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.