HSC homing, quiescence, and self-renewal depend on the bone marrow HSC niche. A large proportion of solid tumor metastases are bone metastases, known to usurp HSC homing pathways to establish footholds in the bone marrow. However, it is not clear whether tumors target the HSC niche during metastasis. Here we have shown in a mouse model of metastasis that human prostate cancer (PCa) cells directly compete with HSCs for occupancy of the mouse HSC niche. Importantly, increasing the niche size promoted metastasis, whereas decreasing the niche size compromised dissemination. Furthermore, disseminated PCa cells could be mobilized out of the niche and back into the circulation using HSC mobilization protocols. Finally, once in the niche, tumor cells reduced HSC numbers by driving their terminal differentiation. These data provide what we believe to be the first evidence that the HSC niche serves as a direct target for PCa during dissemination and plays a central role in bone metastases. Our work may lead to better understanding of the molecular events involved in bone metastases and new therapeutic avenues for an incurable disease. IntroductionMetastases represent the most common malignant tumors involving the skeleton: nearly 70% of patients with breast cancer or prostate cancer (PCa) -and approximately 15%-30% of patients with carcinomas of the lung, colon, stomach, bladder, uterus, rectum, thyroid, or kidney -have bone lesions (1). Several mechanisms are thought to account for the organ-specific nature of bone metastases, including direct tumor extensions, retrograde venous flow, and tumor embolization. It is also clear, however, that anatomy alone does not explain the organ-specific pattern of metastasis.One hypothesis that has gained favor is that the metastatic process is functionally similar to the homing behavior of HSCs to the BM (2, 3). HSC homing, quiescence, and self-renewal in the BM are now known to depend on a region termed the HSC niche (4, 5). Recent studies identified cells of the osteoblastic and endothelial lineages as key components of the niche (6-11). Molecules that play critical roles in HSC niche selection are now thought to be used by metastases to establish footholds in the BM (2, 3), including chemoattractants (CXCL12; also referred to as stromal-derived factor-1; refs. 3, 12), attachment factors (annexin II [Anxa2]; ref. 13), regulators of cell growth, and vascular recruitment ref. 14). Once in the BM, tumor cells parasitize the bone microenvironment to regulate long-term survival/dormancy and, ultimately, metastatic growth. However, it is not known whether metastatic cells specifically target the HSC niche during dissemination.In the present work, we used a PCa model to demonstrate that tumors directly compete with HSCs for occupancy of the endosteal HSC niche during BM transplantation (BMT). Critically, HSCs

Clusterin is an enigmatic glycoprotein that is overexpressed in several human cancers such as prostate and breast cancers, and squamous cell carcinoma. Because the suppression of clusterin expression renders human cancer cells sensitive to chemotherapeutic drug-mediated apoptosis, it is currently an antisense target in clinical trials for prostate cancer. However, the molecular mechanisms by which clusterin inhibits apoptosis in human cancer cells are unknown. Here we report that intracellular clusterin inhibits apoptosis by interfering with Bax activation in mitochondria. Intriguingly, in contrast to other inhibitors of Bax, clusterin specifically interacts with conformation-altered Bax in response to chemotherapeutic drugs. This interaction impedes Bax oligomerization, which leads to the release of cytochrome c from mitochondria and caspase activation. Moreover, we also find that clusterin inhibits oncogenic c-Myc-mediated apoptosis by interacting with conformation-altered Bax. Clusterin promotes c-Myc-mediated transformation in vitro and tumour progression in vivo. Taken together, our results suggest that the elevated level of clusterin in human cancers may promote oncogenic transformation and tumour progression by interfering with Bax pro-apoptotic activities.

Human embryonic stem cells (hESCs) have great potentials for future cell-based therapeutics. However, their mechanosensitivity to biophysical signals from the cellular microenvironment is not well characterized. Here we introduced an effective microfabrication strategy for accurate control and patterning of nanoroughness on glass surfaces. Our results demonstrated that nanotopography could provide a potent regulatory signal over different hESC behaviors, including cell morphology, adhesion, proliferation, clonal expansion, and self-renewal. Our results indicated that topological sensing of hESCs might include feedback regulation involving mechanosensory integrin-mediated cell-matrix adhesion, myosin II, and E-cadherin. Our results also demonstrated that cellular responses to nanotopography were cell-type specific and as such, we could generate a spatially segregated co-culture system for hESCs and NIH/3T3 fibroblasts using patterned nanorough glass surfaces.