Stabilization of tubulin mRNAs by insulin and insulin-like growth factor I during neurite formation

Neurotrophic factors may increase axon and dendrite growth in part by regulating the content of cytoskeletal elements such as microtubules, which are comprised of tubulin subunits. The mechanism by which insulin, insulin-like growth factors (IGFs), and nerve growth factor (NGF) can increase the relative abundance of tubulin mRNAs as a prelude to neurite formation was studied. Insulin significantly increased the abundance of tubulin mRNAs relative to total RNA in cultured human neuroblastoma SH-SY5Y cells. This increase was not the result of a generalized elevation of all transcripts, because tubulin mRNAs were elevated relative to poly(A)⁺ RNA as well. Moreover, whereas polymerases I and III were elevated in activity, polymerase II was not. Tubulin mRNAs were stabilized against degradation in the presence of actinomycin D by both insulin and IGF-I. In contrast, actin and histone 3.3 mRNAs were neither increased nor stabilized. Insulin did not alter α- or β-tubulin gene transcription rates in nuclear run-off experiments, and did increase the relative synthesis of tubulin proteins. These results suggest that tubulin mRNA levels are increased mainly through selective stabilization by insulin and IGFs. Because NGF is known to stabilize tubulin mRNA levels also, stabilization of tubulin mRNAs is suggested to be a common event in the pathway leading to neurite elongation directed by neuritogenic polypeptides.