The semaphorins were initially characterized as axon guidance factors, but have subsequently been implicated also in the regulation of immune responses, angiogenesis, organ formation, and a variety of additional physiological and developmental functions. The semaphorin family contains more then 20 genes divided into 7 subfamilies, all of which contain the signature sema domain. The semaphorins transduce signals by binding to receptors belonging to the neuropilin or plexin families. Additional receptors which form complexes with these primary semaphorin receptors are also frequently involved in semaphorin signaling. Recent evidence suggests that semaphorins also fulfill important roles in the etiology of multiple forms of cancer. Some semaphorins have been found to function as bona-fide tumor suppressors and to inhibit tumor progression by various mechanisms while other semaphorins function as inducers and promoters of tumor progression. The semaphorin familyThe semaphorin family members are divided into 8 subclasses of which subclasses 1 and 2 contain invertebrate semaphorins while subclasses 3-7 contain the 22 vertebrate semaphorins. The 8th subclass contains viral semaphorins. In early publications, semaphorins were assigned confusing names. This situation was rectified by the adoption of a unified nomenclature in which sema is followed by the subclass number and by alphabetic designation within the subclass.1 Semaphorins are characterized by the presence of a Β»500 amino-acids long sema domain located close to their N-termini which is also present in semaphorin receptors of the plexin family, and by a plexin-semaphorin-integrin (PSI) domain located downstream to the sema domain. The sema domain is essential for semaphorin activity and plays a role in the determination of the receptor binding specificity.2 The sema domains of several different semaphorins were characterized by X-ray crystallography revealing a b propeller topology.3-5 Different semaphorin subclasses are characterized by class specific structural motifs. Thus, the vertebrate semaphorins belonging to classes 3, 4 and 7 contain immunoglobulin like domains, class-5 semaphorins contain thrombospondin repeats and class-3 semaphorins contain a basic domain. Class-3 semaphorins are the only vertebrate semaphorins produced as secreted proteins while other vertebrate semaphorins are membrane anchored or trans-membrane proteins that can be further processed into soluble forms by proteolytic cleavage (Fig.
Semaphorins are the products of a large gene family containing 28 genes of which 21 are found in vertebrates. Class-3 semaphorins constitute a subfamily of seven vertebrate semaphorins which differ from the other vertebrate semaphorins in that they are the only secreted semaphorins and are distinguished from other semaphorins by the presence of a basic domain at their C termini. Class-3 semaphorins were initially characterized as axon guidance factors, but have subsequently been found to regulate immune responses, angiogenesis, lymphangiogenesis, and a variety of additional physiological and developmental functions. Most class-3 semaphorins transduce their signals by binding to receptors belonging to the neuropilin family which subsequently associate with receptors of the plexin family to form functional class-3 semaphorin receptors. Recent evidence suggests that class-3 semaphorins also fulfill important regulatory roles in multiple forms of cancer. Several class-3 semaphorins function as endogenous inhibitors of tumor angiogenesis. Others were found to inhibit tumor metastasis by inhibition of tumor lymphangiogenesis, by direct effects on the behavior of tumor cells, or by modulation of immune responses. Notably, some semaphorins such as sema3C and sema3E have also been found to potentiate tumor progression using various mechanisms. This review focuses on the roles of the different class-3 semaphorins in tumor progression.
Class-3 semaphorin guidance factors bind to receptor complexes containing neuropilin and plexin receptors. A semaphorin may bind to several receptor complexes containing somewhat different constituents, resulting in diverse effects on cell migration. U87MG glioblastoma cells express both neuropilins and the four class-A plexins. Here, we show that these cells respond to Sema3A or Sema3B by cytoskeletal collapse and cell contraction but fail to contract in response to Sema3C, Sema3D, Sema3G or Sema3E, even when class-A plexins are overexpressed in the cells. In contrast, expression of recombinant plexin-D1 enabled contraction in response to these semaphorins. Surprisingly, unlike Sema3D and Sema3G, Sema3C also induced the contraction and repulsion of plexin-D1-expressing U87MG cells in which both neuropilins were knocked out using CRISPR/Cas9. In the absence of neuropilins, the EC50 of Sema3C was 5.5 times higher, indicating that the neuropilins function as enhancers of plexin-D1-mediated Sema3C signaling but are not absolutely required for Sema3C signal transduction. Interestingly, in the absence of neuropilins, plexin-A4 formed complexes with plexin-D1, and was required in addition to plexin-D1 to enable Sema3C-induced signal transduction.
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