ABSTRACT
The aim of this study was the characterization of the human Gbeta4 subunit of heterotrimeric G proteins. Human Gbeta4 is widely expressed. Its gene is located on chromosome 3 with a genomic structure indistinguishable from that of the genes of Gbeta1 to Gbeta3, but entirely different from Gbeta5. In vitro translation co-precipitation analyses revealed that Gbeta4 can form stable dimers with Ggamma1, Ggamma2, Ggamma3, Ggamma4, Ggamma5, Ggamma7, Ggamma10, Ggamma11, Ggamma12, and Ggamma13, dimers which were also able to stimulate phospholipase beta2.
Subject(s)
Heterotrimeric GTP-Binding Proteins/chemistry , Heterotrimeric GTP-Binding Proteins/genetics , Heterotrimeric GTP-Binding Proteins/metabolism , Amino Acid Sequence , Base Sequence , Chromosomes, Human, Pair 3 , Cloning, Molecular , DNA, Complementary/metabolism , Dimerization , Enzyme Activation , Exons , Humans , Models, Genetic , Molecular Sequence Data , Precipitin Tests , Protein Biosynthesis , Protein Structure, Tertiary , RNA, Messenger/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction , Tissue DistributionABSTRACT
Somatostatin (SST) and somatostatin receptors (SSTR) are widely distributed in lymphoid tissues. Here, we report on the stimulatory effects of SST in Epstein-Barr virus-immortalized B lymphoblasts. By RT-PCR, we demonstrated the exclusive expression of the somatostatin receptor isoform 2A (SSTR2A) in B lymphoblasts. Addition of SST rapidly increased the cytosolic free calcium concentration [Ca(2+)](i) maximally by about 200 nM, with an EC(50) of 1.3 nM, and stimulated the formation of inositol phosphates. Furthermore, SST increased binding of guanosine 5'-O-(3-thiotriphosphate) by 50% above basal. These effects were partly inhibited by pertussis toxin (PTX), which indicates the involvement of PTX-sensitive G proteins. We provide further evidence that Galpha(16,) a PTX-insensitive G protein confined to lymphohematopoietic cells, is involved in the otherwise unusual coupling of SSTR2A to phospholipase C activation. In addition, SST activated extracellular regulated kinases and induced a 3.5-fold stimulation of DNA synthesis and a 4.4-fold stimulation of B lymphoblast proliferation, which was accompanied by an enhanced immunoglobulin formation. Thus SST exerts a growth factor-like activity on human B lymphoblasts.
