The regulation of beta-actin mRNA localization through signal transduction pathways
Latham, Vaughan Milton, Jr.
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Serum or growth factors such as LPA, PDGF, and thrombin are necessary to maintain beta-actin mRNA in the leading lamellae of chicken embryo fibroblasts (CEFs) (Latham et al., 1994). Specific growth factors can induce the localization of specific mRNAs to a specific compartment within the cell. The induction of beta-actin mRNA localization by serum or growth factors is sensitive to inhibitors and activators of major cell signaling pathways such as PKC, PKA, and receptor tyrosine kinases. These signals necessary for moving beta-actin mRNA to the leading edge act through the actin cytoskeleton, since the rapid (within a few minutes) induction of localization is blocked by cytochalasin D, but not by nocodazole. A major pathway regulating the actin cytoskeleton, the small GTPases of the Rho family, can also regulate beta-actin mRNA localization. Activators of Rho such as LPA, sphingosine-1-phosphate (SPP), and thrombin, all induce localization rapidly. This rapid induction can be blocked by C3 exoenzyme. A significant increase in the percentage of cells showing localized beta-actin mRNA at the leading edge is seen after transfection of CEFs with activated V14RhoA, while a significant decrease in localized cells is seen in cells transfected with inactive N19RhoA. This induction of localization through Rho GTPases may involve an actomyosin interaction since BDM (2,3-butanedione 2-monoxime), a myosin light chain kinase (MLCK) and myosin II ATPase inhibitor, ML-7, a MLCK inhibitor, and H7 a MLCK and serine/threonine kinase inhibitor, all inhibit localization. More specifically, myosin II B may be involved since embryonic fibroblasts from a myosin II B knockout mouse have decreased levels of beta-actin mRNA localization. These fibroblasts fail to localize beta-actin mRNA to the leading edge after serum stimulation, as compared to embryonic fibroblasts from wild-type mice. This data suggests that an actomyosin interaction is involved in an aspect of the localization process such as transport of the mRNA on actin filaments, or a structural interaction in the lamella necessary for localization. These results demonstrate that growth factors acting through signaling pathways can regulate cytoskeletal components of spatial gene expression.