Hyper-responsiveness to 5-HT2 agonists by tumour-linked arterioles in mice: consequences for tumor growth

Expression of Serotonin Receptors 2B and 4 in Human Prostate Cancer Tissue and Effects of Their Antagonists on Prostate Cancer Cell Lines

OBJECTIVES

Overexpression of receptors to neuroendocrine (NE) cell products has been suggested to contribute to development of hormone-refractory prostate cancer (HRPC). In this study, we evaluated the expression of 5-HTR2B and 5-HTR4 in HRPC, and the effects of their antagonist on PC cell line growth.

METHODS

Proteins and mRNA expression was determined by immunohistochemistry, western blot and RT-PCR. Growth inhibition of PC cell lines was determined in vitro using ELISA-BrdU proliferation assay and cell cycle was evaluated by flow cytometry.

RESULTS

Immunostaining of 5-HTR2B was observed in low-grade and high-grade tumours, PIN and BPH cells, and in vascular endothelial cells, whereas 5-HTR4 was found predominantly in high-grade tumours. This result was confirmed by western blot analysis. At the mRNA level, 5-HTR4 mRNA was expressed in DU145 and LNCaP cells. Antagonists to both receptor subtypes inhibited proliferation of PC cells in a dose-dependent manner.

CONCLUSIONS

The present result indicate that 5-HTRs are present at various tumour stages and that antagonists to these receptors can inhibit the proliferative activity of androgen-independent PC cell lines.

A novel drug to reduce tumor perfusion: antitumor effect alone and with hyperthermia

We have investigated the feasibility of enhancing damage induced by hyperthermia in SCK murine tumors by reducing tumor blood perfusion using a new agent, KB-R8498. Within several minutes of an i.v. injection, the tumor perfusion was reduced to less than 20% of the control value, and it recovered to 40-70% of the control value by 1 h after injection. The perfusion in normal tissues decreased or increased soon after drug administration depending on the tissue type. However, by 1 h after drug treatment, perfusion in five of the seven tissues examined had returned to the control level. The tumor pH was also reduced after i.v. drug administration. Control tumors grew to four times the initial volume in 6 days. Tumors that were heated at 42.5 degrees C for 60 min were delayed in growth by 4 days compared to control tumors. There was a growth delay of 14 days when an i.v. injection of KB-R8498 was given and the tumors were heated at 42.5 degrees C either immediately or 1 h later. In drug-alone studies, the tumor growth was delayed by 4 days when the drug was infused continuously at a rate of 30-50 mg/kg day(-1) for 7 days or about 2 days when mice were treated with five daily injections of 30 mg/kg KB-R8498.

Impact of serotonin on tumour growth

Several opposite effects of serotonin (5HT) on tumour growth have been reported. On one hand, 5HT is known as a growth factor for several types of nontumoural cells, and it has been proposed to take part in the autocrine loops of growth factors contributing to cell proliferation in aggressive tumours such as small cell lung carcinoma. Depending on the tumour type either 5HT2 or 5HT1 receptor antagonist have been found to inhibit the 5HT-induced increase in tumour growth. In contrast, several authors have also reported that 5HT and 5HT2 agonist can inhibit tumour growth. Most often this effect has been considered to be related with the specific vasoconstrictive effect of 5HT or 5HT2 agonists on the vessels irrigating the tumour, which has been evidenced by intravital microscopy. Intravital microscopy studies have also shown that vessels perfusing the tumour exhibit a specific vasconstrictive response to 5HT1 agonists. In addition, 5HT has been shown to be involved in the effects of several anticancer treatments associated with the reduction of tumour flow. Finally, the specific vasoconstrictive effect of 5HT or 5HT receptor subtype agonists might also be useful in inducing hypoxia in tumours, which could be exploited in a strategy using hypoxia-selective cytotoxins or hypoxia-selective gene therapy.

Reduction of tumour blood flow with KB-R8498 potentiates the response of tumours to hyperthermia

The anti-tumour activity and the effect on tumour and normal tissue perfusion of a newly discovered anticancer agent, KB-R8498 (Kanebo Ltd., Osaka, Japan), were investigated in FSa II tumours of C3H mice. The tumour perfusion, as measured by the 86Rb-uptake method, markedly decreased with relatively little change in the normal tissue perfusion after an i.v. injection of KB-R8498. Furthermore, the drug potentiated the effect of hyperthermia at 42.5 degrees C for 60 min to suppress the tumour growth. The results suggest that the preferential reduction in tumour blood flow relative to normal tissue blood flow by KB-R8498 may be exploited to enhance the anti-tumour effect of hyperthermia.

Pharmacological reactivity of neoplastic and non-neoplastic associated neovasculature to vasoconstrictors

Angiogenesis and the pharmacological responses of the tumour and non-tumour associated neovasculature have been investigated. Cannulated sponge discs in mice were used to host the angiogenic stimulators, while 133Xe washout was employed to assess local blood flow. Enhancement of blood flow was detected in implants bearing B16 cells, 3T3 cells and angiotensin II (AII)-treated at day 7. The responses of non-neoplastic associated neovasculature at day 14 post sponge implantation to the vasoconstrictors used endothelin-1 (Et-1), AII, platelet activating factor (PAF) and 5-hydroxytryptamine (5-HT) were dose-dependent. By contrast, the newly formed blood vessels induced by tumour cells were markedly insensitive to the vasoconstrictors agonists Et-1 and AII, while fully responsive to PAF and 5-HT. The vessels resulting from neoplastic stimulus exhibited altered pharmacological reactivity, suggesting that the characteristics of the neovasculature are dependent on the nature of the angiogenic stimuli.

Enhancement of the anti-tumour effects of the antivascular agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) by combination with 5-hydroxytryptamine and bioreductive drugs

The tumour blood flow inhibitor 5,6-dimethylxanthenone-4-acetic acid (DMXAA) causes dramatic haemorrhagic necrosis in murine tumours, but activity is seen only at doses close to the toxic limit. This study investigates two approaches for increasing the therapeutic ratio of DMXAA. The first approach combines DMXAA with a second tumour blood flow inhibitor, 5-hydroxytryptamine (5-HT). Co-administration of 5-HT (700 micromol kg(-1)) to C3H mice caused marked enhancement of DMXAA effects against MDAH-MCa-4 tumours, with dose-modifying factors (DMFs) of >3 for blood flow inhibition (at 4 h), 2.3 for necrosis (at 12 h) and 2.0 for growth delay, without compromising the maximum tolerated dose of DMXAA (90 micromol kg(-1)). The data are consistent with ischaemic injury to the tumour being the major mechanism of anti-tumour activity. The second approach combines DMXAA (+/- 5-HT) with hypoxia-selective bioreductive drugs. Anti-tumour activity of all three bioreductive drugs tested (tirapazamine, CI-1010, SN 23816) was strongly potentiated by DMXAA, suggesting that there is a population of reversibly hypoxic tumour cells after DMXAA treatment. Co-administration of 5-HT further potentiated anti-tumour activity, but also increased host toxicity of tirapazamine and CI-1010 so that little therapeutic benefit was achieved. In contrast, the host toxicity of the dinitrobenzamide mustard SN 23816 was only slightly increased by DMXAA/5-HT, whereas the tumour growth delay at the maximum tolerated dose of SN 23816 was increased from 3.5 to 26.5 days. This study demonstrates that 5-HT and/or bioreductive drugs can improve the therapeutic activity of DMXAA in mice, and that with SN 23816 both approaches can be used together to provide considerably enhanced anti-tumour activity.

Specific response of mouse tumor-feeding arterioles to stimulation by 5-HT1 agonists

Using intravital microscopy, we compared the responses to 5-HT1 receptor stimulation by the host-modified arterioles feeding a Meth-A tumor implanted in the flank of female Balb/c mice with the responses of tumor-independent arterioles (TIA) and those of control arterioles from mice without tumor. Topical administration of 5 x 10(-5) M serotonin in the presence of 10(-4) M ketanserin (5-HT2 receptors inhibitor) induced arteriolar vasodilation in TIA (+13%) and in the control arterioles (+19%), but induced constriction (-14%) in the tumor-feeding arterioles (TFA). Topical administration of the general 5-HT1 agonist 5-carboxamidotryptamine maleate (10(-6) to 10(-4) M) or the 5-HT1A agonist buspirone (2 x 10(-6) to 2 x 10(-4) M) induced vasoconstriction that was dramatically higher in TFA than in TIA or control arterioles (p < 0.0001 in both cases). In addition, topical administration of the 5-HT1B agonist M-trifluoromethylphenylpiperazine (2 x 10(-6) to 2 x 10(-4) M) produced opposite responses, i.e., dose-dependent vasodilation in TIA and control arterioles, and dose-dependent constriction in TFA. Since we observed the same degree of vasodilation in response to 10(-4) M acetylcholine in all three groups of arterioles, the differences between the responses to 5-HT1 receptor stimulation were not due to the absence of endothelial-dependent dilatory mechanisms in the tumor-feeding arterioles. We conclude that 5-HT1 agonists are interesting pharmacologic tools for the modulation of tumoral blood flow, since they more dramatically constrict the microvasculature feeding the tumors than that feeding normal tissue.

Effects of tumour cells on angiogenesis and vasoconstrictor responses in sponge implants in mice

The effects of tumour cells (Colon 26) on the development and response of new blood vessels to different vasoconstrictors (platelet activating factor; PAF, endothelin-1, angiotensin II, adrenalin and 5-hydroxytryptamine) have been investigated. Sponge implants in mice were used to host tumour cells while washout of 133Xe was employed to assess local blood flow in the implanted sponges. By 14 days after implantation the response of vessels in tumour-bearing implants to the various vasoconstrictors generally was decreased compared to that obtained in control sponge implants or adjacent normal skin. Thus at this time point the t1/2 for 133Xe washout from control sponges treated with adrenalin (0.5 micrograms) was 30 +/- 4 min whereas in tumour-bearing sponges it was 5 +/- 1 min. This decreased sensitivity in tumour vessels was probably not due to a complete lack of contractile elements since actin was demonstrated by immunohistochemistry around blood vessels in both types of implant. The results of the present study have shown that the pharmacological responses of blood vessels in a growing tumour, Colon 26, differed from the responses of vessels of a similar age in non-neoplastic tissue. These results appear to suggest that the different angiogenic stimuli released from tumour tissue may markedly influence pharmacological reactivity of newly formed blood vessels.