability to affect cell signaling and block cellular differentiation. Additional emerging evidence reveals that the interaction between tumor and stroma cells can alter tumor metabolism (leading to metabolic reprograming) as well as tumor growth and vascular features. This review will summarize previous and current preclinical, non-invasive, multimodal imaging efforts to characterize the tumor microenvironment, including its stromal components and understand tumor-stroma interaction in cancer development, progression, and treatment response.

In vivo remodelling of vascularizing engineered tissues

A critical aspect of creating vascularized tissues is the remodelling that occurs in vivo, driven in large part by the host response to the tissue construct. Rather than a simple inflammatory response, a beneficial tissue remodelling response results in the formation of vascularised tissue. The characteristics and dynamics of this response are slowly being elucidated, especially as they are modulated by the complex interaction between the biomaterial and cellular components of the tissue constructs and the host. This process has elements that are similar to both wound healing and tumour development, and its features are illustrated by reference to the bottom-up generation of a tissue using modular constructs. These modular constructs consist of mesenchymal stromal cells (MSC) embedded in endothelial cell (EC)-covered collagen gel rods that are a few hundred microns in size. Particular attention is paid to the role of hypoxia and macrophage recruitment, as well as the paracrine effects of the MSC and EC in this host response.

Interstitial diffusion and the relationship between compartment modelling and multi-scale spatial-temporal modelling of (18)F-FLT tumour uptake dynamics

Tumour cell proliferation can be imaged via positron emission tomography of the radiotracer 3'-deoxy-3'-18F-fluorothymidine (18F-FLT). Conceptually, the number of proliferating cells might be expected to correlate more closely with the kinetics of 18F-FLT uptake than with uptake at a fixed time. Radiotracer uptake kinetics are standardly visualized using parametric maps of compartment model fits to time-activity-curves (TACs) of individual voxels. However the relationship between the underlying spatiotemporal accumulation of FLT and the kinetics described by compartment models has not yet been explored. In this work tumour tracer uptake is simulated using a mechanistic spatial-temporal model based on a convection-diffusion-reaction equation solved via the finite difference method. The model describes a chain of processes: the flow of FLT between the spatially heterogeneous tumour vasculature and interstitium; diffusion and convection of FLT within the interstitium; transport of FLT into cells; and intracellular phosphorylation. Using values of model parameters estimated from the biological literature, simulated FLT TACs are generated with shapes and magnitudes similar to those seen clinically. Results show that the kinetics of the spatial-temporal model can be recovered accurately by fitting a 3-tissue compartment model to FLT TACs simulated for those tumours or tumour sub-volumes that can be viewed as approximately closed, for which tracer diffusion throughout the interstitium makes only a small fractional change to the quantity of FLT they contain. For a single PET voxel of width 2.5-5 mm we show that this condition is roughly equivalent to requiring that the relative difference in tracer uptake between the voxel and its neighbours is much less than one.

Acute versus chronic hypoxia in tumors: Controversial data concerning time frames and biological consequences

BACKGROUND

Many tumors contain hypoxic regions. Hypoxia, in turn, is known to increase aggressiveness and to be associated with treatment resistance. The two most frequently described and investigated subtypes of tumor hypoxia are acute and chronic. These two subtypes can lead to completely different hypoxia-related responses within the tumor, which could have a direct effect on tumor development and response to treatment. In order to accurately assess the specific biological consequences, it is important to understand which time frames best define acute and chronic hypoxia.

MATERIALS AND METHODS

This article provides an overview of the kinetics of in vitro and in vivo acute and chronic tumor hypoxia. Special attention was paid to differentiate between methods to detect spontaneous in vivo hypoxia and to describe the biological effects of experimental in vitro and in vivo acute and chronic tumor hypoxia.

RESULTS AND CONCLUSIONS

There are large variations in reported spontaneous fluctuations in acute hypoxia that are dependent on the cell lines investigated and the detection method used. In addition to differing hypoxia levels, exposure times used to induce in vitro and in vivo experimental acute and chronic hypoxia range from 30 min to several weeks with no clear boundaries separating the two. Evaluation of the biological consequences of each hypoxia subtype revealed a general trend that acute hypoxia leads to a more aggressive phenotype. Importantly, more information on the occurrence of acute and chronic hypoxia in human tumors is needed to help our understanding of the clinical consequences.

Autophagy patterns and prognosis in uveal melanomas

Autophagy is a self-degradation mechanism by which cells recycle their own cytoplasmic constituents. It has been claimed that, under certain conditions, such a process may be associated with tumor progression. In this study, the autophagic activity was investigated in a series of 99 uveal melanomas after immunohistochemical staining for the autophagy-associated proteins MAP1LC3A and BECN1, most commonly known as LC3A and Beclin 1, respectively. These were assessed in parallel with the hypoxia-inducible factor 1α (HIF1A) and its downstream protein lactate dehydrogenase 5 (composed by five LDHA subunits). Increased autophagic reactivity, detected by MAP1LC3A or BECN1, was associated with intense pigmentation and tumor hypoxia. Uveal melanomas with extensive overexpression of BECN1 or those with underexpression of this protein were associated with the worst prognosis, but the former manifested metastases much earlier than the latter; only 58% of patients with extensive BECN1 overexpression were alive at 4 years, compared with 80% of patients with underexpressed patterns. It is concluded that autophagy is commonly upregulated in uveal melanomas, and may be associated with hypoxia and intense pigmentation. There is a strong association between extensive BECN1 overexpression and early metastases/poor prognosis, and between underexpression of this protein and late metastases/better prognosis.

Quantitative assessment of hypoxia subtypes in microcirculatory supply units of malignant tumors using (immuno-)fluorescence techniques

BACKGROUND AND PURPOSE

Hypoxia is a characteristic of tumors, is known to increase aggressiveness, and causes treatment resistance. Traditional classification suggests two types of hypoxia: chronic and acute. Acute hypoxia is mostly caused by transient disruptions in perfusion, while chronic hypoxia is caused by diffusion limitations. This classification may be insufficient in terms of pathogenetic and pathophysiological mechanisms. Therefore, we quantified hypoxia subtypes in tumors based on (immuno-)fluorescent marker distribution patterns in microcirculatory supply units (MCSUs).

MATERIAL AND METHODS

Cryosections from hSCC lines (SAS, FaDu, UT-SCC-5, UT-SCC-14, UT-SCC-15) were analyzed. Hypoxia was identified by pimonidazole, perfusion by Hoechst 33342, and endothelial cells by CD31. The following patterns were identified in vital tumor tissue: (1) normoxia: Hoechst 33342 fluorescence around microvessels, no pimonidazole, (2) chronic hypoxia: Hoechst 33342 fluorescence around microvessels, pimonidazole distant from microvessels, (3) acute hypoxia: no Hoechst 33342 fluorescence around microvessels, pimonidazole in immediate vicinity of microvessels, and (4) hypoxemic hypoxia: Hoechst 33342 fluorescence and pimonidazole directly around microvessels.

RESULTS

Quantitative assessment of MCSUs show predominance for normoxia in 4 out of 5 tumor lines (50.1-72.8%). Total hypoxia slightly prevails in UT-SCC-15 (56.9%). Chronic hypoxia is the dominant subtype (65.4-85.9% of total hypoxia). Acute hypoxia only accounts for 12.9-29.8% and hypoxemic hypoxia for 1.2-6.4% of total hypoxia. The fraction of perfused microvessels ranged from 82.5-96.6%.

CONCLUSION

Chronic hypoxia is the prevailing subtype in MCSUs. Acute hypoxia and hypoxemic hypoxia account for only a small fraction. This approach enables assessment and recognition of different hypoxia subtypes including hypoxemic hypoxia and may facilitate methods to (clinically) identify and eliminate hypoxia.

Radiocurability is associated with interstitial fluid pressure in human tumor xenografts

Interstitial fluid pressure (IFP) has been shown to be an independent prognostic parameter for disease-free survival in cervical carcinoma patients treated with radiation therapy. However, the underlying mechanisms are not fully understood. The main aims of this study were to investigate whether tumor radiocurability may be associated with IFP and, if so, to identify possible mechanisms. Human melanoma xenografts transplanted intradermally or in window chamber preparations in BALB/c nu/nu mice were used as preclinical tumor models. Radiation dose resulting in 50% local tumor control was higher by a factor of 1.19 +/- 0.06 in tumors with IFP > or = 9 mm Hg than in tumors with IFP < or = 7 mm Hg. Tumor IFP was positively correlated to vessel segment length and vessel tortuosity and was inversely correlated to vessel density. Compared with tumors with low IFP, tumors with high IFP showed high resistance to blood flow, high frequency of Po(2) fluctuations, and high fractions of acutely hypoxic cells, whereas the fraction of radiobiologically hypoxic cells and the fraction of chronically hypoxic cells did not differ between tumors with high and tumors with low IFP. IFP showed a significant correlation to the fraction of acutely hypoxic cells, probably because both parameters were determined primarily by the microvascular resistance to blood flow. Therefore, the observed association between tumor radiocurability and IFP was most likely an indirect consequence of a strong relationship between IFP and the fraction of acutely hypoxic cells.