Growth hormone promotion of tubulin polymerization stabilizes the microtubule network and protects against colchicine-induced apoptosis

We have investigated the effect of GH on microtubular physiology in Chinese hamster ovary (CHO) cells stably transfected with the complementary DNA for the rat GH receptor (CHO-GHR(1-638)). We show here that after 30 min of human GH (hGH) treatment of CHO-GHR(1-638) cells, there was a significant increase in the level of polymerization of all four tubulin isoforms (alpha-, beta-, gamma-, and tyrosinated alpha-tubulin) compared with the serum-deprived state. However, this transient increase in the levels of polymerized tubulin after hGH treatment was particularly pronounced for beta- and tyr alpha-tubulin. For alpha- and gamma-tubulin, the hGH induced increase in polymerization state lasted to approximately 3 h and then declined by 7 h, whereas for beta- and tyr alpha-tubulin there was a decrease in the polymerization state at 1-2 h after hGH treatment compared with the level at 30 min (but still greater than the serum-deprived state) followed by a second but lesser wave of increased polymerization lasting to 7 h. The changes in the polymerization state of the tubulins were not accompanied by comparative changes in the level of total cellular tubulin. The proline rich box 1 region of the GH receptor was required for hGH to stimulate tubulin polymerization indicative that this event is JAK dependent. Increased tubulin polymerization still occurred in response to hGH in a receptor truncation lacking the carboxyl terminal half of the intracellular domain of the GH receptor indicative that hGH induced changes in intracellular calcium concentration is not required for tubulin polymerization. Prior treatment of CHO-GHR(1-638) cells with hGH retarded colchicine induced microtubule depolymerization and also prevented colchicine induced apoptotic cell death. The integrity of the microtubule network was not required for GH-induced STAT5 mediated transcription as treatment of cells with colchicine, vincristine, or vinblastine did not alter the fold stimulation of the STAT5 mediated transcriptional response to GH. Thus one consequence of cellular treatment with GH is alteration in microtubule physiology.