Induction of apoptosis by PQ1, a gap junction enhancer that upregulates connexin 43 and activates the MAPK signaling pathway in mammary carcinoma cells

Abstract

The mechanism of gap junction enhancer (PQ1) induced cytotoxicity is thought to be attributed to the change in connexin 43 (C×43) expression; therefore, the effects of C×43 modulation in cell survival were investigated in mammary carcinoma cells (FMC2u) derived from a malignant neoplasm of a female FVB/N-Tg(MMTV-PyVT)634Mul/J (PyVT) transgenic mouse. PQ1 was determined to have an IC50 of 6.5 μM in FMC2u cells, while inducing an upregulation in C×43 expression. The effects of C×43 modulation in FMC2u cell survival was determined through transfection experiments with C×43 cDNA, which induced an elevated level of protein expression similar to that seen with PQ1 exposure, or siRNA to silence C×43 protein expression. Overexpression or silencing of C×43 led to a reduction or an increase in cell viability, respectively. The mitogen-activated protein kinase (MAPK) family has been implicated in the regulation of cell survival and cell death; therefore, the gap junctional intercellular communication (GJIC)-independent function of PQ1 and C×43 in the Raf/Mitogen-activated protein kinase/ERK kinase/extracellular-signal-regulated kinase (Raf-MEK-ERK) cascade of cellular survival and p38 MAPK-dependent pathway of apoptosis were explored. PQ1 treatment activated p44/42 MAPK, while the overexpression of C×43 resulted in a reduced expression. This suggests that PQ1 affects the Raf-MEK-ERK cascade independent of C×43 upregulation. Both overexpression of C×43 and PQ1 treatment stimulated an increase in the phosphorylated form of p38-MAPK, reduced levels of the anti-apoptotic protein Bcl-2, and increased the cleavage of pro-caspase-3. Silencing of C×43 protein expression led to a reduction in the phosphorylation of p38-MAPK and an increase in Bcl-2 expression. The mechanism behind PQ1-induced cytotoxicity in FMC2u mammary carcinoma cells is thought to be attributed to the change in C×43 expression. Furthermore, PQ1-induced apoptosis through the upregulation of C×43 may depend on p38 MAPK, highlighting that the effect of PQ1 on gap junctions as well as cellular survival via a MAPK-dependent pathway.