In T cells, cAMP-dependent protein kinase (PKA) type I colocalizes with the T cell receptor–CD3 complex (TCR/CD3) and inhibits T cell function via a previously unknown proximal target. Here we examine the mechanism for this PKA-mediated immunomodulation. cAMP treatment of Jurkat and normal T cells reduces Lck-mediated tyrosine phosphorylation of the TCR/CD3 ζ chain after T cell activation, and decreases Lck activity. Phosphorylation of residue Y505 in Lck by COOH-terminal Src kinase (Csk), which negatively regulates Lck, is essential for the inhibitory effect of cAMP on ζ chain phosphorylation. PKA phosphorylates Csk at S364 in vitro and in vivo leading to a two- to fourfold increase in Csk activity that is necessary for cAMP-mediated inhibition of TCR-induced interleukin 2 secretion. Both PKA type I and Csk are targeted to lipid rafts where proximal T cell activation occurs, and phosphorylation of raft-associated Lck by Csk is increased in cells treated with forskolin. We propose a mechanism whereby PKA through activation of Csk intersects signaling by Src kinases and inhibits T cell activation.

2. Cyclic AMP and the cAMP-dependent protein kinase (PKA) signaling system 3. Isozymes of PKA 3.1. Multiple isoforms of regulatory and catalytic subunits of PKA 3.2. Features of regulatory and catalytic subunits of PKA 3.2.1. Structure of regulatory subunits 3.2.2. Structure of catalytic subunits 4. Levels and expression of the regulatory and catalytic subunits 5. PKA isozyme composition and characteristics 6. Specific effects of cAMP are mediated through subcellularly anchored PKA isozymes. 6.1. PKAI mediates specific effects of cAMP at distinct subcellular sites. 6.2. PKAII is targeted to subcellular structures via A kinase anchoring proteins (AKAPs) and mediates discrete cAMP responses. 7. AKAPs assemble signal complexes important for intracellular signaling.

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We demonstrate here the functional reprogramming of a somatic cell using a nuclear and cytoplasmic extract derived from another somatic cell type. Reprogramming of 293T fibroblasts in an extract from primary human T cells or from a transformed T-cell line is evidenced by nuclear uptake and assembly of transcription factors, induction of activity of a chromatin remodeling complex, histone acetylation, and activation of lymphoid cell specific genes. Reprogrammed cells express T cell specific receptors and assemble the interleukin-2 receptor in response to T cell receptor CD3 (TCR CD3) complex stimulation. Reprogrammed primary skin fibroblasts also express T cell specific antigens. After exposure to a neuronal precursor extract, 293T fibroblasts express a neurofilament protein and extend neurite-like outgrowths. In vitro reprogramming of differentiated somatic cells creates possibilities for producing isogenic replacement cells for therapeutic applications.

In addition to generating movement, skeletal muscle may have a function as a secretory organ. The aim of the present study was to identify novel proteins with signaling capabilities secreted from skeletal muscle cells. IL-7 was detected in media conditioned by primary cultures of human myotubes differentiated from satellite cells, and concentrations increased with incubation time. By immunoblotting and real-time RT-PCR IL-7 expression was confirmed at both protein and mRNA levels. Furthermore, with immunofluorescence and specific antisera, multinucleated myotubes were found to coexpress IL-7 and myosin heavy chain. During differentiation of human myotubes from satellite cells, IL-7 expression increased at mRNA and protein levels. In contrast, mRNA expression of the IL-7 receptor was 80% lower in myotubes compared with satellite cells. Incubations with recombinant IL-7 under differentiation conditions caused approximately 35% reduction in mRNA for the terminal myogenic markers myosin heavy chain 2 (MYH2) and myogenin (MYOG), suggesting that IL-7 may act on satellite cells to inhibit development of the muscle fiber phenotype. Alternative routes of cell development were investigated, and IL-7 increased migration of satellite cells by 40% after 48 h in a Transwell system, whereas cell proliferation remained unchanged. In vivo, real-time RT-PCR analysis of musculus vastus lateralis (n = 10) and musculus trapezius (n = 7) biopsies taken from male individuals undergoing a strength training program demonstrated that after 11 wk mean IL-7 mRNA increased by threefold (P = 0.01) and fourfold (P = 0.04), respectively. In conclusion, we have demonstrated that IL-7 is a novel myokine regulated both in vitro and in vivo, and it may play a role in the regulation of muscle cell development.