PKA is a cAMP-dependent serine/threonine kinase. Several years ago Kammer et al, who were the first to search for biochemical defects in SLE T cells, identified abnormalities in cAMP metabolism and PKA-catalyzed protein phosphorylation (Mandler et al., 1982; Hasler et al., 1990). In the inactive state, PKA forms a tetramer consisting of two regulatory and two catalytic subunits (R2C2). Depending on the isoform of the R-subunit, PKA exists as either PKA-I or PKA-II isoenzyme, localized to the plasma membrane and the cytoplasm, respectively. Binding of cAMP to the regulatory subunits alters their confirmation, causing them to dissociate from the PKA complex (R2C2 + 4 cAMP o R cAMP4 + 2C). The released catalytic subunits are thereby activated to phospho-rylate a diverse panel of target proteins both in the cytoplasmic and the nuclear compartments (Torgersen et al., 2002).
Initial studies revealed that cAMP-primed SLE T cells display impaired PKA-catalyzed protein phosphorylation mainly due to decreased activity of the isoenzyme PKA-I (Kammer et al., 1994). First-order enzyme kinetics studies subsequently showed that in SLE T cells the binding capacity of cAMP for the R-I subunit was significantly impaired (Kammer et al., 1996). A mutated form of R-I subunit with lower cAMP-binding activity could provide a logical explanation for this finding. Investigators found that SLE T cells have decreased protein expression of the R-I subunits (a and P isoforms) that paralleled decreased amounts of R-I transcripts, which may reflect defects at the transcriptional level (Laxminarayana et al., 1999). Lower levels of functional R-I possibly denote lower levels of PKA-I (RI2C2) complexes and thus a smaller pool of PKA-I for cAMP to act on, leading to impaired performance of this signaling pathway. On the other hand, since the expression of the catalytic subunits appears to be intact, it is logical to expect increased amounts of unrelated (and thus possibly active) C subunits that could spontaneously promote PKA-mediated actions. Nevertheless, when R-I transcripts from SLE T cells were analyzed several mutations were found in the absence of any genetic mutations, implying defective mRNA editing. Whether altered R-I subunits from SLE T cells display reduced ability to bind cAMP remains to be addressed. Along with a better characterization of the nature of PKA-I deficiency, a more thorough understanding of its contribution to cell and cytokine defects in SLE T cells is anticipated with great interest.
Moreover, a series of experiments indicate that the P isoform of the R-sub-unit (RIIB) of PKA-II isoenzyme could mediate impaired cAMP-initiated responses in the nucleus of lupus T cells (Kammer, 2002). In the nucleus, tran-scriptional regulation by cAMP is mediated by a family of cAMP-responsive nuclear factors, which bind to and regulate the expression of genes containing the cAMP-responsive element (CRE) consensus in their promoters. Phosphorylation of these CRE-binding proteins (CREBs) by the C subunit of PKA that translocates into the nucleus following activation by cAMP modulates their activity (Skalhegg and Tasken, 2000). In SLE T cells, deficient PKA-II activity is characterized by spontaneous dissociation of the cytosolic RIIP2C2 holoenzyme, aberrant RIIP translocation to the nucleus from the cytosol, and retention of RIIP
in the nucleus. Experiments addressing a possible interplay with extracellular stimuli in the lupus microenvironment showed that RIIß disorder was primary to the SLE T cell (Mishra et al., 2000). Interestingly, forced expression of RIIß in Jurkat cells revealed a novel, specific, and direct interaction between RIIß and CREB that led to inhibition of CRE-dependent expression of the proto-oncogene c-fos (Elliott et al., 2003). In addition, Jurkat cells overexpressing a phosphory-lated form of RIIß were shown to suppress the production of IL-2 in parallel with upregulation of the costimulatory molecule CD40L (Elliott et al, 2004). Taken together these results indicate that accumulation of RIIß in the nucleus of SLE T cells could contribute to some phenotypic abnormalities observed in these cells, such as reduced production of IL-2 and overexpression of CD40L.
Importantly, PKA-I and -II abnormalities encountered in SLE T cells do not correlate with disease activity, therapy, or demographic characteristics, and are sustained over time (Kammer, 2002). In addition, the fact that the genes encoding RIß and RIIß map to 7p22 and 7q22 regions, which have been identified as susceptibility loci for SLE (Gaffney et al., 2000), underlines the importance of the study of PKA function in SLE.
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