Rh. Hackett et al., MAPPING OF THE CYTOPLASMIC DOMAIN OF THE HUMAN GROWTH-HORMONE RECEPTOR REQUIRED FOR THE ACTIVATION OF JAK2 AND STAT PROTEINS, The Journal of biological chemistry, 270(36), 1995, pp. 21326-21330
Incubation of cells with growth hormone (GH) stimulates both tyrosine
phosphorylation of the Jak2 tyrosine kinase and, in some cells, the tr
anscription factor Stat1 alpha (1-4). When the promyeloid cell line FD
C-P1 is transfected with the human growth hormone receptor, these cell
s can grow in the presence of GH and in the absence of interleukin-3.
Growth hormone treatment of cells expressing the human growth hormone
receptor did not activate Stat1 alpha. However, a complex is present i
n extracts prepared from growth hormone treated cells that binds to th
e gamma response region, an enhancer present in the promoter of the hi
gh affinity Fc gamma R1 receptor to which cytokine-activated Stat comp
lexes bind. When truncations of the cytoplasmic domain of the receptor
are expressed in FDC-P1 cells only the membrane-proximal 80 amino aci
ds (containing box 1 and box 2) are required for activation of both a
GH-stimulated binding activity (GHSF) and tyrosine phosphorylation of
Jak2. Activation of GHSF can be inhibited in a cell-free system by the
addition of a glutathione S-transferase fusion protein containing the
se SO amino acids. Replacement of the one tyrosine in this region of t
he receptor with a phenylalanine does not alter the activation of eith
er GHSF or Jak2, suggesting that tyrosine phosphorylation of the recep
tor is not required for GH activation of GHSF. Moreover, a cell line e
xpressing a receptor with only the 54 membrane-proximal amino acids of
the intracellular domain (including box 1) shows constitutively tyros
ine phosphorylated Jak2 as well as GHSF binding. With this truncated r
eceptor, there is little if any additional GH-induced tyrosine phospho
rylation of Jak2 or induced binding to the gamma response region. Thes
e results define the importance of the membrane-proximal 80 amino acid
s of the GH receptor (with the conserved box 1 and box 2 domains) with
regard to GH activation of both Jak2 and Stat(s). They also suggest t
hat within these domains there may be positive and negative elements t
hat regulate Jak2 function.