Tubulin stimulates adenylyl cyclase activity in C6 glioma cells by bypassing the β‐adrenergic receptor: a potential mechanism of G protein activation

Abstract

While the cytoskeleton is known to play several roles in the biology of the cell, one role, which has been revealed only recently, is that of a participant in the signal transduction process. Tubulin binds speci®cally to the a subunits of Gs (stimulatory GTP-binding regulatory protein of adenylyl cyclase), Gi1 (inhibitory protein of adenylyl cyclase), and Gq and transactivates those molecules through direct transfer of GTP. The relevance of this transactivation process to G proteins which are normally activated by a neurotransmitteroccupied receptor is the subject of this study. C6 glioma cells, made permeable with saponin, retained tight coupling between Gs and the b-adrenergic receptor. Although 5-guanylylimidodiphosphate (GppNHp) was incapable of activating Gs (and subsequently, adenylyl cyclase) in the absence of agonist, tubulin with GppNHp bound (tubulinGppNHp) activated adenylyl cyclase with an EC 50 of 30 nM. Desensitization of b-adrenergic receptors by isoproterenol exposure had no effect on the ability of tubulin-GppNHp to activate Gs and adenylyl cyclase. When the photoaf®nity GTP analog, azidoanilido GTP (AAGTP; P3(4-azidoanilido)-P1-5'-GTP), was added to C6 membranes or permeable C6 cells, it was only weakly incorporated by Gas in the absence of isoproterenol. When the same concentration of dimeric tubulin with AAGTP bound was introduced, AAGTP was transferred from tubulin to Gas, activating the latter species. Similar preferential' activation of Gas by tubulin-AAGTP versus the free nucleotide was seen using puri®ed components. Thus, membrane-associated tubulin may serve to activate Gas, independent of signals not normally coupled to that protein.Tubulin may act as an agent to link a variety of membraneassociated signalling systems.

RETRACTED ARTICLE: β adrenergic receptor modulated signaling in glioma models: promoting β adrenergic receptor-β arrestin scaffold-mediated activation of extracellular-regulated kinase 1/2 may prove to be a panacea in the treatment of intracranial and spinal malignancy and extra-neuraxial carcinoma

“…Treatment with the myelosuppressive non-neuropathic microtubule synthesis inhibitor vinblastine at doses insufficient to modulate protein synthesis prevents isoproterenol mediated enhancement of phosphodiesterase synthesis, though fails to prevent β agonist-mediated upregulation of nerve growth factor [42]. Crossmodal modulation between molecular compounds modulating polymerization and depolymerization of microtubules and βAR modulated signaling may be critically implicated in glioma initiation, promotion, progression, invasion, and metastasis [93]. NG 108-15 rat neuroblastoma cells express βARK isotypes 1 and 2 mRNA and exhibit Gβγ-dependent phosphorylation of rhodopsin and agonist-bound delta opioid receptor, recapitulating effects mediated by βAR activation in non-transformed cells [94,95].…”

Antidepressants Accumulate in Lipid Rafts Independent of Monoamine Transporters to Modulate Redistribution of the G Protein, Gαs

“…Estimates of the ratio of glia to neurons vary considerably, but evidence suggests that glia control the number and stability of neuronal synapses formed (31,32). Furthermore, chronic antidepressant treatment has been shown to increase the expression and release of glial cell-derived neurotrophic factor (GDNF) (33)(34)(35).…”

Exploratory Visual Analysis of Statistical Results from Microarray Experiments Comparing High and Low Grade Glioma

“…Many of the genes in these groups also had marginally significant individual statistical results. For example, disruption of microtubule dynamics by genes in the “Tubulin” group can affect apoptotic pathways (Yan et al 2001). …”