Stromal senescence contributes to age-related increases in cancer

Ageing is a process characterized by a wide array of cellular and systemic changes that together increase the risk of developing cancer. While cell-autonomous mutations within incipient tumour cells are important, age-related changes in the microenvironment are critical partners in the transformation process and response to therapy. However, aspects of ageing that are important and the degree to which they contribute to cancer remain obscure. One of the factors that impacts ageing is increased cellular senescence but it is important to note that ageing and cellular senescence are not synonymous. We highlight open questions, including if senescent cells have phenotypically distinct impacts in aged versus young tissue, or if it is the cell type that dictates the impact of senescence on tissue homeostasis and disease. Finally, it is probable that our current definition of cellular senescence encompasses more than one mechanistically distinct cellular state; thus, we highlight phenotypic differences that have been noted across cell types and tissues of origin. This Review focuses on the role that senescent stromal cells have in cancer, with a particular emphasis on fibroblasts given the amount of work that has focused on them.