Nucleocytoplasmic distribution of opioid growth factor and its receptor in tongue epithelium

The subcellular distributions of the opioid growth factor (OGF), [Met ⁵]-enkephalin, and opioid growth factor receptor (OGFr) in the epithelium of the rat tongue were determined in order to reveal structure-function relationships. Laser scanning confocal microscopic analysis showed that both OGF and OGFr were colocalized in the paranuclear cytoplasm and in the nuclei of keratinocytes in the stratum basale. Using immunoelectron microscopy and postem-bedding techniques, double labeling experiments disclosed that complexes of OGF-OGFr were colocalized on the outer nuclear envelope, in the paranuclear cytoplasm, perpendicular to the nuclear envelope in a putative nuclear pore complex, and in the nucleus adjacent to heterochromatin. Anti-OGF IgG alone was detected in the cytoplasm, and anti-OGFr IgG alone was associated with the outer nuclear envelope. Study of chronic treatment with the opioid antagonist, naltrexone (NTX), which blocks opioid-receptor binding, revealed the presence of OGFr immunoreactivity alone in the cytoplasm and the nucleus; some OGF-OGFr complexes were also observed. Colocalization of OGFr and karyopherin (importin) β was recorded in the cytoplasm and nucleus. These results in tongue epithelium are the first to suggest that OGFr resides on the outer nuclear envelope, where OGF interacts with OGFr; that the OGF-OGFr complex translocates between cytoplasm and nucleus at the nuclear pore; and that the nuclear localization signal of OGFr interacts with karyopherin β for nuclear transport. These novel data also indicate that signal transduction for cell proliferation appears to involve an OGF-OGFr complex that interfaces with chromatin in the nucleus. Moreover, the unique finding that OGFr was found in the cytoplasm and nucleus in NTX-treated specimens may suggest that NTX-OGFr complexes have the same pathway as OGF-OGFr.