ACTIVATION OF THE GLYCOPROTEIN HORMONE ALPHA-SUBUNIT PROMOTER BY A LIM-HOMEODOMAIN TRANSCRIPTION FACTOR

Recently, a pituitary-specific enhancer was identified within the 5' f lanking region of the mouse glycoprotein hormone alpha-subunit gene. T his enhancer is active in pituitary cells of the gonadotrope and thyro trope lineages and has been designated the pituitary glycoprotein horm one basal element (PGBE). In the present studies, we sought to isolate and characterize proteins which interact with the PGBE. Mutagenesis e xperiments identified a 14-bp imperfect palindrome that is required fo r binding of a factor which is present in cells of gonadotrope and thy rotrope lineages but not in other cells. Screening of a mouse cDNA lib rary with a DNA probe containing the imperfect palindrome resulted in the isolation of a LIM-homeodomain transcription factor. The cDNA pred icts a mouse protein which is 94% identical to the recently described rat LIM-homeodomain protein LH-2. LH-2 contains two zinc fingers (LIM domain) and a consensus homeodomain. Hybridization analysis revealed r elatively high expression of LH-2 mRNA in the central nervous system a nd in pituitary cells of the gonadotrope and thyrotrope lineages. Lowe r or nondetectable levels of LH-2 mRNA were found in other pituitary c ells and tissues, including placental cells. Recombinant LH-2 homeodom ain was found to selectively bind to the previously identified imperfe ct palindrome in the PGBE. Point mutations in the PGBE resulted in par allel losses in the binding of a nuclear factor from a cell line of th e gonadotrope lineage and recombinant LH-2-binding activity. Use of an antibody to LH-2 provided evidence that endogenous PGBE-binding activ ity from cells of the gonadotrope lineage involves a protein which is immunologically related to LH-2. Expression of LH-2 in two heterologou s cell types resulted in activation of a reporter gene containing the mouse alpha promoter. These data suggest that the LIM-homeodomain fact or LH-2 plays a role in stimulating tissue-specific expression of the mouse glycoprotein hormone alpha subunit. The finding that a LIM-homeo domain protein can stimulate expression of one of the earliest markers of pituitary differentiation raises the possibility that this factor plays a role in cell lineage determination in the pituitary.