Tolbutamide reduces glioma cell proliferation by increasing connexin43, which promotes the up‐regulation of p21 and p27 and subsequent changes in retinoblastoma phosphorylation

Abstract

Our previous work has shown that tolbutamide increases gap junctional permeability in poorly coupled C6 glioma cells and that this effect is similar and additive to that found with dbcAMP, a well-known activator of gap junctional communication. Furthermore, the increase in gap junctional communication promoted by tolbutamide or dbcAMP is concurrent with the inhibition of proliferation of C6 glioma cells. In the present work, we show that tolbutamide and dbcAMP increase the synthesis of the tumor suppressor protein Cx43 and that they decrease the level of Ki-67, a protein expressed when cells are proliferating. These effects were accompanied by a reduction in the phosphorylation of pRb, mainly on Ser-795, a residue critical for the control of cell proliferation. The decrease in the phosphorylation of pRb is not likely to be mediated by a reduction in the levels of D-type cyclins, since instead of decreasing the expression of cyclins, D1 and D3 increased slightly after treatment with tolbutamide or dbcAMP. However, the Cdk inhibitors p21 and p27 were up-regulated after treatment with tolbutamide and dbcAMP, suggesting that they would be involved in the decrease in pRb phosphorylation. When Cx43 was silenced by siRNA, neither tolbutamide nor dbcAMP were able to up-regulate p21 and consequently to reduce glioma cell proliferation, as judged by Ki-67 expression. In conclusion, tolbutamide and dbcAMP inhibit C6-glioma cell proliferation by increasing Cx43, which correlates with a reduction in pRb phosphorylation due to the up-regulation of the Cdk inhibitors p21 and p27.

Involvement of connexin 43 in angiotensin II-induced migration and proliferation of saphenous vein smooth muscle cells via the MAPK-AP-1 signaling pathway

“…In this study, we found that cyclin E was involved in the Cx43-induced proliferation of VSMC after Ang II stimulation. Cx43 may also regulate cell cycle inhibitor proteins via p27 and p21, since the upregulation of Cx43 accompanies an increase in p27 and p21 expression levels in cancer cells [29]. These results provide insight into the direct correlation of Cx43 expression and cell cycle proteins.…”

Serial In vivo Spectroscopic Nuclear Magnetic Resonance Imaging of Lactate and Extracellular pH in Rat Gliomas Shows Redistribution of Protons Away from Sites of Glycolysis

“…The cells of C6 gliomas are weakly coupled by gap junctions (50) and we hypothesize that these junctions might allow some flow of intracellular protons from one cell to another. In this scheme, pHe would not necessarily change where lactate was extruded, and Figure 6.…”

Selective β2 adrenergic agonist increases Cx43 and miR-451 expression via cAMP-Epac

“…The gap junction, which is required for maintaining cell growth, has been reported to be absent in gliomas and astrocytomas. The gap junction is made of connexin proteins, which have been proposed to act as tumor suppressors (2,3). Connexin 43 (Cx43) is the major protein forming gap junction channels in astrocytes and has been proposed to have growth inhibitory effects.…”

“…Connexin 43 (Cx43) is the major protein forming gap junction channels in astrocytes and has been proposed to have growth inhibitory effects. Expression of Cx43 is inversely correlated with the degree of malignancy (3). It is accepted that connexins not only act as critical gatekeepers of cell proliferation by controlling the intercellular exchange of essential growth regulators, but they may also affect cell cycling by non-gap junctional intercellular communication (4).…”