Patricia Juárez

Regulation of NKCC2 by a chloride-sensing mechanism involving the WNK3 and SPAK kinases

The Na+:K+:2Cl− cotransporter (NKCC2) is the target of loop diuretics and is mutated in Bartters syndrome, a heterogeneous autosomal recessive disease that impairs salt reabsorption in the kidneys thick ascending limb (TAL). Despite the importance of this cation/chloride cotransporter (CCC), the mechanisms that underlie its regulation are largely unknown. Here, we show that intracellular chloride depletion in Xenopus laevis oocytes, achieved by either coexpression of the K-Cl cotransporter KCC2 or low-chloride hypotonic stress, activates NKCC2 by promoting the phosphorylation of three highly conserved threonines (96, 101, and 111) in the amino terminus. Elimination of these residues renders NKCC2 unresponsive to reductions of [Cl−]i. The chloride-sensitive activation of NKCC2 requires the interaction of two serine-threonine kinases, WNK3 (related to WNK1 and WNK4, genes mutated in a Mendelian form of hypertension) and SPAK (a Ste20-type kinase known to interact with and phosphorylate other CCCs). WNK3 is positioned upstream of SPAK and appears to be the chloride-sensitive kinase. Elimination of WNK3s unique SPAK-binding motif prevents its activation of NKCC2, as does the mutation of threonines 96, 101, and 111. A catalytically inactive WNK3 mutant also completely prevents NKCC2 activation by intracellular chloride depletion. Together these data reveal a chloride-sensing mechanism that regulates NKCC2 and provide insight into how increases in the level of intracellular chloride in TAL cells, as seen in certain pathological states, could drastically impair renal salt reabsorption.

GLI2-Mediated Melanoma Invasion and Metastasis

BACKGROUND The transforming growth factor-beta (TGF-beta) pathway, which has both tumor suppressor and pro-oncogenic activities, is often constitutively active in melanoma and is a marker of poor prognosis. Recently, we identified GLI2, a mediator of the hedgehog pathway, as a transcriptional target of TGF-beta signaling. METHODS We used real-time reverse transcription-polymerase chain reaction (RT-PCR) and western blotting to determine GLI2 expression in human melanoma cell lines and subsequently classified them as GLI2high or as GLI2low according to their relative GLI2 mRNA and protein expression levels. GLI2 expression was reduced in a GLI2high cell line with lentiviral expression of short hairpin RNA targeting GLI2. We assessed the role of GLI2 in melanoma cell invasiveness in Matrigel assays. We measured secretion of matrix metalloproteinase (MMP)-2 and MMP-9 by gelatin zymography and expression of E-cadherin by western blotting and RT-PCR. The role of GLI2 in development of bone metastases was determined following intracardiac injection of melanoma cells in immunocompromised mice (n = 5-13). Human melanoma samples (n = 79) at various stages of disease progression were analyzed for GLI2 and E-cadherin expression by immunohistochemistry, in situ hybridization, or RT-PCR. All statistical tests were two-sided. RESULTS Among melanoma cell lines, increased GLI2 expression was associated with loss of E-cadherin expression and with increased capacity to invade Matrigel and to form bone metastases in mice (mean osteolytic tumor area: GLI2high vs GLI2low, 2.81 vs 0.93 mm(2), difference = 1.88 mm(2), 95% confidence interval [CI] = 1.16 to 2.60, P < .001). Reduction of GLI2 expression in melanoma cells that had expressed high levels of GLI2 substantially inhibited both basal and TGF-beta-induced cell migration, invasion (mean number of Matrigel invading cells: shGLI2 vs shCtrl (control), 52.6 vs 100, difference = 47.4, 95% CI = 37.0 to 57.8, P = .024; for shGLI2 + TGF-beta vs shCtrl + TGF-beta, 31.0 vs 161.9, difference = -130.9, 95% CI = -96.2 to -165.5, P = .002), and MMP secretion in vitro and the development of experimental bone metastases in mice. Within human melanoma lesions, GLI2 expression was heterogeneous, associated with tumor regions in which E-cadherin was lost and increased in the most aggressive tumors. CONCLUSION GLI2 was directly involved in driving melanoma invasion and metastasis in this preclinical study.

TGF-β Pathway as a Therapeutic Target in Bone Metastases

Breast and prostate cancer frequently metastasizes to the skeleton and causes bone destruction. In skeletal tissue, transforming growth factor-beta (TGF-beta) is a major bone-derived factor responsible for driving a feed-forward vicious cycle of breast cancer growth in bone. TGF-beta is released from bone in active form by osteoclastic resorption and increases the tumor secretion of factors, which stimulate osteolytic destruction of the bone adjacent to the tumor. Moreover it activates epithelial-mesenchymal transition and tumor cell invasion, increases angiogenesis and induces immunosuppression. Blocking the TGF-beta signaling pathway to interrupt this vicious cycle between tumor and bone offers a target for therapeutic intervention to decrease skeletal metastasis. Here we summarize the current knowledge of TGF-beta in bone metastases, the use of TGF-beta inhibitors and its potential for clinical use and consequences.

Plant-derived anticancer agents: a promising treatment for bone metastasis

Bone metastasis is a very frequent complication of advanced cancer, and it remains an incurable disease. Current therapies that have been approved for the treatment of bone metastases delay the occurrence of skeletal-related events and can extend the patients lifespan by a few years. However, they will not cure or cause the regression of established bone metastases, and new side effects are emerging after prolonged treatment. Thus, new therapies are severely needed. There are compelling evidences from in vitro and in vivo preclinical studies that support the use of compounds derived from plants to treat several forms of cancers including bone metastasis. More than 25% of the drugs used during the past 20 years were directly derived from plants, whereas another 25% are chemically altered natural products. Still, only 5-15% of the ∼250 000 higher plants have ever been investigated for bioactive compounds. There is a growing interest for the study of anticancer drugs with relatively low side effects that target specific key signaling pathways that control the establishment and progression of the cancer metastasis. Therefore, further studies are needed to identify new natural compounds with high efficiency in cancer prevention and treatment. Extensive reviews about plant-derived agents and their use in cancer have been published, but none when it comes to the treatment of bone metastases. Only a few of these compounds have been evaluated for the treatment of bone metastasis; here we describe some of the most prominent ones that are having the potential to reach the clinic soon.

Therapeutic strategies to target TGF-β in the treatment of bone metastases.

Bone is one of the most common organs to be affected in patients with metastatic cancer. These bone metastases are often accompanied by bone destruction, bone fractures, pain, and hypercalcemia. Transforming growth factor-β (TGF-β) is a major bone-derived factor that is released in active form upon osteoclastic bone resorption. TGF-β, in turn, stimulates bone metastatic cells to secrete factors that further drive osteolytic destruction of the bone adjacent to the tumor, categorizing TGF-β as a crucial factor responsible for driving the feed-forward vicious cycle of cancer growth in bone. Moreover, TGF-β activates epithelial-to-mesenchymal transition, increases tumor cell invasiveness and angiogenesis and induces immunosuppression. Blocking the TGF-β signaling pathway to interrupt this vicious cycle between tumor cells and bone offers a promising target for therapeutic intervention to decrease skeletal metastasis. In this review, preclinical and clinical data are evaluated for the potential use of TGF-β inhibitors in clinical practice to treat bone metastases.

Halofuginone inhibits TGF-β/BMP signaling and in combination with zoledronic acid enhances inhibition of breast cancer bone metastasis

More efficient therapies that target multiple molecular mechanisms are needed for the treatment of incurable bone metastases. Halofuginone is a plant alkaloid-derivative with antiangiogenic and antiproliferative effects. Here we demonstrate that halofuginone is an effective therapy for the treatment of bone metastases, through multiple actions that include inhibition of TGFβ and BMP-signaling. Halofuginone blocked TGF-β-signaling in MDA-MB-231 and PC3 cells showed by inhibition of TGF-β–induced Smad-reporter, phosphorylation of Smad-proteins, and expression of TGF-β-regulated metastatic genes. Halofuginone increased inhibitory Smad7-mRNA and reduced TGF-β-receptor II protein. Proline supplementation but not Smad7-knockdown reversed halofuginone-inhibition of TGF-β-signaling. Halofuginone also decreased BMP-signaling. Treatment of MDA-MB-231 and PC3 cells with halofuginone reduced the BMP-Smad-reporter (BRE)4, Smad1/5/8-phosphorylation and mRNA of the BMP-regulated gene Id-1. Halofuginone decreased immunostaining of phospho-Smad2/3 and phospho-Smad1/5/8 in cancer cells in vivo. Furthermore, halofuginone decreased tumor-take and growth of orthotopic-tumors. Mice with breast or prostate bone metastases treated with halofuginone had significantly less osteolysis than control mice. Combined treatment with halofuginone and zoledronic-acid significantly reduced osteolytic area more than either treatment alone. Thus, halofuginone reduces breast and prostate cancer bone metastases in mice and combined with treatment currently approved by the FDA is an effective treatment for this devastating complication of breast and prostate-cancer.

Tumor-bone interactions: There is no place like bone

Cancer whether from solid tumors or in the form of hematologic malignancies frequently spreads to bone where it will disrupt the balance of bone remodeling between bone formation and resorption. Cancer patients with skeletal complications will experience severe clinical consequences affecting their quality of life, the worst of all being that cancer in bone remains incurable. As research and our understanding of these pathologies are progressing, the intricate interactions between cancer cells and bones are being revealed showing us that the affinity of cancer cells for bone is already determined in the primary tumor or how cancer cells can prepare the metastatic niche prior to colonizing the skeleton. These complex processes are regulated by multiple genes that cooperate to ensure cancer cell growth and survival, despite the treatments developed. In this chapter, we will review the mechanisms controlling the different steps leading to overt metastases and how these could be used to develop new therapies to prevent skeletal complications of malignancies or to cure cancer from bone.

Regulation of the Transforming Growth Factor-β Superfamily by Betaglycan

The type III transforming growth factor-β (TGF-β) receptor (TβRIII), also known as betaglycan (BG), is a membrane proteoglycan coreceptor that modulates the action of diverse members of the TGF-β superfamily of growth factors. Membrane bound BG is the precursor of a soluble receptor form generated by the proteolytic cleavage (shedding) of its extracellular region. While membrane BG enhances TGF-β binding to the type II TGF-β receptor, thereby increasing the activity of the factor (1), the soluble form sequesters the ligand, acting as a neutralizing agent of TGF-β (2). Recombinant soluble BG (rSBG) has been used successfully for treatment of diverse experimental pathologies in which TGF-β plays a physiopathological role. Given the recent discovery that BG shedding can be regulated, TGF-β modulatory properties of BG make it an attractive target for pathologies in which TGF-β action. Despite its reputation as an “accessory” TGF-β receptor, the mice lacking BG (BG-null or TβRIII−/−) exhibited an embryonic lethal phenotype (3), indicating that this coreceptor has essential, not yet identified, cellular functions. Here, we review the current knowledge of BG and discuss future scenarios for BG research and applications.