Geoffrey Brown

The Emerging Oncogenic Role of RARγ: From Stem Cell Regulation to a Potential Cancer Therapy

Retinoic acid receptor (RAR) γ expression is restricted during adult haematopoiesis to haematopoietic stem cells and their immediate offspring and is required for their maintenance. From zebrafish studies, RARγ is selectively expressed by stem cells and agonism in the absence of exogenous all-trans retinoic acid blocked stem cell development. Recent findings for the expression of RARγ have revealed an oncogenic role in acute myeloid leukaemia and cholangiocarcinoma and colorectal, head and neck, hepatocellular, ovarian, pancreatic, prostate, and renal cancer. Overexpression and agonism of RARγ enhanced cell proliferation for head and neck, hepatocellular, and prostate cancer. RARγ antagonism, pan-RAR antagonism, and RARγ downregulation led to cell growth which was often followed by cell death for acute myeloid leukaemia, astrocytoma, and cholangiocarcinoma as well as hepatocellular, primitive, neuroectodermal ovarian, and prostate cancer. Histological studies have associated high level RARγ expression with high-grade disease, metastasis, and a poor prognosis for cholangiocarcinoma and ovarian, pancreatic, and prostate cancer. RARγ is expressed by cancer stem cells and is a targetable drive of cancer cell growth and survival.

Targeting the Retinoic Acid Pathway to Eradicate Cancer Stem Cells

All-trans retinoic acid is a morphogen during embryogenesis and a teratogen. Cancer is an error of development, and the retinoic acid receptors (RAR) for all-trans retinoic acid play a role in cancer. Expression of the cytosolic aldehyde dehydrogenases, which mediate the last step to the synthesis of all-trans retinoic acid, is deregulated in various human cancers. Inhibiting these enzymes using a variety of agents reduced the proliferation of lung cancer cells, reduced the proliferation and induced apoptosis of ovarian, prostate, squamous, and uterine cancer cells, and sensitised breast, colorectal and ovarian cancer cells to chemotherapeutic agents. RARγ is an oncogene within some cases of AML, cholangiocarcinoma, colorectal cancer, clear cell renal cell carcinoma, hepatocellular carcinoma, pancreatic ductal adenocarcinoma, prostate cancer, and ovarian cancer. Pan-RAR and RARγ antagonist inhibition of the action of RARγ led to necroptosis of human prostate and pediatric brain tumour cancer stem cells. Treatment of hepatocellular carcinoma cells with the flavenoid acacetin, which interferes with the action of RARγ, decreased cell growth and induced apoptosis. Targeting the retinoic acid pathway is promising regarding the development of new drugs to eradicate cancer stem cells.