At various stages during embryogenesis and cancer cells are exposed to tension, compression and shear stress; forces that can regulate cell proliferation and differentiation. In the present study, we show that shear stress blocks cell cycle progression in colon cancer cells and regulates the expression of genes linked to the Wnt/β-catenin, mitogen-activated protein kinase (MAPK) and NFκB pathways. The shear stress-induced increase of the secreted Wnt inhibitor DKK1 requires p38 and activation of NFκB requires IκB kinase-β. Activation of β-catenin, important in Wnt signaling and the cause of most colon cancers, is inhibited by shear stress through a pathway involving laminin-5, α6β4 integrin, phosphoinositide 3-kinase (PI 3-kinase) and Rac1 coupled with changes in the distribution of dephosphorylated β-catenin. These data show that colon cancer cells respond to fluid shear stress by activation of specific signal transduction pathways and genetic regulatory circuits to affect cell proliferation, and indicate that the response of colon cancers to mechanical forces such as fluid shear stress should be taken into account in the management of the disease.

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Basally located tight junctions between Sertoli cells in the postpubertal testis are the largest and most complex junctional complexes known. They form at puberty and are thought to be the major structural component of the "blood-testis" barrier. We have now examined the development of these structures in the immature mouse testis in conjunction with immunolocalization of the tight-junction-associated protein ZO-1 (zonula occludens 1). In testes from 5-day-old mice, tight junctional complexes are absent and ZO-1 is distributed generally over the apicolateral, but not basal, Sertoli cell membrane. As cytoskeletal and reticular elements characteristic of the mature junction are recruited to the developing junctions, between 7 and 14 days, ZO-1 becomes progressively restricted to tight junctional regions. Immunogold labeling of ZO-1 on Sertoli cell plasma membrane preparations revealed specific localization to the cytoplasmic surface of tight junctional regions. In the mature animal, ZO-1 is similarly associated with tight junctional complexes in the basal aspects of the epithelium. In addition, it is also localized to Sertoli cell ectoplasmic specializations adjacent to early elongating, but not late, spermatids just prior to sperm release. Although these structures are not tight junctions, they do have a similar cytoskeletal arrangement, suggesting that ZO-1 interacts with the submembrane cytoskeleton. These results show that, in the immature mouse testis, ZO-1 is present on the Sertoli cell plasma membrane in the absence of recognizable tight junctions. In the presence of tight junctions, however, ZO-1 is found only at the sites of junctional specializations associated with tight junctions and with elongating spermatids.

The inner ear of the sea lamprey, Petromyzon marinus, was examined using scanning and transmission electron microscopy. Many of the nonsensory surfaces of the ear chamber are lined by numerous, noninnervated, multiciliated epithelial cells. Each multiciliated epithelial cell has 43–66 true cilia projecting from its apical surface into the lumen of the ear. Although the cilia leave the cell individually, all of the cilia from a single cell come together just above the apical cell surface and are held together by a cross-network of fibrillar material. The cell bodies of the multiciliated cells sit upon a basal lamina which overlies a collagen-filled matrix. Petromyzon has typical vertebrate sensory hair cells on the cristae of the two semicircular canals as well as on the main sensory epithelium, the macula communis. Cell bodies of the sensory hair cells are similar to hair cells of other vertebrates. However, unlike other fishes, the sensory hair cells in Petromyzon have striated organelles between the nucleus and the apical cell membrane. The hair cells are innervated by afferent and efferent nerve fibers.

Eukaryotic initiation factor 6 (eIF6), an essential protein important in ribosome biosynthesis and assembly, was identified as an interacting partner of the b-catenin C terminus in the yeast two-hybrid assay. Independent studies identified Drosophila eIF6 (DeIF6) in a genetic screen designed to detect new genes involved in the regulation of the Wnt/Wg (wingless) pathway. Ectopic expression of DeIF6 in wing discs results in a Wg phenotype. Expression of eIF6 in adenomatous polyposis coli (APC)-mutant colon cancer cells, which express high levels of active b-catenin, showed that eIF6 selectively inhibits the Wnt pathway at the level of b-catenin protein independently of proteasomal degradation. Incorporation of radiolabeled amino acids into b-catenin was selectively decreased in cells that overexpressed eIF6. A similar inverse relationship of the two proteins was observed in the APC min/ þ mouse intestine, in which b-catenin levels are very high. Taken together these data reveal a link between eIF6 and Wnt signaling, perhaps at the level of ribosome recycling on b-catenin mRNA.