The role of Connexin 46 promoter in lens and other hypoxic tissues

Abstract

Gap junctions are multimeric membrane protein channels that connect the cytoplasm of one cell to another. Much information about connexins regards electrophysiology and channel function but relatively little information is known about non-channel functions of connexins. Lens connexins, Cx43, Cx46 and Cx50, have been extensively studied for their role in lens homeostasis. Connexins allow the movement of small metabolically relevant molecules and ions between cells and this action in the lens prevents cataract formation. Interruption of Cx46 channel function leads to cataract formation due to dysregulation of lens homeostasis. The loss of Cx46 upregulates Cx43 in lens cell culture and suppresses tumor growth in breast and retinoblastoma tumor xenografts. Upregulation of Cx46 in hypoxic tissues has been noted and may be due in part to the effects of hypoxia and HIF activators. Here, we report that the Cx46 promoter is regulated by hypoxia and also offer speculation about the role of Cx46 in lens differentiation and solid tumor growth.

Keywords: Connexin 43; Connexin 46; cellular differentiation; connexin promoter; hypoxia; lens; tumors; xenografts.

Figure 1. Schematic model of lens connexin expression zones. Cx43 is expressed primarily in the epithelial and short differentiating fiber cells. Cx46 expression begins relatively early in lens differentiation and small amounts can be found in the short differentiating fiber cells. Cx50 expression remains relatively constant throughout lens development. Lens epithelial cells begin to elongate and migrate inward when the transition of Cx43 to Cx46 expression occurs as oxygen levels fall from 10–15 mmHg to ~3–5 mmHg within the lens.

Figure 2. The promoter of human Connexin 46 is transiently responsive to 1% oxygen in human lens cells. Various fragments 5′ to the predicted transcription start site of the human Cx46 promoter were cloned into a promoterless luciferase reporter vector and tested for responsiveness to 1% oxygen in human lens epithelial cells. The varying activity correlated with the length of the promoter indicates the presence of regulatory elements encoded within the promoter. The promoter did not respond to hypoxia in N2A cells in the same assay (unpublished data). Error bars represent standard error of the mean (n = 6).