Exogenous regucalcin suppresses the proliferation of human breast cancer MDA-MB-231 bone metastatic cells in vitro

Regucalcin Is a Potential Regulator in Human Cancer: Aiming to Expand into Cancer Therapy

Regucalcin, a calcium-binding protein lacking the EF-hand motif, was initially discovered in 1978. Its name is indicative of its function in calcium signaling regulation. The rgn gene encodes for regucalcin and is situated on the X chromosome in both humans and vertebrates. Regucalcin regulates pivotal enzymes involved in signal transduction and has an inhibitory function, which includes protein kinases, protein phosphatases, cysteinyl protease, nitric oxide dynthetase, aminoacyl-transfer ribonucleic acid (tRNA) synthetase, and protein synthesis. This cytoplasmic protein is transported to the nucleus where it regulates deoxyribonucleic acid and RNA synthesis as well as gene expression. Overexpression of regucalcin inhibits proliferation in both normal and cancer cells in vitro, independent of apoptosis. During liver regeneration in vivo, endogenous regucalcin suppresses cell growth when overexpressed. Regucalcin mRNA and protein expressions are significantly downregulated in tumor tissues of patients with various types of cancers. Patients exhibiting upregulated regucalcin in tumor tissue have shown prolonged survival. The decrease of regucalcin expression is linked to the advancement of cancer. Overexpression of regucalcin carries the potential for preventing and treating carcinogenesis. Additionally, extracellular regucalcin has displayed control over various types of human cancer cells. Regucalcin may hold a prominent role as a regulatory factor in cancer development. Supplying the regucalcin gene could prove to be a valuable asset in cancer treatment. The therapeutic value of regucalcin suggests its potential significance in treating cancer patients. This review delves into the most recent research on the regulatory role of regucalcin in human cancer development, providing a novel approach for treatment.

Functional pleiotropy of calcium binding protein Regucalcin in signaling and diseases

Regucalcin (Mr ∼ 33.38 kDa) is a calcium binding protein, discovered in rat liver. In humans, gene for regucalcin is located on chromosome-11 (p11.3-q11.2) consisting of seven exons and six introns. The protein differs from other calcium binding protein in the way that it lacks EF-hand motif of calcium binding domain. It is also called as Senescence Marker Protein-30 (SMP-30) as previously its weight assumes to be 30 kDa and expression of this protein decreases with aging in androgen independent manner. Among vertebrates, it is a highly conserved protein showing gene homology in Drosophila, Xenopus, fireflies and others too. It is primarily expressed in liver and kidney in addition to brain, lungs, and skeletal muscles. Regucalcin acts as a Ca²⁺ regulatory protein and controls various cellular functions in liver and other organs. It suppresses protein phosphatase, protein kinase, DNA and RNA synthesis. Published evidences suggest regucalcin to be a reliable biomarker in various disorders of liver, kidney, brain and ocular. In over expressed state, it subdues apoptosis in cloned rat hepatoma cells and also induces hyperlipidemia and osteoblastogenesis by regulating various factors. Owing to the multi-functionality of regucalcin this review is presented to elaborate its importance in order to understand its involvement in cellular signaling during various pathologies.

Extracellular Regucalcin Suppresses the Growth, Migration, Invasion and Adhesion of Metastatic Human Prostate Cancer Cells

Regucalcin plays a multifunctional role in the regulation of cellular function including metabolism, signaling process and transcriptional activity in maintaining cell homeostasis. Downregulated expression or activity of regucalcin contributes to the development of malignancies in various types of human cancer. Survival of cancer patients, including metastatic prostate cancer, is prolonged with high expression of regucalcin in the tumor tissues. Furthermore, we elucidate whether extracellular regucalcin conquers the growth, migration, invasion and adhesion of metastatic human prostate cancer PC-3 and DU-145 cells. Extracellular regucalcin (0.1, 1, and 10 nM) of physiologic levels inhibited colony formation and growth of PC-3 and DU-145 cells, while it did not have an effect on cell death. Repressive effects of extracellular regucalcin on the proliferation were not exhibited by the presence of inhibitors of cell cycle, intracellular signaling process and transcriptional activity, suggesting that the signals of extracellular regucalcin are transmitted to block cell growth. Furthermore, extracellular regucalcin (0.1, 1, or 10 nM) inhibited migration, invasion and adhesion of PC-3 and DU-145 cells. Mechanistically, extracellular regucalcin (10 nM) decreased the levels of various signaling proteins including Ras, hosphatidylinositol-3 kinase, mitogen-activated protein kinase, mTOR, RSK-2, caveolin-1 and integrin β1 in PC-3 cells. Thus, extracellular regucalcin may play a suppressive role in growth, migration, invasion and adhesion, which are involved in metastatic activity of human prostate cancer cells, via affecting diverse signaling processes. This study may provide a new strategy in preventing metastatic prostate cancer with exogenous regucalcin.

Extracellular regucalcin suppresses colony formation and growth independent of tumor suppressor p53 in human mammary epithelial cells

Regucalcin plays a multifunctional role in cell regulation as a suppressor in the processes of intracellular signaling and transcription, leading to inhibition of cell growth. The downregulated expression or activity of regucalcin has been shown to contribute to the development of carcinogenesis in various types of human cancer. The wild-type tumor suppressor TP53 gene encodes for a transcriptional factor p53. This protein may play a role in cell proliferation. Loss of p53 function may induce cell transformation during carcinogenesis and tumor progression of human cancer. We investigate whether or not extracellular regucalcin suppresses the proliferation of non-tumorigenic human mammary epithelial MCF 10A cells with loss of p53 in vitro. Loss of p53 did not impact colony formation and proliferation of the cells. Interestingly, p53 loss caused decrease in the cell cycle suppressor p21, but not retinoblastoma and regucalcin, as compared with those of wild-type MCF 10A cells. Notably, extracellular regucalcin suppressed colony formation and proliferation of wild-type MCF 10A cells and p53 (-/-) cells, while it did not have an effect on cell death. Mechanistically, extracellular regucalcin decreased levels of various signaling factors including Ras, phosphatidylinositol-3 kinase, mitogen-activated protein kinase (MAPK), phospho-MAPK, and signal transducer and activator of transcription 3 in wild-type MCF 10A cells and p53 (-/-) cells. Thus, extracellular regucalcin was found to suppress the growth of MCF 10A cells with loss of p53. Extracellular regucalcin may play a role as a suppressor in the growth of human mammary epithelial cells with p53 loss, providing a novel strategy for cancer.

Overexpression of Regucalcin Suppresses the Growth of Human Osteosarcoma Cells in Vitro: Repressive Effect of Extracellular Regucalcin

Regucalcin plays a pivotal role as a suppressor of human carcinogenesis, and downregulation of regucalcin expression may contribute to the promotion of human osteosarcoma. Overexpression of regucalcin suppressed the proliferation of Saos-2 human osteosarcoma cells in vitro and decreased the protein levels of multiple signaling components, transcription factors, and tumor suppressors. Interestingly, extracellular regucalcin repressed colony formation and proliferation of Saos-2 cells, and reduced the protein levels of multiple signaling components, cell cycle inhibitor, and various transcription factors. Thus, regucalcin suppressed the growth of human osteosarcoma cells, providing a novel strategy with the gene therapy for treatment of osteosarcoma.

The current status and perspectives regarding the clinical implication of intracellular calcium in breast cancer

Calcium ions (Ca²⁺ ) act as second messengers in intracellular signaling. Ca²⁺ pumps, channels, sensors, and calcium binding proteins, regulate the concentrations of intracellular Ca²⁺ as a key regulator of important cellular processes such as gene expression, proliferation, differentiation, DNA repair, apoptosis, metastasis, and hormone secretion. Intracellular Ca²⁺ also influences the functions of several organelles, that include: the endoplasmic reticulum, mitochondria, the Golgi, and cell membrane both in normal and breast cancer cells. In breast cancer, the disruption of intracellular: Ca²⁺ homeostasis may cause tumor progression by affecting key factors/pathways including phospholipase C (PLC), inositol 1,4,5-trisphosphate (IP3), calmodulin (CaM), nuclear factor of activated T-cells (NFAT), calpain, calmodulin-dependent protein kinase II (CaMKII), mitogen-activated protein kinase (MAPK), epithelial-mesenchymal transition (EMT), vascular endothelial growth factor (VEGF), poly (ADP-Ribose) polymerase-1 (PARP1), estrogen, and estrogen receptor. Because the foregoing molecules play crucial roles in breast cancer, the factors/pathways influencing intracellular Ca²⁺ concentrations are putative targets for cancer treatment, using drugs such as Mephebrindole, Tilapia piscidin 4, Nifetepimine, Paricalcitol, and Prednisolone. We have explored the factors/pathways which are related to breast cancer and Ca²⁺ homeostasis and signaling in this review, and also discussed their potential as biomarkers for breast cancer staging, prognosis, and therapy.