Altered efficacy of AT1R-targeted treatment after spontaneous cancer cell-AT1R upregulation.

BACKGROUND: Targeting of the renin angiotensin system (RAS) reduces tumour growth in experimental models of cancer. We aimed to establish if combined targeting of the 'classical' and 'alternative' arms of the RAS could result in synergistic inhibition of colorectal cancer (CRC) liver metastases. METHODS: Immediately following induction of CRC liver metastases through intrasplenic injection of murine CRC cells, treatment with irbesartan (AT1R blocker; 50 mg/kg/day s.c.), captopril (ACE inhibitor; 750 mg/kg/day i.p.), CGP42112A (AT2R agonist; 0.6 µg/kg/hr i.p.), and/or ANG-(1-7) (24 µg/kg/hr i.p.) began and continued for 21 days. Liver to body weight ratio and/or stereology were used as a measure of tumour burden. Immunohistochemistry was used to determine AT1R and VEGF expression as well as proliferation (Ki67), apoptosis (active caspase 3) and angiogenesis (CD34). RESULTS: Combined RAS therapies failed to improve upon single arm therapies. However, while irbesartan previously inhibited tumour growth in this model, in the current experiments irbesartan failed to affect tumour burden. Subsequent analysis showed a cancer-cell specific upregulation of the angiotensin II type I receptor (AT1R) in irbesartan-insensitive compared to irbesartan-sensitive tumours. The upregulation of AT1R was associated with an increase in proliferation and VEGF expression by cancer cells. While animals bearing irbesartan-sensitive tumours showed a marked decrease in the number of proliferating cells in the liver and VEGF-expressing infiltrating cells in the tumour following AT1R treatment, these were unchanged by treatment in animals bearing irbesartan-insensitive (high AT1R expressing) tumours. CONCLUSIONS: Although the results do not support increased efficacy of combined treatment, they provide intriguing evidence of the importance of RAS expression in determining patient response and tumour growth potential and suggest that components of the RAS could be used as biomarkers to aid in patient selection.

Changes in the renin angiotensin system during the development of colorectal cancer liver metastases.

BACKGROUND: Blockade of the renin angiotensin system (RAS) via angiotensin I converting enzyme (ACE) inhibition reduces growth of colorectal cancer (CRC) liver metastases in a mouse model. In this work we defined the expression of the various components of the RAS in both tumor and liver during the progression of this disease. METHODS: Immunohistochemistry and quantitative RT-PCR was used to examine RAS expression in a mouse CRC liver metastases model. CRC metastases and liver tissue was assessed separately at key stages of CRC liver metastases development in untreated (control) mice and in mice treated with the ACE inhibitor captopril (750 mg/kg/day). Non-tumor induced (sham) mice indicated the effect of tumors on normal liver RAS. The statistical significance of multiple comparisons was determined using one-way analysis of variance followed by Bonferroni adjustment with SAS/STAT software. RESULTS: Reduced volume of CRC liver metastases with captopril treatment was evident. Local RAS of CRC metastases differed from the surrounding liver, with lower angiotensin II type 1 receptor (AT1R) expression but increased ANG-(1-7) receptor (MasR) compared to the liver. The AT1R localised to cancer and stromal infiltrating cells, while other RAS receptors were detected in cancer cells only. Tumor induction led to an initial increase in AT1R and ACE expression while captopril treatment significantly increased ACE expression in the final stages of tumor growth. Conversely, captopril treatment decreased expression of AT1R and angiotensinogen. CONCLUSIONS: These results demonstrate significant changes in RAS expression in the tumor-bearing captopril treated liver and in CRC metastases. The data suggests the existence of a tumor-specific RAS that can be independently targeted by RAS blockade.

Effect of ACE inhibitors and angiotensin II receptor antagonists in a mouse model of colorectal cancer liver metastases.

BACKGROUND AND AIMS: Angiotensin converting enzyme inhibitors (ACE-I) and angiotensin II type I receptor (AT1R) antagonists are commonly used as a treatment for hypertension. Recent experimental and population studies have suggested that these agents may exert an inhibitory effect on malignancy, possibly through anti-angiogenic pathways. The aim of this study was to investigate the effect of an ACE-I (captopril) and an AT1R antagonist (irbesartan) in colorectal cancer liver metastases. METHODS: The effect of captopril and irbesartan on tumor growth was investigated in a mouse model using quantitative stereological and histological analysis. Tumor microcirculation was assessed by microvascular resin casting. A survival study was also carried out. RESULTS: Both captopril and irbesartan markedly decreased tumor growth when compared to control (P = 0.003 and P = 0.004, respectively). However, there was no significant difference in survival or tumor necrosis for either of the drugs. Tumor microvasculature exhibited a reduction in central microvascular density, with constriction and tapering of vessels. CONCLUSION: Captopril and irbesartan caused a marked reduction in volume of colorectal cancer liver metastases and caused changes in tumor microvasculature. However, there was no difference in percentage tumor necrosis or improvements in survival. Further investigation is needed to identify the mode of action of these agents.