Cell-to-cell communication in osteoblastic networks: Cell line–dependent hormonal regulation of gap junction function

Abstract

We have characterized the distribution, expression, and hormonal regulation of gap junctions in primary cultures of rat osteoblast‐like cells (ROBs), and three osteosarcoma cell lines, ROS 17/2.8, UMR‐106, and SAOS‐2, and a continuous osteoblastic cell line, MC3T3‐E1. All cell lines we examined were functionally coupled. ROS 17/2.8 were the more strongly coupled, while ROB and MC3T3‐E1 were moderately coupled and UMR‐106 and SAOS‐2 were weakly coupled. Exposure to parathyroid hormone (PTH) for 1 h increased functional coupling in ROB cells in a concentration‐dependent manner. Furthermore, PTH (3–34), an analog of PTH with binds to the PTH receptor and thus attenuates PTH‐stimulated cAMP accumulation, also attenuated PTH‐stimulated functional coupling in ROB. This suggests that PTH increases functional coupling partly through a cAMP‐dependent mechanism. A 1 h exposure to PTH did not affect coupling in ROS 17/2.8, UMR‐106, MC3T3‐E1, or SAOS‐2. To examine whether connexin43 (Cx43), a specific gap junction protein, is present in functionally coupled osteoblastic cells, we characterized Cx43 distribution and expression. Indirect immunofluorescence with antibodies to Cx43 revealed that ROS 17/2.8, ROB, and to a lesser extent MC3T3‐E1 and UMR‐106, expressed Cx43 immunoreactivity. SAOS‐2 showed little if any Cx43 immunoreactivity. Cx43 mRNA and Cx43 protein were detected by Northern blot analysis and immunoblot analysis, respectively, in all cell lines examined, including SAOS‐2. Our findings suggest that acute exposure to PTH regulates gap junction coupling, in a cell‐line dependent manner, in osteoblastic cells. Furthermore, Cx43 is expressed in osteoblastic cells regardless of the degree to which the cells are functionally coupled or the degree to which coupling is regulated by PTH.