Deficiency or blockade of angiotensin II type 2 receptor delays tumorigenesis by inhibiting malignant cell proliferation and angiogenesis

A Novel Cellular Model to Study Angiotensin II AT2 Receptor Function in Breast Cancer Cells

Recent studies have highlighted the AT1 receptor as a potential therapeutic target in breast cancer, while the role of the AT2 subtype in this disease has remained largely neglected. The present study describes the generation and characterization of a new cellular model of human invasive breast cancer cells (D3H2LN-AT2) stably expressing high levels of Flag-tagged human AT2 receptor (Flag-hAT2). These cells exhibit high-affinity binding sites for AngII, and total binding can be displaced by the AT2-selective antagonist PD123319 but not by the AT1-selective antagonist losartan. Of interest, high levels of expression of luciferase and green fluorescent protein make these cells suitable for bioluminescence and fluorescence studies in vitro and in vivo. We provide here a novel tool to investigate the AT2 receptor functions in breast cancer cells, independently of AT1 receptor activation.

Is there a role of the local tissue RAS in the regulation of physiologic and pathophysiologic conditions in the reproductive tract?

The renin-angiotensin system is well known as a systemic endocrine pathway that regulates blood pressure and salt-water metabolism. In addition to the systemic renin-angiotensin system there is evidence in different species for the presence of a local tissue renin-angiotensin system, which allows local production of the bioactive peptides angiotensin II and angiotensin (1-7). The local renin-angiotensin system has been found in a variety of tissues including tissue of the human reproductive tract. Thus, it was suspected that it may have important functions in the local hormonal microenvironment. Here, a systematic literature search was undertaken to review whether there is evidence for regulatory functions of the local tissue renin-angiotensin system in the human reproductive tract under physiological and pathological conditions.

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.

Regulation of androgen receptor expression through angiotensin II type 1 receptor in prostate cancer cells

BACKGROUND

Although the local renin-angiotensin system (RAS) of the prostate gland is related to cell proliferation and angiogenesis, the detailed mechanism remains unclear. We examined the effects of the angiotensin II type 1 receptor (AT1R) on androgen receptor (AR) expression in prostate cancer cells.

METHODS

AR modulation by AT1R was examined by Western blot analysis, luciferase assay, and Immunocytochemical staining. The influence of AR expression by angiotensin II (Ang-II) and AT1R inhibition using siRNA was determined. Furthermore, using angiotensinogen or AT1R knockout (KO) mice, we performed quantitative real-time PCR to investigate the expression of AR.

RESULTS

Ang-II induced cell proliferation with enhancement of AR, prostate specific antigen (PSA), NF-κB, and c-myc, and the activity of AR and PSA promoter. Cell proliferation of LNCaP transfected with AT1R siRNA was decreased by 75% at 7 days by inhibition of AR, PSA, NF-κB, and c-myc. Immunocytochemical staining confirmed the suppression of AR translocation into the nucleus in AT1R siRNA cells. AT1R KO mice showed a decrease in AR expression in the prostate gland. We also found that the expression level of AT1R could modulate the transcriptional level of AR by affecting NF-κB and c-myc expression.

CONCLUSIONS

Knocking down of the AT1R protein resulted in significant inhibition of cell growth, associated with a marked decrease of AR protein. These results indicate that inhibition of AT1R has the potential to influence AR expression in prostate cells, and is anticipated to contribute to the development of novel therapeutic agents for prostate cancer.

A current evaluation of the safety of angiotensin receptor blockers and direct renin inhibitors

The safety of angiotensin II receptor blockers (ARBs) for the treatment of hypertension and cardiovascular and renal diseases has been well documented in numerous randomized clinical trials involving thousands of patients. However, recent concerns have surfaced about possible links between ARBs and increased risks of myocardial infarction and cancer. Less is known about the safety of the direct renin inhibitor aliskiren, which was approved as an antihypertensive in 2007. This article provides a detailed review of the safety of ARBs and aliskiren, with an emphasis on the risks of cancer and myocardial infarction associated with ARBs. Safety data were identified by searching PubMed and Food and Drug Administration (FDA) Web sites through April 2011. ARBs are generally well tolerated, with no known class-specific adverse events. The possibility of an increased risk of myocardial infarction associated with ARBs was suggested predominantly because the Valsartan Antihypertensive Long-Term Use Evaluation (VALUE) trial reported a statistically significant increase in the incidence of myocardial infarction with valsartan compared with amlodipine. However, no large-scale, randomized clinical trials published after the VALUE study have shown a statistically significant increase in the incidence of myocardial infarction associated with ARBs compared with placebo or non-ARBs. Meta-analyses examining the risk of cancer associated with ARBs have produced conflicting results, most likely due to the inherent limitations of analyzing heterogeneous data and a lack of published cancer data. An ongoing safety investigation by the FDA has not concluded that ARBs increase the risk of cancer. Pooled safety results from clinical trials indicate that aliskiren is well tolerated, with a safety profile similar to that of placebo. ARBs and aliskiren are well tolerated in patients with hypertension and certain cardiovascular and renal conditions; their benefits outweigh possible safety concerns.

Angiotensin receptor blockers and angiogenesis: clinical and experimental evidence

Angiotensin II type 1 receptor antagonists [ARBs (angiotensin receptor blockers)] are indicated for BP (blood pressure)-lowering, renal protection and cardioprotection in patients unable to tolerate ACEIs (angiotensin-converting enzyme inhibitors). A recent meta-analysis revealed an association between ARBs and tumour development, possibly due to enhancement of angiogenesis. However, published evidence is conflicting on the effects of ARBs on angiogenesis or the expansion of the existing vascular network. ARBs have been shown to exert primarily anti-angiogenic effects in basic science studies of cancer, retinopathy, peripheral artery disease and some models of cardiovascular disease. In animal and cellular models of myocardial infarction and stroke, however, ARB administration has been associated with robust increases in vascular density and improved recovery. The aim of the present review is to examine the angiogenic effects of ARBs in animal and cellular models of relevant disease states, including proposed molecular mechanisms of action of ARBs and the clinical consequences of ARB use.

Use of angiotensin receptor blockers and the risk of cancer

BACKGROUND

A recent meta-analysis of randomized trials suggested that use of angiotensin receptor blockers (ARBs) may be associated with a modestly increased risk of incident cancer, particularly lung cancer.

METHODS AND RESULTS

We linked individual-level data from Danish registries on filled drug prescriptions, diagnostic information, and covariates. In a nationwide cohort of new users of ARBs and angiotensin-converting enzyme inhibitors ≥35 years of age during 1998 to 2006, we compared incidence rates of all cancer, cancer subgroups by anatomic site, and cancer mortality. Among 107 466 ARB users, 3954 cases of cancer were detected during 312 753 person-years of follow-up compared with 6214 cases during 435 207 person-years of follow-up in 209 692 angiotensin-converting enzyme inhibitor users (adjusted rate ratio, 0.99; 95% confidence interval, 0.95 to 1.03). Cancer risk did not increase with increasing duration of ARB exposure (increase in rate ratio per year, 0.99; 95% confidence interval, 0.99 to 1.00,) and was similar across individual ARBs. In subgroup analyses, there was a significant association between ARB use and cancer of male genital organs (rate ratio, 1.15; 95% confidence interval, 1.02 to 1.28), but no significantly increased risk of any of the other 15 cancer subgroups, including lung cancer (rate ratio, 0.92; 95% confidence interval, 0.82 to 1.02). For cancer mortality, the rate ratio was 0.77 (95% confidence interval, 0.72 to 0.82).

CONCLUSION

In this large nationwide cohort, use of ARBs was not significantly associated with increased risk of incident cancer overall or of lung cancer.

The renin-angiotensin system and cancer: old dog, new tricks

For cancers to develop, sustain and spread, the appropriation of key homeostatic physiological systems that influence cell growth, migration and death, as well as inflammation and the expansion of vascular networks are required. There is accumulating molecular and in vivo evidence to indicate that the expression and actions of the renin-angiotensin system (RAS) influence malignancy and also predict that RAS inhibitors, which are currently used to treat hypertension and cardiovascular disease, might augment cancer therapies. To appreciate this potential hegemony of the RAS in cancer, an expanded comprehension of the cellular actions of this system is needed, as well as a greater focus on translational and in vivo research.

Targeting the angiotensin II type 2 receptor (AT2R) in colorectal liver metastases

BACKGROUND

Blockade of the angiotensin (ANG) II type 1 receptor (AT1R) inhibits tumour growth in several cancers, including colorectal cancer (CRC) liver metastases. While AT1R blockade has been extensively studied, the potential of targeting the antagonistically acting AT2R in cancer has not been investigated. This study examined the effect of AT2R activation with the agonist CGP42112A in a mouse model of CRC liver metastases.

RESULTS

In vitro, mouse CRC cell (MoCR) proliferation was inhibited by treatment with CGP42112A in a dose dependent manner while apoptosis was increased. Immunofluorescent staining for key signalling and secondary messengers, PLA2 and iNOS, were also increased by CGP42112A treatment in vitro. Immunohistochemical staining for proliferation (PCNA) and the apoptosis (active caspase 3) markers confirmed a CGP42112A-associated inhibition of proliferation and induction of apoptosis of mouse CRC cells (MoCR) in vivo. However, angiogenesis and vascular endothelial growth factor (VEGF) appeared to be increased by CGP42112A treatment in vivo. This increase in VEGF secretion by MoCRs was confirmed in vitro. Despite this apparent pro-angiogenic effect, a syngenic orthotopic mouse model of CRC liver metastases showed a reduction in liver to body weight ratio, an indication of tumour burden, following CGP42112A treatment compared to untreated controls.

CONCLUSIONS

These results suggest that AT2R activation might provide a novel target to inhibit tumour growth. Its potential to stimulate angiogenesis could be compensated by combination with anti-angiogenic agents.