Introduction

Prostate Cancer (PCa) is the most common cancer in males and the second leading cause of death from cancer in men. When PCa progress from localized disease to castration resistance, the formation of incurable metastases, primarily in the bone, is almost inevitable. Therefore, understanding the factors that regulate homing and survival of metastatic cancer cells in the bone is important for the identification of new therapeutic targets. High MCAM expression has been detected in the stroma of lytic and blastic lesions in preclinical models of PCa bone metastasis. The objective of this study is to characterize the role of MCAM in the maintenance of the aggressive phenotype in human PCa.

Methods

We used shRNAs to knockdown the expression of MCAM in the lytic PC-3M-Pro4Luc2_dTomato and in the blastic C4-2B_dTomato PCa cell lines. We validated the knockdown at protein level and tested the effect with functional assays such as migration, proliferation. RT-qPCR was used to test MCAM knockdown on EMT markers. The effect of the knockdown on the maintenance of cancer stem/progenitor-like cells was measured by ALDEFLUOR.

Results

MCAM knockdown reduced proliferation in PC-3M-Pro4Luc2_dTomato PCa cells and resulted in increased E-Cadherin expression. Conversely, no effect on proliferation was measured on C4-2B_dTomato cells. It has been described that metastatic human PCa cells target the hematopoietic stem cell (HSC) niche in the bone marrow at the level of an &[raquo]endosteal/osteoblast&[laquo] niche and a &[raquo]vascular/perivascular&[laquo] niche. We set-up an in vitro model of &[raquo]osteoblast niche&[laquo] to study the behavior of prostate cancer cells upon co-culture with osteoblasts and to measure the resulting effects on cancer stem/progenitor-like markers. We found that MCAM is required for the osteoblast-mediated induction of ALDH activity on PCa cells and MCAM knockdown prevented the increase in the size of the ALDH^high subpopulation in PC-3M-Pro4Luc2_dTomato, mediated by human osteoblasts. Additionally, MCAM knockdown in PCa cells co-culture with osteoblast, prevented the induction of MCAM expression by osteoblasts compared to non-targeted control. Finally, we showed that MCAM is significantly increased in the highly metastatic ALDH^high cells and identified a new subset of ALDH^high / MCAM^high cells which could be depleted upon MCAM knockdown.

Conclusions

We detected a new subset of ALDH^high/MCAM^high cells and demonstrated that MCAM influences the maintenance of an aggressive-mesenchymal phenotype in human PCa. Therefore, MCAM represent an interesting target molecule to modulate the behavior of aggressive PCa cells.

Funding

SNF Grant 310030_156933