promote macrophage cell division. We conclude that cAMPdependent signal transduction as well as other signaling cascades are essential for ␣ 2 M*-induced cell proliferation.1 is part of a large superfamily that includes proteinase inhibitors and complement components (1). ␣ 2 M is a homotetramer, and, like C3 and C4, each subunit contains a -cysteinyl-␥-glutamyl thiolester (2, 3). Upon reaction of ␣ 2 M with proteinases, the thiolesters rupture, and the molecule undergoes a large conformational change (2, 3). This exposes a cryptic determinant located in the carboxyl-terminal domain of each subunit, which constitutes the receptor recognition site (2, 3). Direct reaction of the thiolesters with small nucleophiles, such as NH 3 or CH 3 NH 2 also triggers exposure of the receptor recognition sites (2, 3). ␣ 2 M* binds to the low density lipoprotein receptor-related protein and to the ␣ 2 M signaling receptor (␣ 2 MSR), which appears to consist of a coreceptor complexed to lipoprotein receptor-related protein (4 -11). Binding of ␣ 2 M* to ␣ 2 MSR activates a pertussis toxininsensitive phospholipase C, which hydrolyzes membrane phosphoinositides, generating two second messengers, inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG). IP 3 raises cytosolic free Ca 2ϩ , [Ca 2ϩ ] i , by releasing Ca 2ϩ sequestered in the endoplasmic reticulum, thus triggering the onset of several Ca 2ϩ -dependent signaling cascades (4 -13). DAG, on the other hand, activates protein kinase C (PKC), thus triggering the activation of phosphorylation-dependent signaling components. Ligation of ␣ 2 MSR induces DNA and protein synthesis, which is Ca 2ϩ -dependent and requires participation of activated tyrosine kinases, p21ras -dependent MAPK, and PI 3-kinase signaling cascades (4 -18, 12-17). Treatment of macrophages with ␣ 2 M* also causes a 2-2.5-fold increase in cell number (10).cAMP-response element-binding protein (CREB) is a nuclear transcription factor which is a downstream target of cAMP signaling (18,19). Protein kinase A (PKA) phosphorylates CREB at Ser-133 within the kinase-inducible domain (18,19). This increases its transcriptional activity by promoting its association with CREB-binding protein, leading to activation of the transcriptional machinery. CREB also can be phosphorylated at Ser-133 by multiple signaling mechanisms including ERK 1/2, PKC, Ca 2ϩ /calmodulin-dependent protein kinases, p38 MAPK, and ribosomal S6 kinase (p70s6k) (18 -27). MAPKs activate CREB kinase (p90s6k), which in turn phosphorylates and activates CREB. To elucidate the role of cAMP signaling in cellular physiology and homeostasis, several studies have used genetic manipulations in intact animals and cell systems. These include gene knockout and gene overexpression. In the last several years, the use of posttranscriptional gene silencing and RNA interference techniques have been employed to block protein expression in a variety of in vitro systems (28 -36). The techniques of RNA interference employ sequence-specific posttranslational gene sil...
