Semaphorins and their receptors in stem and cancer cells

There is a growing body of evidence that links cancer with genes and pathways that are required for normal embryonic development, increasing the possibility that cancer cells with stem cell properties, particularly self-renewal and multipotentiality, are primarily involved in tumor formation and progression. One novel pathway that is important in regulating the morphogenesis, proliferation, survival and growth in a variety of adult and embryonic tissues is the semaphoring signaling pathway. Semaphorins are a large family of secreted, transmembrane and GPI-linked proteins with a broad spectrum of functions. Semaphorin signaling is transduced by plexins which, in the case of most class 3 semaphorins, require high affinity neuropilin receptors. The neuropilins also function as receptors for VEGF and other growth factors, and their expression is abnormal in tumors. Various semaphorins can either promote or inhibit tumor progression through the promotion or inhibition of processes such as tumor angiogenesis, metastasis and tumor cell survival. In normal tissues, semaphoring signaling is mainly active in precursor cells. This increases the possibility of tumors being derived from such cells, possibly even stem cells, which are unable to differentiate and/or to stop proliferating. In this review, we summarize the molecular mechanisms of semaphorin signal transduction involved in the stem cell compartment, and describe the evidence that links semaphorins to the control of tumor progression.