Down-Regulation of Cannabinoid Type 1 (CB1) Receptor and its Downstream Signaling Pathways in Metastatic Colorectal Cancer
Valeria Tutino, Maria Gabriella Caruso, Valentina De Nunzio, Dionigi Lorusso, Nicola Veronese, Isabella Gigante, Maria Notarnicola and Gianluigi Giannelli
Cancers, 2019, 11, 708, 1-14.
Changes in the regulation of endocannabinoid production, together with an altered expression of their receptors are hallmarks of cancer, including colorectal cancer (CRC). Although several studies have been conducted to understand the biological role of the CB1 receptor in cancer, little is known about its involvement in the metastatic process of CRC. The aim of this study was to investigate the possible link between CB1 receptor expression and the presence of metastasis in patients with CRC, investigating the main signaling pathways elicited downstream of CB1 receptor in colon cancer. Fifty-nine consecutive patients, with histologically proven colorectal cancer, were enrolled in the study, of which 30 patients with synchronous metastasis, at first diagnosis and 29 without metastasis. A low expression of CB1 receptor were detected in primary tumor tissue of CRC patients with metastasis and consequently, we observed an alteration of CB1 receptor downstream signaling. These signaling routes were also altered in intestinal normal mucosa, suggesting that, normal mucosa surrounding the tumor provides a realistic picture of the molecules involved in tissue malignant transformation. These observations contribute to the idea that drugs able to induce CB1 receptor expression can be helpful in order to set new anticancer therapeutic strategies.
Keywords : endocannabinoid system; cannabinoid type 1 (CB1) receptor; colorectal cancer; metastasis
Cannabinoid receptors and their endogenous ligands, the endocannabinoids, constitute the endocannabinoids system (ECS), known to be important in regulating gastrointestinal motility, secretion, inflammation, and immunity . ECS has been demonstrated to have a role in the regulation of signaling pathways involved in cancer pathogenesis [2,3]. Numerous studies have provided evidence that a deregulation of endocannabinoids production, together with an altered expression of their receptors are hallmarks of cancer [4–6]. The role of ECS in the onset of cancer is dierent and depends on the cancer type and tissue . A high expression of Cannabinoid type 1 (CB1) receptor, but not cannabinoid type 2 (CB2), has been observed in pancreatic cancer , whereas an over-expression of both CB1 and CB2 receptors seems to be correlated with an improved prognosis in hepatocellular carcinoma . In colon cancer, CB1 receptor has been detected at low levels and its inhibition has been demonstrated to accelerate intestinal adenoma growth . In particular, CB1 deficient mice exhibited a more severe inflammatory status in the colon tissue, suggesting a protective role of this receptor against colonic tissue inflammation and malignant transformation [9,10]. Previously, we demonstrated a significant reduction in CB1 receptor gene expression levels in cancer tissue compared to normal surrounding mucosa of patients with colorectal cancer (CRC), confirming that the negative modulation of cell proliferation, mediated by CB1 receptor, is lost in cancer . CB1 receptor induction seems to inhibit the proliferation of colon cancer cell lines, aecting cell cycle, in particular reducing the number
of cells in the S phase and decreasing the cell polyamines content [12,13].
CB1 receptor gene up-regulation in ApcMin/+ mice fed a diet enriched with omega 3-PUFAs, was correlated with a significant inhibition of intestinal polyps growth mediated by a concurrent inactivation of the Wnt/-catenin pathway . The growth suppressing eect of CB1 receptor is due to a down-regulation of the epidermal growth factor receptor (EGFR) in prostate cancer cell lines  or via the activation of cAMP/protein kinase A pathway in breast cancer cells .
Experimental evidence suggests the use of cannabinoids as potential anticancer agents, given their ability to exert antitumoral eects in vitro and in animal models of cancer [11–16]. Cannabinoids via their receptors, especially CB1 receptor, can modulate signaling pathways that control cell survival and apoptosis [12,17]. Although several studies have been conducted to understand the biological role of the CB1 receptor in cancer, little is known about its involvement in metastatic process of CRC. Metastatic CRC has limited treatment options and thus results in higher mortality rates compared to non-metastatic CRC . The cancer cells that are able to survival in circulation, and to migrate and invade another organ, are characterized by the expression of cellular oncogenes or the loss of tumor suppressor gene function . Identifying the cellular mechanisms that regulate metastasis onset may be useful to develop eective anticancer therapies.
It has demonstrated that endogenous ligands of CB1 receptor, as anandamide, are able to regulate several stages of the metastatic process, modulating both cell migration and invasion . Cannabinoids seem also to have a regulatory role in angiogenesis, inhibiting the formation and tumor-induced angiogenesis in a model of endothelial tumor cells .
On the basis of these evidences and considering our previous data showing a significant reduction vof CB1 receptor gene expression in cancer compared to normal surrounding mucosa from patients with CRC , here we hypothesize that CB1 receptor may be associated with disease severity of CRC. Therefore, the aim of the present study was to address a possible link between CB1 receptor expression and the presence of metastasis in patients with CRC. Moreover, we investigated the main signaling pathways at the basis of the action of CB1 receptor in colon cancer.