Dynorphin B is an agonist of nuclear opioid receptors coupling nuclear protein kinase C activation to the transcription of cardiogenic genes in GTR1 embryonic stem cells.

The cardiac differentiation of embryonic stem (ES) cells was found to involve prodynorphin gene and dynorphin B expression and was associated with the interaction of secreted dynorphin B with cell surface opioid receptors coupled with protein kinase C (PKC) signaling and complex subcellular redistribution patterning of selected PKC isozymes. Here, confocal microscopy revealed the presence of immunoreactive dynorphin B-like material in GTR1 ES cells, suggesting that dynorphin peptides may also act intracellularly. Opioid binding sites were identified in ES cell nuclei, with a single dissociation constant in the low nanomolar range. A significant increase in Bmax for a kappa opioid receptor ligand was observed in nuclei isolated from ES-derived cardiomyocytes compared with nuclei from undifferentiated cells. Direct exposure of nuclei isolated from undifferentiated ES cells to dynorphin B or U-50,488H, a synthetic kappa opioid receptor agonist, time- and dose-dependently activated the transcription of GATA-4 and Nkx-2.5, 2 cardiac lineage-promoting genes. Nuclear exposure to dynorphin B also enhanced the rate of prodynorphin gene transcription. These responses were abolished in a stereospecific fashion by the incubation of isolated nuclei with selective opioid receptor antagonists. Nuclei isolated from undifferentiated cells were able to phosphorylate the acrylodan-labeled MARCKS peptide, a high-affinity fluorescent PKC substrate. Exposure of isolated nuclei to dynorphin B induced a remarkable increase in nuclear PKC activity, which was suppressed by opioid receptor antagonists. Nuclear treatment with PKC inhibitors abolished the capability of dynorphin B to prime the transcription of cardiogenic genes.