Voltage-insensitive Ca2+ channels and Ca2+/calmodulin-dependent protein kinases propagate signals from endothelin-1 receptors to the c-fos promoter.

Endothelin-1 (ET-1) triggers poorly understood nuclear signaling cascades that control gene expression, cell growth, and differentiation. To better understand how ET-1 regulates gene expression, we asked whether voltage-insensitive Ca2+ channels and Ca2+/calmodulin-dependent protein kinases (CaMKs) propagate signals from ET-1 receptors to the c-fos promoter in mesangial cells. Ca2+ influx through voltage-insensitive Ca2+ channels, one of the earliest postreceptor events in ET-1 signaling, mediated induction of c-fos mRNA and activation of the c-fos promoter by ET-1. A CaMK inhibitor (KN-93) blocked activation of the c-fos promoter by ET-1. Ectopic expression of CaMKII potentiated stimulation by ET-1, providing further evidence that CaMKs contribute to c-fos promoter activation by ET-1. The c-fos serum response element was necessary but not sufficient for CaMKII to activate the c-fos promoter. Activation of the c-fos promoter by ET-1 and CaMKII also required the FAP cis element, an AP-1-like sequence adjacent to the serum response element. Thus, voltage-insensitive Ca2+ channels and CaMKs apparently propagate ET-1 signals to the c-fos promoter that require multiple, interdependent cis elements. Moreover, these experiments suggest an important role for voltage-insensitive Ca2+ channels in nuclear signal transduction in nonexcitable cells.     

Demonstration in living cells of an intragenic negative regulatory element within the rodent c-fos gene

We studied c-fos gene expression in rat fibroblasts by microinjection of regulatory DNA sequences, such as the serum response element (SRE) present in c-fos promotor, in order to compete directly with such sequences for binding of putative regulatory factors. We show that an additional fos intragenic regulatory element (FIRE) is located at the end of exon 1. When coinjected with an SRE oligonucleotide, it induced c-fos expression in quiescent cells, whereas injection of SRE sequence alone failed to do so. Moreover, injection in quiescent cells of an SRE oligonucleotide together with a p-fos-lacZ construct containing the c-fos SRE as well as an in-frame insertion of FIRE resulted in a block to beta-galactosidase expression that can be relieved by coinjection of the FIRE sequence.