Key questions in metastasis: new insights in molecular pathways and therapeutic implications

The metastatic cascade and colonization remains a major challenge in clinical therapeutics. The formation of metastasis has many rate limiting steps. The expression of metastases initiation genes in primary tumors is driven by the need for cell motility, invasiveness, handling the shear stress in the vasculature and lymphatic circulation, and the survival and persistent growth in the distant organ. However, the expression of the progression genes in the primary tumors has a more complex basis. These metastasis-prone genes support primary tumor growth through one particular effect, whereas they enhance distant metastasis through another effect. The boundaries between metastasis initiation and metastasis progression genes are not rigid. In this review, we examine novel gene signatures identified in metastases, address key inflammatory factors mastering homing selection, gain further mechanistic insights into cell plasticity and evaluate the role of microRNAs. Moreover, we also describe the recent progress in developing nanoparticle imaging substantiating a promising theranostic platform for future cancer diagnostics and treatment, and assess the relevance of the bioinformatic analysis of metastasis-related proteins with an eye toward the metastatic niche. All these tools will provide valuable biological information of the progression of the disease, helping find potential therapeutic targets and improving surgical procedures. In a near future the understanding of the molecular mechanisms in tumor dissemination will be pivotal for the translation of these methods to the clinic and will help to overcome the barriers in clinical therapy of metastases.