Growth hormone induces tyrosine phosphorylation but does not alter insulin-like growth factor-I gene expression in human IM9 lymphocytes.
GH induces hepatic IGF-I synthesis by increasing transcription of its gene. IGF-I is synthesized, however, in many other tissues where the effect of GH on its gene expression is less well characterized. IGF-I and GH are produced by human lymphocytes and may function as autocrine regulators of lymphoproliferation. We have therefore used the human IM9 lymphocyte cell line to (A) define the IGF-I gene transcripts expressed and (B) investigate the effect of GH on early (protein tyrosine phosphorylation) and late (changes in IGF-I mRNA levels) events in intracellular signal transduction. Multiple IGF-I mRNA species, ranging in size from 0.9 to 5.8 kb, were detected by Northern hybridization of poly(A)+ mRNA from IM9 cells. The human IGF-I gene contains at least six exons and alternative splicing produces a number of transcripts. Solution hybridization with exon-specific riboprobes and amplification by PCR using exon-specific primers revealed that multiple transcripts were expressed in IM9 cells, and that exon 2 was the dominant leader exon. Treatment of IM9 cells with 200 ng recombinant human (rh)GH/ml led to the specific tyrosine phosphorylation of three intracellular proteins (93, 120 and 134 kDa), which are involved in the initial signalling of the GH transduction pathway. However a solution hybridization assay using the IGF-IA specific riboprobe on IM9 cell RNA from similar experiments revealed that GH treatment did not change IGF-I gene expression. This study has demonstrated (A) that the IGF-I gene is expressed in human IM9 lymphocytes, (B) that in contrast to other human tissue, exon 2 is the major leader exon, and (C) that rhGH induces tyrosine phosphorylation of 93, 120 and 134 kDa proteins but does not alter IGF-I gene expression. The IM9 cell may form an important model to investigate a GH transduction pathway not coupled to the IGF-I gene.
Department of Medicine, University of Virginia Medical School, Charlottesville 22908.
P E
PE
Clayton
R N
RN
Day
C M
CM
Silva
P
P
Hellmann
K H
KH
Day
M O
MO
Thorner
D002460:Cell Line; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D005091:Exons; D015870:Gene Expression / Q000187:drug effects; D013006:Growth Hormone / Q000494:pharmacology*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D008214:Lymphocytes / Q000187:drug effects* / Q000378:metabolism*; D010766:Phosphorylation; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism*
D017931:DNA Primers; D018076:DNA, Complementary; D012333:RNA, Messenger; D014443:Tyrosine; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone