We found that the magnesium salt of ilimaquinone, named 201-F, specifically disassembled dynamically unstable microtubules in fibroblasts and various epithelial cell lines. Unlike classical tubulin- interacting drugs such as nocodazole or colchicine which affect all classes of microtubules, 201-F did not depolymerize stable microtubules. In WIF-B–polarized hepatic cells, 201-F disrupted the Golgi complex and inhibited albumin and alpha1-antitrypsin secretion to the same extent as nocodazole. By contrast, 201-F did not impair the transport of membrane proteins to the basolateral surface, which was only affected by the total disassembly of cellular microtubules. Transcytosis of two apical membrane proteins—the alkaline phosphodiesterase B10 and dipeptidyl peptidase IV—was affected to the same extent by 201-F and nocodazole. Taken together, these results indicate that only dynamically unstable microtubules are involved in the transport of secretory proteins to the plasma membrane, and in the transcytosis of membrane proteins to the apical surface. By contrast, stable microtubules, which are not functionally affected by 201-F treatment, are involved in the transport of membrane proteins to the basolateral surface. By specifically disassembling highly dynamic microtubules, 201-F is an invaluable tool with which to study the functional specialization of stable and dynamic microtubules in living cells.

In the last decade, the notion that microtubules are critical to the spatial organization of signal transduction and contribute to the transmission of signals to downstream targets has been proposed. Because the STAT5B transduction and transcription factor is the major STAT protein activated by growth hormone stimulation in hepatocytes and is a crossroads between many signaling pathways, we studied the involvement of microtubules in STAT5B-mediated growth hormone signaling pathway in the highly differentiated and polarized WIF-B hepatic cell line. We showed that depolymerization of the microtubule network impaired STAT5B translocation to the nucleus upon growth hormone treatment. A significant amount of STAT5B binds to microtubules, while STAT5A and STAT3 are exclusively compartmentalized in the cytosol. Moreover, taxol-induced stabilization of microtubules released STAT5B from its binding, and we show that STAT5B binds specifically to the highly dynamic microtubules and is absent of the stable microtubule subpopulation. The specific involvement of dynamic microtubule subpopulation in growth hormone signaling pathway was confirmed by the inhibition of growth hormone-induced STAT5B nuclear translocation after stabilization of microtubules or specific disruption of highly dynamic microtubules. Upon growth hormone treatment, MT-bound STAT5B was rapidly released from microtubules by a dynein-dependent transport to the nucleus. Altogether, our findings indicate that the labile microtubule subpopulation specifically and dynamically organizes STAT5B-mediated growth hormone signaling in hepatic cells.Once bound to specific receptors, a large number of cytokines, growth factors, and hormones act through a sequence of steps allowing the rapid transport of information from the plasma membrane to subcellular targets. This information often results in the phosphorylation of transcription factors that subsequently migrate to the nucleus where they initiate specific gene transcription. How transcription factors are organized as molecular targets downstream of receptor activation and how they move rapidly through the cytoplasm from their activation site to the nucleus often remains unclear. The notion that microtubules (MTs) 1 might be involved in the cytoplasmic activation and/or trafficking of various transcription factors and protein kinases has emerged, as many signal transduction molecules, including transcription factors and protein kinases, have been shown to interact with MTs (1). For example, the glucocorticoid receptor was shown to colocalize with the MT network in mammalian cells (2, 3). The tumor suppressor protein p53 also uses MT tracks to migrate to the nucleus (4). Various protein kinases such as the MAP kinase kinase kinase MLK2 (5) or the MAP kinases ERK1 and ERK2 (6) were also shown to interact or to colocalize with MTs or with microtubule-associated proteins (7).In this study, we investigated the putative involvement of the MT network in GH signaling in the highly differentiated WIF-B hepatic cell line (8, 9...