A high cannabinoid CB₁ receptor immunoreactivity is associated with disease severity and outcome in prostate cancer

Abstract

In the light of findings indicating that cannabinoids can affect the proliferation of a number of cancer cell types and that cannabinoid receptor expression is higher in prostate cancer cell lines than in non-malignant cells, we investigated whether the level of cannabinoid 1 receptor immunoreactivity (CB₁IR) in prostate cancer tissues is associated with disease severity and outcome. Formalin-fixed paraffin-embedded non-malignant and tumour tissue samples from patients who were diagnosed with prostate cancer at a transurethral resection for voiding problems were used. CB₁IR, which was scored in a total of 399 cases, was associated with the epithelial cell membranes, with little staining in the stroma. Patients with a tumour CB₁IR score greater or equal to the median (2) had a significantly higher proportion of Gleason scores 8–10, metastases at diagnosis, tumour size and rate of cell proliferation at diagnosis than patients with a score < 2. For 269 cases, tumour CB₁IR was measured for patients who only received palliative therapy at the end stages of the disease, allowing the influence of CB₁IR upon the disease outcome to be determined. Receiver operating characteristic (ROC) curves showed an area under the curve of 0.67 (95% confidence limits 0.59–0.74) for CB₁IR in the tumour. CB₁IR in non-malignant tissue was not associated with disease outcome. A tumour CB₁IR score ⩾ 2 was associated with a significantly lower disease specific survival. A Cox proportional hazards regression indicated that the tumour CB₁IR score and the Gleason score were independent prognostic variables. It is concluded that a high tumour CB₁IR score is associated with prostate cancer severity and outcome.

Introduction

Prostate cancer is the major cancer form afflicting males. In the United States of America (USA), for example, the American Cancer Society listed 218,219 new cancer cases in their estimates for 2007, a number approximately equal to the number of male cases for lung and bronchus, colon and rectum, and melanoma of the skin put together.¹ There is a wide range of treatment options for prostate cancer depending on tumour characteristics, patient age and status. However, the curative treatments for localised prostate cancer (prostatectomy and radiation therapy) are associated with the risks of side-effects, including erectile dysfunction and incontinence,2, 3, 4 and both novel treatment strategies and prognostic markers (to avoid the overtreatment of patients who would better have been served by surveillance alone⁵) are much needed.

Δ⁹-Tetrahydrocannabinol (Δ⁹-THC), the main psychoactive ingredient of cannabis, produces most of its effects via the activation of two G-protein coupled cannabinoid (CB) receptors termed CB₁ and CB₂. CB₁ receptors are among the most common of all receptor types in the brain, and are also located peripherally in both neurons and non-neuronal tissue, whilst CB₂ receptors are mainly found in immune cells.⁶ The endogenous ligands for these receptors (‘endocannabinoids’) anandamide and 2-arachidonoylglycerol are synthesised on demand and mimic many of the actions of Δ⁹-THC, but have short-lived effects due to effective metabolic pathways.7, 8

Synthetic Δ⁹-THC (Marinol™) and its analogue nabilone (Cesamet™) are licensed in the USA, Canada and the United Kingdom (UK) for their palliative effects upon chemotherapy-induced nausea and vomiting. However, cannabinoids also affect the viability, proliferation, adhesion and migration of cancer cells, including prostate cancer cells, and can reduce angiogenesis and the growth of tumour cells implanted into nude mice.9, 10, 11, 12, 13, 14, 15, 16, 17, 18 In 2005, Sarfaraz and colleagues¹³ reported that CB₁ receptor expression by the human prostate cancer cell lines, LNCaP (androgen-sensitive), DU145 and PC3 (androgen-independent), was higher than that seen in normal human prostate epithelial cells. Furthermore, the CB₁ receptor expression was higher in CA-HPAV10 cells (virally transformed cells from the adenocarcinoma of human prostate tissue) than in the corresponding CA-HPV-7 cells, also virally transformed, but from normal human tissue.¹³ It is not known, however, whether the level of CB₁ receptor immunoreactivity (CB₁IR) is associated with disease severity and outcome in prostate cancer tissue samples from patients. This question has been addressed in the present study.