Cytoskeletal Control of Gene Expression : Depolymerization of Microtubules Activates NF-xB

Cell shape changes exert specific effects on gene expression. It has been speculated that the cytoskeleton is responsible for converting changes in the cytoarchitecture to effects on gene transcription. However, the signal transduction pathways responsible for cytoskeletal-nuclear communication remained unknown. We now provide evidence that a variety of agents and conditions that depolymerize microtubules activate the sequence-specific transcription factor NF-xB and induce NFKB-dependent gene expression. These effects are caused by depolymerization of microtubule because they are blocked by the microtubulestabilizing agent taxol. In nonstimulated cells, the majority of NF-xB resides in the cytosplasm as a complex with its inhibitor IxB. Upon cell stimulation, NF-KB translocates to the nucleus with concomitant degradation of IKB. We show that cold-induced depolymerization of microtubules also leads to IKB degradation and activation of NF-~B. However, the activated factor remains in the cytoplasm and translocates to the nucleus only upon warming to 37°C, thus revealing two distinct steps in NF-KB activation. These findings establish a new role for NF-KB in sensing changes in the state of the cytoskeleton and converting them to changes in gene activity. variety of observations suggest that changes in cell shape or architecture can regulate gene expression (6, 35). The shape of a cell is affected by interactions with either the extracellular matrix or neighboring cells, which lead to restructuring of the cytoplasmic cytoskeleton. Altering cytoskeletal structure may in turn change the availability of regulatory or catalytic sites of key signal-transducing molecules. Some of the classical examples illustrating the effect of cell shape changes on nuclear gene expression include a switch from type I to type II collagen expression by condriocytes upon a shift from growth on fibronectin to growth in suspension (7), as well as the repression of liver-specific gene expression in dispersed hepatocytes and its resumption upon cell aggregation (14). The exact mechanisms by which changes in cell shape are converted to changes in the pattern of gene expression have not been explained, but the cytoskeleton is a good candidate to have a key role in this mysterious signal transduction process. It has been suggested that the cytoskeleton may regulate gene expression by interacting with the nuclear matrix (9), and that this interaction may lead to physical expansion of nuclear pores and thereby increase the rate of nuclear transport in spreading cells (35). One of the major components of the cytoskeleton is the microtubule network. Because of the dynamic instability of tubulin dimers, this structure is subject to constant remodelAddress correspondence to Michael Karin, Department of Pharmacology, School of Medicine, University of California at San Diego, La Jolla, CA. Tel. : (619) 534-1361. Fax: (619) 534-8158. ing. Massive microtubule reorganization occurs in the mitotic phase of the cell cycle (46), during cell differentiation (36), upon specific binding of cytotoxic T lymphocytes to its target cell (24), and during injury-induced migration of endothelial cells to wounded tissue (27). This massive microtubule reorganization correlates with changes in gene expression. Depolymerization of microtubules with drugs can stimulate cell proliferation in the absence of other signals (15), whereas stabilization of microtubules can inhibit the action of certain mitogens (16). It was shown that transcription of several genes, including the ones that encode urokinase type plasminogen activator (uPA)' (11, 38) and interleukin IL-lfl (23, 44) is activated by microtubuledepolymerizing agents such as nocodazole and colchicine. One plausible way by which the cytoplasmic cytoskeleton can affect nuclear gene expression is by modulating the activity of transcription factors that reside in the cytoplasm of unstimulated cells in an inactive form and migrate to the nucleus in response to various stimuli (34). Such factors are the NF-KBs, a collection of structurally and functionally related dimeric complexes that activate transcription of immunoregulatory genes that encode cell surface receptors, cytokines, acute phase reactants, and cell adhesion molecules (reviewed in reference 28). NF-rB was originally characterized as a heterodimer of two subunits, p50 and p65, each of which is also capable of homodimerizing and binding 1. Abbreviations used in this paper: CAT, chloramphenicoI acetyl transferase; TPA, tetradecanoyl phorbol acetate; uPA, urokinase plasminogen activator. © The Rockefeller University Press, 0021-9525/95/03/11 i 1/9 $2.00 The Journal of Cell Biology, Volume 128, Number 6, March 1995 1111-1119 1111 on Jne 7, 2017 D ow nladed fom Published March 15, 1995