Selective thrombosis of tumor blood vessels in mammary adenocarcinoma implants in rats

Tumor rim cells: From resistance to vascular targeting agents to complete tumor ablation

Current vascular targeting strategies pursue two main goals: anti-angiogenesis agents aim to halt sprouting and the formation of new blood vessels, while vascular disrupting agents along with coaguligands seek to compromise blood circulation in the vessels. The ultimate goal of such therapies is to deprive tumor cells out of oxygen and nutrients long enough to succumb cancer cells to death. Most of vascular targeting agents presented promising therapeutic potential, but the final goal which is cure is rarely achieved. Nevertheless, in both preclinical and clinical settings, tumors tend to grow back, featuring a highly invasive, metastatic, and extremely resistant form. This review highlights the critical significance of tumor rim cells as the main factor, determining therapy success with vascular targeting agents. We present an overview of different single and combination treatments with vascular targeting agents that enable efficient targeting of tumor rim cells and long-lasting tumor cure. Understanding the nature of tumor rim cells, how they establish, how they manage to survive of vascular targeting agents, and how they contribute in tumor refractoriness, may open new avenues to the development of beneficial strategies, capable to eliminate residual rim cells, and enable tumor ablation once and forever.

C-type lectin-like receptor 2 promotes hematogenous tumor metastasis and prothrombotic state in tumor-bearing mice

Essentials The role of C-type lectin-like receptor-2 (CLEC-2) in cancer progression is unclear. CLEC-2-depleted mouse model is generated by using a rat anti-mouse CLEC-2 monoclonal antibody. CLEC-2 depletion inhibits hematogenous tumor metastasis of podoplanin-expressing B16F10 cells. CLEC-2 depletion prolongs cancer survival by suppressing thrombosis and inflammation.

SUMMARY

Background C-type lectin-like receptor 2 (CLEC-2) is a platelet activation receptor of sialoglycoprotein podoplanin, which is expressed on the surface of certain types of tumor cells. CLEC-2-podoplanin interactions facilitate hematogenous tumor metastasis. However, direct evidence of the role of CLEC-2 in hematogenous metastasis and cancer progression is lacking. Objective and methods We generated immunological CLEC-2-depleted mice by using anti-mouse CLEC-2 monoclonal antibody 2A2B10 and investigated whether CLEC-2 promoted hematogenous tumor metastasis and tumor growth and exacerbated the prognosis of mice bearing podoplanin-expressing B16F10 melanoma cells. Results Our results showed that hematogenous metastasis was significantly inhibited in CLEC-2-depleted mice. B16F10 cells co-cultured with wild-type platelets, but not with CLEC-2-deficient platelets, showed increased proliferation. However, B16F10 cell proliferation was not inhibited in CLEC-2-depleted mice. Histological analysis showed that thrombus formation in tumor vessels was significantly inhibited and functional vessel density was significantly increased in CLEC-2-depleted mice. These data suggest that CLEC-2 deficiency may inhibit thrombus formation in tumor vessels and increase the density of functional vessels, thus improving oxygen and nutrient supply to tumors, indirectly promoting tumor proliferation. Furthermore, the overall survival of CLEC-2-depleted mice was significantly prolonged, which may be due to the suppression of thrombus formation in the lungs and subsequent inhibition of systemic inflammation and cachexia. Conclusions These data provide a rationale for the targeted inhibition of CLEC-2 as a new strategy for preventing hematogenous tumor metastasis and for inhibiting cancer-related thromboembolism.

The significant role of mast cells in cancer

Mast cells (MC) are a bone marrow-derived, long-lived, heterogeneous cellular population that function both as positive and negative regulators of immune responses. They are arguably the most productive chemical factory in the body and influence other cells through both soluble mediators and cell-to-cell interaction. MC are commonly seen in various tumors and have been attributed alternatively with tumor rejection or tumor promotion. Tumor-infiltrating MC are derived both from sentinel and recruited progenitor cells. MC can directly influence tumor cell proliferation and invasion but also help tumors indirectly by organizing its microenvironment and modulating immune responses to tumor cells. Best known for orchestrating inflammation and angiogenesis, the role of MC in shaping adaptive immune responses has become a focus of recent investigations. MC mobilize T cells and antigen-presenting dendritic cells. They function as intermediaries in regulatory T cells (Treg)-induced tolerance but can also modify or reverse Treg-suppressive properties. The central role of MC in the control of innate and adaptive immunity endows them with the ability to tune the nature of host responses to cancer and ultimately influence the outcome of disease and fate of the cancer patient.

Synthesis and characterization of a novel paramagnetic macromolecular complex [Gd(TTDASQ–protamine)]

Adenocarcinomas in rats and humans frequently contain perivascular, degranulating mast cells that release heparin. Protamine is a low-molecular weight, cationic polypeptide that binds to heparin and neutralizes its anticoagulant properties. A novel magnetic resonance imaging (MRI) contrast agent containing protamine was synthesized. TTDASQ, the derivative of TTDA (3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid), was also synthesized and the kinetic stability of [Gd(TTDASQ)]- chelate containing phosphate buffer and ZnCl2 to measure the relaxation rate (R1) at 20 MHz was studied by transmetallation with Zn(II). The water-exchange rate (k(ex)298) of [Gd(TTDASQ)]- is 6.4 x 10(6) s(-1) at 25.0 +/- 0.1 degrees C which was obtained from the reduced 17O relaxation rates (1/T(1r) and 1/T(2r)) and chemical shift (omega(r)) of H(2)17O, and it is compared with that previously reported for the other gadolinium(III) complex, [Gd(DO3ASQ)]. The binding affinity assay showed that the (TTDASQ)3-pro19 has higher activity toward heparin. On the other hand, the effect of heparin on the relaxivity of the [Gd(TTDASQ)3-pro19] conjugate shows the binding strength (K(A)) is 7669 dm3 mol(-1) at pH 7.4 and the relaxivity (r(b)1) of the [Gd(TTDASQ)3-pro19]-heparin adduct is 30.9 dm3 mmol(-1) s(-1).

Protamine enhances uptake of cationic liposomes in angiogenic microvessels of solid tumours

INTRODUCTION

Cationic liposomes have been shown to target angiogenic endothelial cells of solid tumours. Supposing a charge-related mechanism might be responsible for liposome-endothelial interaction, we investigated the effect of intravenous pre-injection of the charged molecules protamine, a polycationic protein, and fucoidan, a polyanionic polysaccharide on the accumulation of cationic liposomes within the blood vessels of a solid tumour.

MATERIALS AND METHODS

Experiments were performed using the amelanotic hamster melanoma A-Mel-3 growing in a dorsal skinfold chamber of hamsters. Accumulation of fluorescently-labelled cationic liposomes was quantified by intravital macroscopy and digital image analysis of tumour (t) and surrounding normal host tissue (n) over an observation period of 6 h. All animals received an i.v. injection of cationic liposomes. Animals of the control group were pre-treated with an i.v. injection of 0.9% saline, while animals of group 2 received positively charged protamine and animals of group 3 negatively charged fucoidan prior to liposome injection.

RESULTS

In control animals i.v. injection of cationic liposomes revealed a preferential targeting of the tumour vessels, indicated by a maximal t/n ratio of 2.2 +/- 0.24 and a maximal fluorescence intensity (fmax) corresponding to the tumour of 66 +/- 12 [% standard]. While there were no significant differences of liposome accumulation within normal host tissue, accumulation of cationic liposomes within the tumour was significantly enhanced after the pre-administration of protamine (fmax: 117 +/- 12 [% standard]). The t/n ratio was significantly increased in protamine pre-treated animals (5.3 +/- 1.7) in comparison to control and fucoidan treated animals. In contrast, pre-injection of fucoidan resulted in reduced maximal fluorescence intensities in tumour (47 +/- 8 [% standard]) and normal surrounding host tissue.

CONCLUSION

Pre-administration of protamine increases the accumulation of cationic liposomes in a solid tumour animal model causing an increased selectivity of cationic liposomes in targeting angiogenic microvessels.

Mast cell inhibitor cromolyn increases blood clotting and hypoxia in murine breast cancer

Human breast cancer is extensively infiltrated by mast cells that contain powerful anticoagulants such as heparin, tryptase and chymase. To determine if human breast cancer is associated with mast cell activation, we measured the levels of mast cell tryptase (an indicator of mast cell activation) in the blood of 20 women with varying stages of breast cancer. The mean level of tryptase in women with breast cancer (10.3 +/- 4.2 microg/L) was significantly higher than in 50 normal healthy women without breast cancer (3.0 +/- 2.5 microg/L, p < 0.05 by two-tailed t-test). To explore the role of mast cells in breast cancer in more detail, we then carried out experiments that were aimed at determining if an inhibitor of mast cell function, sodium cromolyn, could increase blood clotting and hypoxia within subcutaneous implants of the 4T1 mammary adenocarcinoma cell line in mice. We treated tumor-bearing mice with 5 consecutive daily doses of sodium cromolyn (10 mg/kg, i.p.). An average of 30% of the periphery of the tumors from the 5 drug-treated mice contained large lakes of clotted blood that were not evident in any of the tumors from the control (untreated) mice. By computerized image analysis of tumors immunostained for a hypoxia marker (pimonidazole), the tumors from the treated mice had significantly more hypoxia (35 +/-12 % hypoxic regions, n = 5) than the tumors from untreated (control) mice (16 +/- 7%, n = 5). We conclude that sodium cromolyn enhanced peri-tumoral blood clotting and intratumoral hypoxia. These results suggest that mast cells may play an important role in regulating blood clotting and hypoxia within breast cancer.

Assessment of protamine-induced thrombosis of tumor vessels for cancer therapy using dynamic contrast-enhanced MRI

Since the role of angiogenesis in cancer development has been recognized, the study of anti-angiogenic or anti-vascular therapeutic agents has become increasingly important for cancer treatment. Selective thrombosis is one approach towards this goal. Since many types of carcinoma accumulate large numbers of degranulating mast cells which will release heparin, intravenously injected protamine may bind to heparin, neutralize its anti-coagulant effect and induce thrombosis. In this work we studied the formation of thrombosis by using dynamic contrast enhanced MRI. The enhancement kinetics of the contrast medium measured before and after protamine treatment were compared to assess the thrombotic effect. The underlying concept was that if the vessels became clotted, the subsequently injected contrast medium could not be delivered into the tissue to cause enhancement. In addition to the tissue-specific changes, protamine may also induce systemic effect in the host. The therapy-induced changes measured in tumors were compared to changes in normal tissues: liver, kidney, and especially the muscle adjacent to tumor. The results showed that protamine induced pronounced changes in the tumor. However, the degree of change measured by MRI was not associated with the results of semiquantitative assessment of thrombosis assessed by histology, perhaps due to the heterogeneous nature of the tumor and the difficulty in sampling sufficient regions histologically. The protamine-induced temporal effects were also studied. We demonstrated that protamine could induce selective thrombosis in tumors, and that the effect could last for several hours. Dynamic contrast-enhanced MRI can serve as a suitable means to investigate the mechanism of this novel approach to induce selective thrombosis for anti-vascular cancer therapy.