Amphiregulin is a novel growth factor involved in normal bone development and in the cellular response to parathyroid hormone stimulation

Parathyroid hormone (PTH) is the major mediator of calcium homeostasis and bone remodeling and is now known to be an effective drug for osteoporosis treatment. Yet the mechanisms responsible for its functions in bone are largely unknown. Here we report that the expression of amphiregulin (AR), a member of the epidermal growth factor (EGF) family, is rapidly and highly up-regulated by PTH in several osteoblastic cell lines and bone tissues. Other osteotropic hormones (1alpha,25-dihydroxyvitamin D3 and prostaglandin E2) also strongly stimulate AR expression. We found all EGF-like ligands and their receptors are expressed in osteoblasts, but AR is the only member that is highly regulated by PTH. Functional studies demonstrated that although AR is a potent growth factor for preosteoblasts, it completely inhibits further differentiation. AR also strongly and quickly stimulated Akt and ERK phosphorylation and c-fos and c-jun expression in an EGF receptor-dependent manner. Moreover, AR null mice displayed significantly less tibial trabecular bone than wild-type mice. Taken together, we have identified a novel growth factor that is PTH-regulated and appears to have an important role in bone metabolism.     

Critical role of cAMP-response element-binding protein for angiotensin II-induced hypertrophy of vascular smooth muscle cells

We reported previously an important role of cyclic AMP-response element (CRE) for the induction of interleukin-6 gene expression by angiotensin II (AngII). We examined signaling pathways that are responsible for AngII-induced phosphorylation of CRE-binding protein (CREB) at serine 133 that is a critical marker for the activation in rat vascular smooth muscle cells (VSMC). AngII time dependently induced phosphorylation of CREB with a peak at 5 min. The AngII-induced phosphorylation of CREB was blocked by CV11974, an AngII type I receptor antagonist, suggesting that AngII type I receptor may mediate the phosphorylation of CREB. Inhibition of extracellular signal-regulated protein kinase (ERK) by PD98059 or inhibition of p38 mitogen-activated protein kinase (MAPK) by SB203580 partially inhibited AngII-induced CREB phosphorylation. A protein kinase A inhibitor, H89, also partially suppressed AngII-induced CREB phosphorylation. Inhibition of epidermal growth factor-receptor by AG1478 suppressed the AngII-induced CREB phosphorylation as well as activation of ERK and p38MAPK. Overexpression of the dominant negative form of CREB by an adenovirus vector suppressed AngII-induced c-fos expression and incorporation of [(3)H]leucine to VSMC. These findings suggest that AngII may activate multiple signaling pathways involving two MAPK pathways and protein kinase A, all of which contribute to the activation of CREB. Transactivation of epidermal growth factor-receptor is also critical for AngII-induced CREB phosphorylation. Activation of CREB may be important for the regulation of gene expression and hypertrophy of VSMC induced by AngII.     

Lysophosphatidic acid-induced c-fos up-regulation involves cyclic AMP response element-binding protein activated by mitogen- and stress-activated protein kinase-1

Lysophosphatidic acid (LPA) is a lipid growth factor that exerts diverse biological effects through its cognate receptor-mediated signaling cascades. Recently, we reported that LPA stimulates cAMP response element-binding protein (CREB) through mitogen- and stress-activated protein kinase-1 (MSK1). Previously, LPA has been shown to stimulate c-fos mRNA expression in Rat-2 fibroblast cells via a serum response element binding protein (SRF). However, involvement of CREB in LPA-stimulated c-fos gene expression is not elucidated yet. To investigate the CREB-mediated c-fos activation by LPA, various c-fos promoter-reporter constructs containing wild-type and mutated SRE and CRE were tested for their inducibility by LPA in transient transfection assays. LPA-stimulated c-fos promoter activation was markedly decreased when SRE and CRE were mutated. A dominant negative CREB significantly down-regulated the LPA-stimulated c-fos promoter activation. Chromatin immunoprecipitation assay revealed that LPA induced an increased binding of phosphorylated CREB and CREB-binding protein (CBP) to the CRE region of the endogenous c-fos promoter. Immunoblot analyses with various pharmacological inhibitors further showed that LPA induces up-regulation of c-fos mRNA level by activation of ERK, p38 MAPK, and MSK1. Taken together, our results suggest that CREB plays an important role in up-regulation of c-fos mRNA level in LPA-stimulated Rat-2 fibroblast cells.     

Inhibition of MuSK expression by CREB interacting with a CRE-like element and MyoD

The type I receptor-like protein tyrosine kinase MuSK is essential for the neuromuscular junction formation. MuSK expression is tightly regulated during development, but the underlying mechanisms were unclear. Here we identified a novel mechanism by which MuSK expression may be regulated. A cyclic AMP response element (CRE)-like element in the 5'-flanking region of the MuSK gene binds to CREB1 (CRE-binding protein 1). Mutation of this element increases the MuSK promoter activity, suggesting a role for CREB1 in attenuation of MuSK expression. Interestingly, CREB mutants unable to bind to DNA also inhibit MuSK promoter activity, suggesting a CRE-independent inhibitory mechanism. In agreement, CREB1 could inhibit a mutant MuSK transgene reporter whose CRE site was mutated. We provide evidence that CREB interacts directly with MyoD, a myogenic factor essential for MuSK expression in muscle cells. Suppression of CREB expression by small interfering RNA increases MuSK promoter activity. These results demonstrate an important role for CREB1 in the regulation of MuSK expression.