Bone metastasis can be osteolytic or osteosclerotic (osteoblastic)2. Osteoclasts are multinucleated cells, which maintain a healthy skeleton by resorbing the bone matrix. Macrophage colony-stimulating factor (MCSF) and RANKL promote the differentiation of osteoclasts from macrophages3. Imbalances in the osteoclastic differentiation pathway or increased osteoclastic activity leads to bone metastases. MCSF enhances osteoclast formation and survival of preosteoclasts and mice lacking MCSF have a complete lack of osteoclasts3. RANKL produced by stromal cells and osteocytes mediates NF-B and c-FOS activation enabling the recruitment of NFATc1, which is essential for osteoclastic differentiation4. NFATc1 stimulates expression of the calcitonin receptor, TRACP, CTSK (Cathepsin K), and OSCAR, which are osteoclast-specific genes. Anti-resorptive therapies, i.e., bisphosphonates and denosumab (a human monoclonal antibody that is a RANKL inhibitor), are used for osteoclastic bone metastasis5. However, bisphosphonates have some serious adverse side effects, e.g., joint pain, jawbone loss, and long-term use can cause hypocalcemia. Targeting only the bone microenvironment or the cancer cells is not capable of effectively or efficiently treating this deadly disease.
Melanoma differentiation-associated gene-7 (MDA-7)/ Interleukin-24 (IL-24) a member of IL-10 gene family, is a well-studied cytokine discovered in our laboratory by subtraction hybridization6, and established as a therapeutic molecule in a broad array of cancers7, 8. This multifunctional protein delivered by adenovirus induces apoptosis or toxic autophagy uniquely in cancer cells, inhibits tumor angiogenesis, stimulates an anti-cancer immune response, promotes killing of primary tumor cells and metastatic tumor cells as well as cancer stem cells through potent “bystander anti-tumor effects”, and synergizes with current therapies such as radiation, chemotherapy and monoclonal antibody-based therapies resulting in enhanced cancer toxicity6, 7. A unique property of MDA-7/IL-24 is its ability to induce production and secretion of MDA-7/IL-24 protein in normal and cancer cells by a paracrine/autocrine loop8. MDA-7/IL-24 delivered as a recombinant protein also exhibits anti-cancer therapeutic action8, 9. In a recent study, we used a pre-clinical bone metastasis experimental model involving intra-cardiac implantation of PC3-ML human prostate cancer cells to study the therapeutic efficacy of recombinant MDA-7/IL-24 protein10. MDA-7/IL-24-treated animals contained less metastatic lesions in their bones in comparison with controls. This therapeutic effect was the result of cancer-specific cell killing and osteoclastic inhibition by MDA-7/IL-24.
The Akt-Mcl-1 pathway plays a decisive role in osteoclastic differentiation. In this study10, we showed that MDA-7/IL-24 inhibits the osteoclastic differentiation pathway by regulating the Akt-Mcl-1 pathway. Previous research from our laboratory revealed that MDA-7/IL-24 (delivered by a therapeutic cancer terminator virus (CTV)) synergistically acted with BI-97D6, a Mcl-1 inhibitor, to inhibit prostate cancer growth and survival in vitro and in vivo11. Similarly, in a recent study BI-97D6 synergized with MDA-7/IL-24 protein and induced robust therapeutic activity in vivo in a pre-clinical bone metastatic prostate cancer model, without any signs of toxicity10. Our present research10, supports further development of MDA-7/IL-24 as a potential anti-cancer and anti-metastatic therapeutic for prostate cancer-mediated bone metastasis. The ability of a small molecule inhibitor of Mcl-1 to synergistically act with MDA-7/IL-24 protein to further enhance therapy of prostate cancer bone metastasis is also worthy of increased scrutiny.
Figure: Regulation of prostate bone metastasis by purified MDA-7/IL-24 protein alone and in combination with Mcl-1 inhibition. RANKL induces Mcl-1, which contributes to the survival of osteoclast and cancer cells. RANKL induces phosphorylation of Akt, which in turn activates NFATc1. NFATc1 activates osteoclast differentiation-specific genes, e.g., Cathepsin K, TRAP, and OSCAR. MDA-7/IL-24 blocks NFATc1 by blocking Akt phosphorylation. Additionally, MDA-7/IL-24 blocks Mcl-1, thereby regulating the survival of osteoclasts. Mcl-1 inhibitor can synergize with MDA-7/IL-24 in this pathway.
- Ye, L., H.G. Kynaston, and W.G. Jiang. 2007. Bone metastasis in prostate cancer: molecular and cellular mechanisms (Review). Int. J. Mol. Med. 20:103-111.
- Tanaka, Y., S. Nakayamada, and Y. Okada. 2005. Osteoblasts and osteoclasts in bone remodeling and inflammation. Current drug targets. Inflammation and allergy. 4:325-328.
- Glantschnig, H., J.E. Fisher, G. Wesolowski, G.A. Rodan, and A.A. Reszka. 2003. M-CSF, TNFalpha and RANK ligand promote osteoclast survival by signaling through mTOR/S6 kinase. Cell Death Differ. 10:1165-1177.
- Kim, K., J.H. Kim, J. Lee, H.M. Jin, S.H. Lee, D.E. Fisher, H. Kook, K.K. Kim, Y. Choi, and N. Kim. 2005. Nuclear factor of activated T cells c1 induces osteoclast-associated receptor gene expression during tumor necrosis factor-related activation-induced cytokine-mediated osteoclastogenesis. J. Biol. Chem. 280:35209-35216.
- Nanes, M.S. 2010. Preventing metastases to bone: denosumab or bisphosphonates? J. Bone Miner. Res. 25:437-439.
- Jiang H, J.J. Lin, Z.-Z. Su, N.I. Goldstein,and P.B. Fisher. 1995. Subtraction hybridization identifies a novel melanoma differentiation associated gene, mda-7, modulated during human melanoma differentiation, growth and progression. Oncogene. 11:2477-2486.
- Menezes, M.E., S. Bhatia, P. Bhoopathi, S.K. Das, L. Emdad, S. Dasgupta, P. Dent, X.-Y. Wang, D. Sarkar, and P.B. Fisher. 2014. MDA-7/IL-24: multifunctional cancer killing cytokine. Adv Exp Med Biol. 818:127-153.
- Menezes, M.E., P. Bhoopathi, A.K. Pradhan, L. Emdad, S.K. Das, C. Guo, X.Y. Wang, D. Sarkar, and P.B. Fisher. 2018. Role of MDA-7/IL-24 a Multifunction Protein in Human Diseases. Adv. Cancer Res. 138:143-182.
- Sauane M, Su ZZ, Gupta P, Lebedeva IV, Dent P, Sarkar D,Fisher PB. 2008. Autocrine regulation of mda-7/IL-24 mediates cancer-specific apoptosis. Proc Natl Acad Sci U S A. 105: 9763-9768.
- Pradhan, A.K., P. Bhoopathi, S. Talukdar, X.N. Shen, L. Emdad, S.K. Das, D. Sarkar, and P.B. Fisher. 2018. Recombinant MDA-7/IL-24 suppresses prostate cancer bone metastasis through down-regulation of the Akt/Mcl-1 pathway. Mol. Cancer Ther. In Press
- Sarkar, S., B.A. Quinn, X.N. Shen, R. Dash, S.K. Das, L. Emdad, A.L. Klibanov, X.Y. Wang, M. Pellecchia, D. Sarkar, and P.B. Fisher. 2015. Therapy of prostate cancer using a novel cancer terminator virus and a small molecule BH-3 mimetic. Oncotarget. 6:10712-10727.
Department of Human and Molecular Genetics, VCU Institute of Molecular Medicine and VCU Massey Cancer Center, Virginia Commonwealth University, School of Medicine, Richmond, VA
Acknowledgment: The present studies were supported by the National Foundation for Cancer Research (PBF), NIH grant P50 CA058236 (to P.B. Fisher and M.G. Pomper) and NCI Cancer Center Support Grant to VCU Massey Cancer Center P30 CA016059 (to P.B. Fisher and D. Sarkar).
Conflicts: PBF is a co-founder of Cancer Targeting Systems (CTS), Inc. and owns stock and was a consultant. PBF is also a co-founder of InterLeukin Combinatorial Therapies (ILCT), Inc. and owns stock and is a consultant. VCU, Johns Hopkins and Columbia University own stock in CTS and VCU owns stock in ILCT. None of the other authors have conflicts associated with these studies.
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