Identification of SP3 as a negative regulatory transcription factor in the monocyte expression of growth hormone

A number of studies from different laboratories clearly show that cells of the immune system produce a GH molecule indistinguishable from that produced in the pituitary. A more recent finding from our studies suggests that monocytes use the same first exon and promoter sequence for the expression of lymphocyte GH as that reported for the expression of pituitary GH. In this report we have extended these results by determining that two members of the SP family of transcription factors, SP1 and SP3, bind to the region at -138/-133 bp containing a GGGAGG motif. Confirmation that this region of the monocyte GH promoter-bound SP1 and SP3 was accomplished using electrophoretic mobility shift assays with SP1 consensus and mutant probes as well as specific antibodies to SP1 and SP3. Selective mutation of the SP1/SP3 site increased basal transcription by 73%, indicating that this site is important in transcriptional inhibition. Overexpression of SP1 had no demonstrable effect on the GH promoter, whereas overexpression of SP3 caused inhibition of expression in P-388 monocyte cells. Cotransfection of P-388 cells with overexpression vectors for both SP1 and SP3 transcription factors also resulted in inhibition of basal expression. Transfection experiments in Drosophila SL-2 cells overexpressing SP1 and/or SP3 suggest that both factors repress the basal expression of GH promoter luciferase constructs and that the effect together was additive. Taken together, the results demonstrate that basal expression of monocyte GH may be negatively regulated by SP3.