Association between cannabinoid CB₁ receptor expression and Akt signalling in prostate cancer

Abstract

Background: In prostate cancer, tumour expression of cannabinoid CB₁ receptors is associated with a poor prognosis. One explanation for this association comes from experiments with transfected astrocytoma cells, where a high CB receptor expression recruits the Akt signalling survival pathway. In the present study, we have investigated the association between CB₁ receptor expression and the Akt pathway in a well-characterised prostate cancer tissue microarray.

Methodology/principal findings: Phosphorylated Akt immunoreactivity (pAkt-IR) scores were available in the database. CB₁ receptor immunoreactivity (CB₁IR) was rescored from previously published data using the same scale as pAkt-IR. There was a highly significant correlation between CB₁IR and pAkt-IR. Further, cases with high expression levels of both biomarkers were much more likely to have a more severe form of the disease at diagnosis than those with low expression levels. The two biomarkers had additive effects, rather than an interaction, upon disease-specific survival.

Conclusions/significance: The present study provides data that is consistent with the hypothesis that at a high CB₁ receptor expression, the Akt signalling pathway becomes operative.

Figure 1. Tumour nCB₁IR: variation with pAkt-IR.

Panel A shows two tumour cores from the same case (Gleason score 7) showing a large variation in the CB₁IR intensity. They had positions 1 and 5 of the tumour series arrowed. The left core was scored 0.75 (50% score 0, 25% score 1, 25% score 2) by one investigator, and 1 (25% score 0, 50% score 1, 25% score 2) by the other. The right core was scored 2.75 by both investigators (50% score 2, 25% score 3, 25% score 4 by one investigator; 25% score 2, 75% score 3 by the other investigator). Panel B and C show the pAkt scores for individual cores in the test (n = 595, Panel B) and validation (n = 297, Panel C) datasets, divided into approximate nCB₁IR quadrants. The number of cases in each quadrant is shown in the graph, together with the median and interquartile ranges. *P<0.05, **P<0.01, ***P<0.001 for the comparisons shown, otherwise not significant (Dunn's multiple comparison test following significant (P<0.0001) Kruskal-Wallis test. The spearman's rho for correlations between the core nCB₁IR and the core pAkt-IR were 0.29 and 0.28 (both P<0.0001) for the test and validation sets, respectively.

Figure 2. Scatter plots of cases scored for tumour nCB₁IR (abscissae) and pAkt-IR (ordinates) and sorted on the basis of A, Gleason score (GS); B, absence or presence of metastases at diagnosis; C, Ki-67 index; D, tumour grade (LT).

The Ki67 index is a continuous variable ranging from 0–48% in the dataset . The tranches were chosen here for illustrative purposes but represent the bottom 50% (“Ki67 Low”), the 50–75% (“Ki67 intermediate”) and the top 25% (“Ki67 high”). The dotted lines in the figures show the median scores for nCB₁IR and pAkt-IR for the dataset.

Figure 3. Scatter plots and Kaplan-Meier plots for the cases who were followed by expectancy and who had been scored for both nCB₁IR and pAkt-IR.

Panel A shows a scatter plot of the individual cases, so that the group names in the other Panels are easier to follow. In the Kaplan-Meier plots shown in Panels B to D, ^†Pca refers to the number of patients who died as a result of their prostate cancer during the follow-up period. The ² values are for the log-rank (Mantel-Cox) tests, with the P values shown. Panel B, all cases; Panel C, Gleason score 4–6 cases; Panel D, Gleason score 7–10 cases.