Bone metastasis is mediated by complex interactions between tumor cells and resident stromal cells in the bone microenvironment. The functions of metalloproteinases in organ-specific metastasis remain poorly defined despite their well-appreciated role in matrix degradation and tumor invasion. Here, we show a mechanism whereby two distinct metalloproteinases, a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS1) and matrix metalloproteinase-1 (MMP1), orchestrate a paracrine signaling cascade to modulate the bone microenvironment in favor of osteoclastogenesis and bone metastasis. Proteolytic release of membrane-bound epidermal growth factor (EGF)-like growth factors, including Amphiregulin (AREG), heparin-binding EGF (HB-EGF), and transforming growth factor α (TGFα) from tumor cells suppress the expression of osteoprotegerin (OPG) in osteoblasts and subsequently potentiate osteoclast differentiation. EGF receptor (EGFR) inhibitors block osteolytic bone metastasis by targeting EGFR signaling in bone stromal cells. Furthermore, elevated MMP1 and ADAMTS1 expression is associated with increased risk of bone metastasis in breast cancer patients. This study established MMP1 and ADAMTS1 in tumor cells, as well as EGFR signaling in osteoblasts, as promising therapeutic targets for inhibiting bone metastasis of breast cancer.

The tumor microenvironment (TME) is being increasingly recognized as a key factor in multiple stages of disease progression, particularly local resistance, immune-escaping, and distant metastasis, thereby substantially impacting the future development of frontline interventions in clinical oncology. An appropriate understanding of the TME promotes evaluation and selection of candidate agents to control malignancies at both the primary sites as well as the metastatic settings. This review presents a timely outline of research advances in TME biology and highlights the prospect of targeting the TME as a critical strategy to overcome acquired resistance, prevent metastasis, and improve therapeutic efficacy. As benign cells in TME niches actively modulate response of cancer cells to a broad range of standard chemotherapies and targeted agents, cancer-oriented therapeutics should be combined with TME-targeting treatments to achieve optimal clinical outcomes. Overall, a body of updated information is delivered to summarize recently emerging and rapidly progressing aspects of TME studies, and to provide a significant guideline for prospective development of personalized medicine, with the long term aim of providing a cure for cancer patients.

Loss of phosphatase and tensin homolog (PTEN) and activation of the PI3K/AKT signaling pathway are hallmarks of prostate cancer (PCa). However, these alterations alone are insufficient for cells to acquire metastatic traits. Here, we have shown that the histone dimethyl transferase WHSC1 critically drives indolent PTEN-null tumors to become metastatic PCa. In a PTEN-null murine PCa model, WHSC1 overexpression in prostate epithelium cooperated with Pten deletion to produce a metastasis-prone tumor. Conversely, genetic ablation of Whsc1 prevented tumor progression in PTEN-null mice. Molecular characterization revealed that increased AKT activity due to PTEN loss directly phosphorylates WHSC1 at S172, preventing WHSC1 degradation by CRL4Cdt2 E3 ligase. Increased WHSC1 expression transcriptionally upregulates expression of RICTOR, a pivotal component of mTOR complex 2 (mTORC2), to further enhance AKT activity. Therefore, the AKT/WHSC1/mTORC2 signaling cascade represents a vicious feedback loop that elicits unrestrained AKT signaling. Furthermore, we determined that WHSC1 positively regulates Rac1 transcription to increase tumor cell motility. The biological importance of a WHSC1-mediated signaling cascade is substantiated by patient sample analysis in which WHSC1 signaling is tightly correlated with disease progression and recurrence. Taken together, our findings highlight a pivotal link between an epigenetic regulator, WHSC1, and key intracellular signaling molecules, AKT, RICTOR, and Rac1, to drive PCa metastasis.

The utilization of upconverting nanophosphors (UCNP) for photodynamic therapy (PDT) has gained significant interests due to its ability to convert deep‐penetrating near‐infra red (NIR) light (i.e., 978 nm) to visible light. Previous attempts to co‐localize UCNPs with photosensitizers suffer from low photo­sensitizer loading and problems with nanoparticle aggregation. Here, the preparation of a novel composite nanoparticle formulation comprising 100 nm β−NaYF4:Yb3+,Er3+ UCNPs, and meso‐tetraphenyl porphine (TPP) photo­sensitizer, stabilized by biocompatible poly(ethylene glycol‐block‐(dl)lactic acid) block copolymers (PEG‐b‐PLA) is presented. A photosensitizer loading of 10 wt% with respect to UCNP crystal was achieved via the Flash NanoPrecipitation (FNP) process. A sterically stabilizing PEG layer on the composite nanoparticle surface prevents nanoparticle aggregation and ensures nanoparticle stability in water, PBS buffer, and culture medium containing serum proteins, resulting in nanoparticle suitable for in vivo applications. Based on in vitro studies utilizing HeLa cervical cancer cell lines, the composite nanoparticles are shown to exhibit low dark toxicity and efficient cancer cell‐killing activity upon NIR excitation. Exposure with 134 W cm−2 of 978 nm light for 45 min resulted in 75% HeLa cell death. This is the first quantification of the cell‐killing capabilities of the UCNP/TPP composite nanoparticles formulated for photodynamic therapy.

Bone metastasis is a frequent complication of breast cancer and a common cause of morbidity and mortality from the disease. During metastasis secreted proteins play crucial roles in the interactions between cancer cells and host stroma. To characterize the secreted proteins that are associated with breast cancer bone metastasis, we preformed a label-free proteomic analysis to compare the secretomes of four MDA-MB-231 (MDA231) derivative cell lines with varied capacities of bone metastasis. A total of 128 proteins were found to be consistently up-/down-regulated in the conditioned medium of bone-tropic cancer cells. The enriched molecular functions of the altered proteins included receptor binding and peptidase inhibition. Through additional transcriptomic analyses of breast cancer cells, we selected cystatin E/M (CST6), a cysteine protease inhibitor down-regulated in bone-metastatic cells, for further functional studies. Our results showed that CST6 suppressed the proliferation, colony formation, migration and invasion of breast cancer cells. The suppressive function against cancer cell motility was carried out by cancer cell-derived soluble CST6. More importantly, ectopic expression of CST6 in cancer cells rescued mice from overt osteolytic metastasis and deaths in the animal study, while CST6 knockdown markedly enhanced cancer cell bone metastasis and shortened animal survival. Overall, our study provided a systemic secretome analysis of breast cancer bone tropism and established secreted CST6 as a bona fide suppressor of breast cancer osteolytic metastasis.