Table 2.

Effect of experimental manipulation of opioid receptor expression on tumour growth and aggressiveness.

Receptor	Cancer	Manipulation/comparison	Findings	Reference
μ-opioid receptor	Squamous Cell Carcinoma of the Head and Neck	siRNA down regulation of μOR in FaDu MDA686Tu and UMSCC47 cultured cells	In FaDu and MDA686Tu cells, downregulating μOR expression inhibited aggressive features	Gorur A (34)
μ-opioid receptor	Lung Cancer	Lewis lung carcinoma (LLC) cells were either transfected with control shRNA or μOR -1 shRNA	Silencing (shRNA) μOR expression in LLC inhibits invasion and anchorage-independent growth in vitro, and experimental metastasis in vivo. μOR -deficient mice inoculated with LLC have reduced tumour formation.	Matthew (21)
μ-opioid receptor	Melanoma	Wild-type and μOR R-deficient mice were inoculated with B16 melanoma cells that secrete endogenous mu-opioid peptides	μOR -deficient mice demonstrated a marked reduction in tumour growth and significantly higher infiltration of immune cells into the tumours when inoculated with B16 melanoma cells	Boehncke (35)
μ-opioid receptor	Lung Cancer	μOR -overexpressing lung cancer xenografts in nude mice μOR was inhibited with the peripheral μOR antagonist MNTX	Overexpression of μOR in cancer cells increased primary tumour growth rates and lung metastases Inhibiting μOR attenuates EGF-induced proliferation and migration	Lennon (36)
μ-opioid receptor	Non-Small Cell Lung Cancer	Stable vector control and μOR 1 overexpressing human bronchioloalveolar carcinoma cells Xenografted to tumour-bearing nude mice	μOR overexpression increased proliferation and extravasation In vivo, overexpression of μOR in human bronchoalveolar carcinoma cells increased primary tumour growth rates in nude mice by approximately 2.5-fold and lung metastasis by approximately 20-fold compared with vector control cells	Lennon (37)
δ-opioid receptor	Breast Cancer	MCF7 cells transduced with δOR siRNA or control siRNA Xenografted to tumour-bearing nude mice	δOR siRNA inhibits tumour growth in vitro and in vivo	Wei Y-C (26)
δ-opioid receptor	Hepatocellular Carcinoma	siRNA down regulation of δOR in cultured cells (LO2, HepG2, and Hep3B)	δOR siRNA inhibits aggressiveness in vitro and in vivo	Tang (27)
μ and δ-opioid receptors	Non-small Cell Lung Cancer	H2009 Non-small Cell Lung Cancer cell line	siRNA down regulation of either μOR and δOR decreases EGFR activation by EGF and EGFR transactivation by morphine	Fujioka (38)
Opioid Growth Factor receptor (OGFr)	Squamous Cell Carcinoma of the Head and Neck	human SCC-1 and clonal lines overexpressing wild type OGFR or empty vector	OGFR overexpression led to decreased cell proliferation	McLaughlin P (33)