Embryonic Expression of the Luteinizing Hormone β Gene Appears to Be Coupled to the Transient Appearance of p8, a High Mobility Group-related Transcription Factor

A comparison between two pituitary-derived cell lines (αT3-1 and LβT2) that represent gonadotropes at early and late stages of development, respectively, was performed to further elucidate the genomic repertoire required for gonadotrope specification and luteinizing hormone β (LHβ) gene expression. One isolated clone that displayed higher expression levels in LβT2 cells encodes p8, a high mobility group-like protein with mitogenic potential that is up-regulated in response to proapoptotic stimuli and in some developing tissues. To test the functional significance of this factor in developing gonadotropes, a knockdown of p8 in LβT2 cells was generated. The loss of p8 mRNA correlated with loss of endogenous LHβ mRNA and the loss of activity of a transfected LHβ promoter-driven reporter, even upon treatment with gonadotropin-releasing hormone. In addition, expression of p8 mRNA in developing mouse pituitary glands mirrored its expression in the gonadotrope-derived cell lines and coincided with the first detectable appearance of LHβ mRNA. In contrast, p8 mRNA was undetectable in the pituitary glands of normal adults. Taken together, our data indicate that p8 is a stage-specific component of the gonadotrope transcriptome that may play a functional role in the initiation of LHβ gene expression during embryonic cellular differentiation.