Differential partitioning of Galphai1 with the cellular microtubules: a possible mechanism of development of Taxol resistance in human ovarian carcinoma cells

Taxol binds to the cellular microtubules and suppresses their dynamic instability. Development of tumor cell resistance to taxol is typically associated with increased expression of the drug efflux pump P-glycoprotein and/or alterations in the microtubules. Recently, changes in the dynamic instability of the microtubules have also been associated with development of taxol resistance in a lung cancer cell line. We have established a 250-fold taxol-resistant human ovarian carcinoma subline (2008/13/4) that does not display the typical alterations associated with development of drug resistance. Utilizing the mRNA differential display technique, we observed increased expression of an alpha subunit of the guanine nucleotide-binding protein, Galphai1, in the taxol-resistant human ovarian carcinoma cell lines compared to the parental 2008 cells. Several isoforms of the alpha-subunit of the G protein have been identified and the Galphai (inhibitory) are so named because they inhibit the activity of adenylate cyclase leading to inactivation of the cAMP-dependent protein kinase A (PKA) pathway. In addition, Galphai1 is also known to bind to microtubules and activates their GTPase activity and thus induces depolymerization of the microtubules. In the present study we demonstrate that the intracellular level of cAMP and the PKA activity were higher in the taxol-resistant 2008/13/4 and the 2008/17/4 cells despite the increased expression of Galphai1 in these cells. Moreover, Galphai1 was found to be localized not on the cell membrane, but in intracellular compartments in both the taxol-sensitive and -resistant human ovarian carcinoma cells. Interestingly, increased association of the Galphai1 protein and the microtubules in the taxol-resistant cells compared to the parental 2008 cells was observed, both prior to and after treatment of these cells with taxol. Based on the opposing effects of taxol and the Galphai1 protein on the microtubule dynamic instability (taxol suppresses microtubule dynamic instability whilst the Galphai1 protein inhibits the suppression) our results indicate the operation of a novel pathway that would enable the cells to escape the cytotoxic effects of taxol.