Transcriptional regulation of connexin 43 expression by retinoids and carotenoids: Similarities and differences

Abstract

Gap junctions, connexons, are formed by assembly of trans-membrane connexin proteins and have multiple functions including the coordination of cell responses. Most human tumors are deficient in gap junctional communication (GJC) and restoration of GJC by forced expression of connexins reduces indices of neoplasia. Expression of connexin 43 (Cx43), the most widely-expressed connexin family member, is upregulated by cancer-preventive retinoids and carotenoids in normal and preneoplastic cells; an action considered of mechanistic significance. However, the molecular mechanism for upregulated expression is poorly understood. The retinoic acid receptor antagonist Ro 41-5253 was capable of suppressing retinoid-induction Cx43 luciferase reporter construct in F9 cells, but did not suppress reporter activity induced by the non-pro-vitamin A carotenoids astaxanthin or lycopene, indicating that retinoids have separate mechanisms of gene activation than non-pro-vitamin A carotenoids. Neither class of compound required protein synthesis for induction of Cx43 mRNA, nor was the 5.0 h half-life of Cx43 mRNA altered, indicating direct transcriptional activation. The responsive region was found within -158 bp and +209 bp of the transcription start site; this contains a Sp1/Sp3 GC-box to which Sp1 and Sp3 were bound, as revealed by electrophoretic mobility shift assays (EMSA), but no retinoic acid response element (RARE). Site directed mutagenesis of this GC-box resulted in increased basal levels of transcription and loss of responsiveness to a synthetic retinoid. In this construct astaxanthin and lycopene produced marginally, but not significantly higher, reporter activity than the control.

Lycopene ameliorates neuropathic pain by upregulating spinal astrocytic connexin 43 expression

“…lycopene's effect on Cx43 upregulation is independent of RAR activity [36]. As observed in previous studies, β-carotene had no effect on Cx43 expression in cultured spinal astrocytes in the current study, which indicates that the rodent Cx43 promoter lacks RARE [19]. The results further indicate that the retinoid-RAR-RARE axis is likely not involved in lycopene-induced upregulation of Cx43 expression in spinal astrocytes.…”

“…Previous studies have shown that pro-vitamin A retinoids such as β-carotene, in addition to lycopene, also upregulate Cx43 expression in cancer cells through activation of the retinoic acid receptor (RAR) [12,19]. The RAR is a nuclear receptor, and normally binds to the retinoic acid response element (RARE), a promoter region found in specific genes, and is involved in transcriptional regulation.…”

“…In addition to lycopene, other carotenoids can induce Cx43 expression as well [19]. Thus, in addition to lycopene (20 μM), the effect of the provitamin A carotenoid β-carotene (10, 20 μM) and the potent antioxidant NAC (0.5, 1 mM) on the TNF-induced downregulation of Cx43 expression in cultured spinal astrocytes was examined.…”

Dietary Intake of Lycopene and Risk of Prostate Cancer

“…Proliferation of transformed cells was shown to be halted as gap junction communication is restored (8). In human prostate cancer, disease severity is proportional to the degree of connexin 43 expression (17). Furthermore, in cell lines where connexin 43 expression is restored in human prostate cancer cell lines, cells became more differentiated and had reduced growth rates (17).…”

“…In human prostate cancer, disease severity is proportional to the degree of connexin 43 expression (17). Furthermore, in cell lines where connexin 43 expression is restored in human prostate cancer cell lines, cells became more differentiated and had reduced growth rates (17). In view of this in vitro evidence a clinical trial, subjects with prostate cancer were supplemented with 15 mg of lycopene twice/d for 3 wk prior to radical prostatectomy (18).…”