Growth Hormone Expression in Murine Bone Marrow Cells Is Independent of the Pituitary Transcription Factor Pit-1*

GH has been shown to promote the development and function of leukocytes. The expression of both GH and GH-receptors in lymphoid cells has led to the hypothesis that GH acts in an autocrine or paracrine fashion. The described effects of GH on hematopoiesis and B cell development, led us to investigate GH expression in bone marrow cells. By immunocytochemistry, we show that bone marrow-derived granulocytes and macrophages contain immunoreactive GH. We found that 65 +/- 24% of the granulocytes were stained with anti-GH, whereas 5.8 +/- 1.5% of the granulocytes contained detectable amounts of GH mRNA as assessed by in situ hybridization. To address a possible alternative regulation mechanism in bone marrow and to establish whether locally derived GH might still play a role in pituitary-deficient dwarf mice, we also addressed GH expression in bone marrow from hypopituitary Snell dwarf mice. These mice have a mutated gene for the pituitary transcription factor Pit-1 that is deficient in DNA binding. Our finding that GH expression (immunoreactive protein and mRNA) in bone marrow cells from dwarf mice is similar to that in normal mice points to a Pit-1 independent regulation of GH in mouse bone marrow.