RANKL


 * Not to be confused with is_associated_with::RANK, the osteoclast cell-surface receptor that binds to RANKL.

Receptor activator of nuclear factor kappa-B ligand (RANKL), also known as tumor necrosis factor ligand superfamily member 11 (TNFSF11), TNF-related activation-induced cytokine (TRANCE), osteoprotegerin ligand (OPGL), and osteoclast differentiation factor (ODF), is a is_associated_with::protein that in humans is encoded by the TNFSF11 is_associated_with::gene.

RANKL is known as a type II membrane protein and is a member of the is_associated_with::tumor necrosis factor (TNF) superfamily. RANKL has been identified to affect the immune system and control bone regeneration and remodeling. RANKL may be used as an apoptosis regulator gene, a binding partner of OPG, a ligand for the receptor RANK and control proliferation by modifying protein levels of Id4, Id2 and cyclin D1. RANKL is expressed in several tissues and organs including: skeletal muscle, thymus, liver, colon, small intestine, adrenal gland, osteoclast, mammary gland epithelial cells, prostate and pancreas. Variation in concentration levels of RANKL throughout several organs, reconfirm the uses and importance of growth (particularly bone growth) and immune functions within the body.

Tissue expression
The level of RANKL expression does not linearly correlate to the effect of this ligand. High protein expression of RANKL are commonly detected in the lung, thymus and lymph nodes, and low protein expression is found in bone marrow, stomach, peripheral blood, spleen, placenta, leukocytes, heart, thyroid and skeletal muscle. While bone marrow expresses low levels of RANKL, it plays a critical role for adequate bone metabolism, this surface-bound molecule (also known as CD254) found on is_associated_with::osteoblasts serves to activate is_associated_with::osteoclasts, which are critically involved in is_associated_with::bone resorption. Osteoclastic activity is triggered via the osteoblasts' surface-bound RANKL activating the osteoclasts' surface-bound receptor activator of nuclear factor kappa-B (RANK).

Gene and expression
RANKL can be expressed in three different molecular forms consisting of either a: (1) trimeric transmembrane protein, (2) primary secreted form, and (3) truncated ectodomain. RANKL is identified as a part of the TNF family; RANKL is specifically categorized under the TNFSF11, the TNF ligand superfamily member. RANKL is composed of 314 amino acids and was originally described to have a gene sequence containing 5 is_associated_with::exons. Among the exons, Exon 1 encoded the intracellular and transmembrane protein domains and Exon 2-5 encoded the extracellular domains. RANKL’s extracellular domains are similar to other TNF family members in regards to the structural homology and are able to cleave from the cell surface. While the function and significance of A kinase anchor protein 11(AKAP11) is presently unknown, AKAP11 is immediately upstream from RANKL for all species that has a RANKL gene. The upstream of AKAP11 may suggest there is a complex regulator process that regulates the level of RANKL expression.

Function
Since RANKL is a member of the is_associated_with::tumor necrosis factor (TNF) is_associated_with::cytokine family, it is a ligand for is_associated_with::osteoprotegerin and functions as a key factor for is_associated_with::osteoclast differentiation and activation. RANKL also has a function in the immune system, where it is expressed by is_associated_with::T helper cells and is thought to be involved in is_associated_with::dendritic cell maturation. This protein was shown to be a is_associated_with::dendritic cell survival factor and is involved in the regulation of is_associated_with::T cell-dependent immune response. T cell activation was reported to induce expression of this gene and lead to an increase of osteoclastogenesis and bone loss. This protein was shown to activate antiapoptotic kinase AKT/PKB through a signaling complex involving SRC kinase and tumor necrosis factor receptor-associated factor 6 (is_associated_with::TRAF6), which indicated this protein may have a role in the regulation of cell is_associated_with::apoptosis.

Animal models
Targeted disruption of the related gene in mice led to severe is_associated_with::osteopetrosis and a lack of osteoclasts. Deficient mice, with an inactivation of RANKL or its receptor RANK, exhibited defects in early differentiation of T and B lymphocytes, and failed to form lobulo-alveolar mammary structures during pregnancy. It was observed that during pregnancy, RANK-RANKL signaling played a critical role in regulating skeletal calcium release; in which contributed to the hormone response that stimulated proliferation in the mammary cells. Ultimately, impaired lobuloalveolar mammary structures resulted in death of the fetus. Those who suffer from osteoporosis often have a cardiovascular defect, such as heart failure. Some studies suggest, since RANK-RANKL pathway regulates calcium release and homeostasis, RANK-RANKL signal could invertedly affect the cardiovascular system; thus, an explanation for the positive correlation between osteoporosis and cardiovascular deficiencies.

Role in cancer
Primary tumors will commonly metastasize into the bone. Myeloma, Breast, and prostate cancers typically have a greater chance of inducing secondary cancers within bone. is_associated_with::Stephen Paget's seed and soil theory suggests, the microenvironment in bone creates a sufficient ‘soil’ for secondary tumors to grow in. Some studies suggest the expression of RANKL allows sufficient micro environmental conditions to influence cancer cell migration (i.e. chronic lymphocytic leukemia (CLL) and multiple myeloma). Among patients with multiple myeloma, RANKL activity was greatly increased. In fact RANKL’s surface expression and RANKL release expression was reported to be an increase of, respectively, 80% and 50%. Therefore, RANKL is considered to be a key signal regulator for cancer-induce bone diseases.

According to the vicious cycle hypothesis, after secondary tumors cells have migrated to bone, the tumor cell will secrete cytokines and growth factors to osteoblast. Since osteoblast control the regulation of RANKL, the stimulation via cytokines and growth factors, will then stimulate osteoblasts to increase the expression of RANKL. The additional binding of RANK/RANKL within an osteoclast will secrete tumor growth factor, which can ultimately increase tumor growth and bone destruction activity.

Clinical significance
Through the binding of RANKL, osteoclasts and osteoblasts play a vital role in normal bone remodeling. Overproduction of RANKL is implicated in a variety of degenerative bone diseases, such as is_associated_with::rheumatoid arthritis and is_associated_with::psoriatic arthritis. In addition to degenerative bone diseases, bone metastases can also induce pain and other abnormal health complexities that can significantly reduce a cancer patient’s quality of life. Some examples of these complications that are a consequence of bone metastasis are: hypercalcemia, pathological fractures and spinal cord compression. Some findings also suggestion some cancers cells, particularly prostate cancer cells, can activate an increase in bone remodeling and ultimately increase overall bone production. This increase in bone remodeling and bone production increases the overall growth of bone metastasizes. The overall control of bone remodeling is regulated by the binding of RANK with its receptor or its decoy receptor, respectively, RANKL and OPG.

Denosumab
Denosumab is an FDA-approved fully human monoclonal antibody to RANKL and during pre-clinical trials was first used to treat postmenopausal patients suffering with osteoporosis (PMO). In denosumab's third stage of the FDA's clinical trial, it was shown to: (1) decrease bone turnover, (2) reduce fractures in the PMO population, and (3) increase bone mineral density. Since then the anti-RANKL antibody, denosumab, has been used in some cancer clinical trials. In both prostate and breast cancer, denosumab has been shown to reduce cancer treatment–induced bone loss.

Prostate cancer
The HALT-prostate cancer trial (also known as NCT00089674) included 1468 non-metastatic prostate cancer patients who were currently receiving is_associated_with::androgen deprivation therapy. Randomly selected patients were given either 60 mg of denosumab or calcium and vitamin D supplements. This was done to measure the effectiveness of preventing treatment-induced bone loss. The patients who received 60 mg of denosumab showed a +5.6% increased in bone mineral density and a 1.5% decrease in bone fracture rates.

Another clinical trial (NCT00321620) was established to determine the safety and effectiveness of using denosumab compared to is_associated_with::zoledronic acid. In this trial, they used 1901 bone metastatic prostate patients whom were also suffering with other complication of bone diseases. Again, patients were randomized and some were given either 120 mg of denosumab or 4 mg of zoledronic acid. Patients who were given 120 mg of denosumab (in comparison to those who were given 4 mg of zoledronic acid) show an greater increase in hypocalcemia, a greater resistance to bone turnover markers uNTx, a delay response in both pathological fractures and spinal cord compression. However, survival rates for both clinical groups were comparable.

Breast cancer
Hormone receptor positive breast cancer patients have a significant increased in developing complications such as is_associated_with::osteopenia and is_associated_with::osteoporosis. According to cancer.org, about two out of every three breast cancer patients are hormone receptor positive. In the past several years, denosumab has been used in several clinical trials primarily because a much large population is affected by bone complication among those who have breast cancer.

There were 252 breast cancer patients enlisted in the HALT-BC clinical trial (also known as NCT00089661). In addition to receiving vitamin D and calcium supplements, half of the patients were randomly given 60 mg of denosumab while the other half were given a placebo. In comparison, patients who were given denosumab had an increase in lumbar spine bone mineral density, a decrease in bone turnover markers; however there was not a significant change in survival rates.

Another phase III clinical trial for breast cancer patients was conducted and was known as the NCT00321464 trial. Similar to the prostate trial NCT00321620, this trial was conducted to measure the safety and efficacy of using denosumab compared to zoledronic acid. Patients were randomized into two groups, one who received zoledronic acid and the other group received denosumab. When comparing survival rates and the frequency of adverse events, both groups show similar outcomes.

Multiple myeloma
Patients whom are diagnosed with multiple myeloma have approximately 80-100% chance of developing bone complications due to an increase in activity and/or formation of osteoclasts and a decrease activity of osteoblasts. In a stage II clinical trial, denosumab had decrease bone turnover markers by blocking the RANKL/RANK pathway. Once this trial was completed, 1176 patients with either multiple myeloma or progressed cancers were entered into the stage III clinical trial (known as NCT00330759). The main objective of the NCT00330759 trial was to compare effects of patients who were given 120 mg of denosumab relative to patients give 4 mg of zoledronic acid. As a result of this trial, during a month period, patients who received denosumab had a decrease in pathological fractures and spinal cord compression; however, as time progressed it appear that denosumab had significantly delayed bone complications. Similar to both breast and prostate cancers, patients in either denosumab or zoledronic acid groups appear to have compare adverse events and survival rates.

Medroxyprogesterone acetate
Women with menopause have often been given various types of postmenopausal hormone therapies to prevent osteoporosis and reduce menopausal symptoms. is_associated_with::Medroxyprogesterone acetate (MPA) is a synthetic progestin and was commonly used as a contraceptive or used as a hormone therapy for endometriosis or osteoporosis. Recent studies suggest, using MPA increases patient risks of developing is_associated_with::breast cancer due to an increase expression of RANKL. MPA causes a substantial induction of RANKL in mammary-gland epithelial cells while deletion of RANKL decreases the incidence MPA-induced breast cancer. Hence inhibition of RANKL has potential for the prevention and treatment of breast cancer.