Growth hormone (GH),

No abstract

For insight into the mechanisms of gene regulation by growth hormone (GH), the regulation of transcription factors associated with the serum response element (SRE) located upstream of c-fos was examined. The SRE can mediate induction of reporter expression in response to GH. For insight into the mechanism by which GH regulates transcription factors, regulation of SREassociated proteins by GH was examined. In nuclear extracts from 3T3-F442A fibroblasts, several SRE-binding complexes were identified by electrophoretic mobility shift assay. GH treatment for 2-10 min transiently increased binding of two complexes; binding returned to control values within 30 min. The two GH-stimulated complexes were supershifted by antibodies against the serum response factor (SRF), indicating that they contained SRF or an antigenically related protein. One of the GH-stimulated complexes was supershifted by antibody against Elk-1, suggesting that it contains a ternary complex factor (TCF) such as Elk-1 in addition to SRF. Induction of binding by GH was lost when the SRF binding site in the SRE was mutated, and mutation of either the SRF or TCF binding site altered the pattern of protein binding to the SRE. Mutation of the SRF or TCF binding site in SRE-luciferase plasmids inhibited the ability of GH to stimulate reporter expression, supporting a role for both SRF and TCF in GH-induced transcription of c-fos via the SRE. The TCF family member Elk-1 is capable of mediating GH-stimulated transcription, since GH-stimulated reporter expression was mediated by the transcriptional activation domain of Elk-1. Consistent with this stimulation, GH rapidly and transiently stimulated the serine phosphorylation of Elk-1. The increase was evident within 10 min and subsided after 30 min. Taken together, these data indicate that SRF and TCF contribute to GH-promoted transcription of c-fos via the SRE and are consistent with GH-promoted phosphorylation of Elk-1 contributing to GH-promoted transcriptional activation via the SRE.

No abstract

To identify mechanisms by which GH receptors (GHR) mediate downstream events representative of growth and metabolic responses to GH, stimulation by GH of c-fos and egr-1 expression and glucose transport activity were examined in Chinese hamster ovary (CHO) cells expressing mutated GHR. In CHO cells expressing wild-type GHR(GHR(1-638)), GH stimulated the expression of c-fos and egr-1, and stimulated 2-deoxyglucose uptake, responses also mediated by endogenous GHR in 3T3-F442A cells. Deletion of the proline-rich box 1 of GHR (GHR(deltaP)) abrogated all of these responses to GH, indicating that box 1, a site of association of GHR with the tyrosine kinase JAK2, is crucial for these GH-stimulated responses. As the C-terminal half of the cytoplasmic domain of GHR is required for GH-stimulated calcium flux and for stimulation of spi-2.1 transcription, GHR lacking this sequence (GHR(1-454)) were examined. Not only did GHR(1-454) mediate stimulation of c-fos and egr-1 expression and 2-deoxyglucose uptake, but they also mediated GH-stimulated transcriptional activation via Elk-1, a transcription factor associated with the c-fos Serum Response Element. Thus, the C-terminal half of the cytoplasmic domain of GHR is not required for GH-stimulated c-fos transcription, suggesting that increased calcium is not required for GH-stimulated c-fos expression. In CHO cells lacking all but five N-terminal residues of the cytoplasmic domain (GHR(1-294)), GH did not induce c-fos or egr-1 expression or stimulate 2-deoxyglucose uptake. Further, in 3T3-F442A fibroblasts with endogenous GHR, GH-stimulated c-fos expression and 2-deoxyglucose uptake were reduced by the tyrosine kinase inhibitors herbimycin A, staurosporine, and P11. Herbimycin A and staurosporine inhibit JAK2 and tyrosyl phosphorylation of all proteins stimulated by GH, whereas P11 inhibits the GH-dependent tyrosyl phosphorylation of only some proteins, including extracellular signal regulated kinases ERK1 and -2, but not JAK2. Taken together, these results implicate association of GHR with JAK2 and GH-stimulated tyrosyl phosphorylation of an additional cellular protein in GH-stimulated glucose transport and c-fos and egr-1 expression. These studies also indicate that, in contrast to spi-2.1, the N-terminal half of the cytoplasmic domain of GHR is sufficient to mediate stimulation of c-fos and egr-1 expression and Elk-1 activation, supporting multiple mechanisms for GH signaling to the nucleus.