Pier Lorenzo Puri

p300 is required for MyoD-dependent cell cycle arrest and muscle-specific gene transcription.

The nuclear phosphoprotein p300 is a new member of a family of ‘co‐activators’ (which also includes the CREB binding protein CBP), that directly modulate transcription by interacting with components of the basal transcriptional machinery. Both p300 and CBP are targeted by the adenovirus E1A protein, and binding to p300 is required for E1A to inhibit terminal differentiation in both keratinocytes and myoblasts. Here we demonstrate that, in differentiating skeletal muscle cells, p300 physically interacts with the myogenic basic helix–loop–helix (bHLH) regulatory protein MyoD at its DNA binding sites. During muscle differentiation, MyoD plays a dual role: besides activating muscle‐specific transcription, it induces permanent cell cycle arrest by up‐regulating the cyclin‐dependent kinase inhibitor p21. We show that p300 is involved in both these activities. Indeed, E1A mutants lacking the ability to bind p300 are greatly impaired in the repression of E‐box‐driven transcription, and p300 overexpression rescues the wild‐type E1A‐mediated repression. Moreover, p300 potentiates MyoD‐ and myogenin‐dependent activation of transcription from E‐box‐containing reporter genes. We also provide evidence, obtained by microinjection of anti‐p300 antibodies, that p300 is required for MyoD‐dependent cell cycle arrest in either myogenic cells induced to differentiate or in MyoD‐converted C3H10T1/2 fibroblasts, but is dispensable for maintenance of the post‐mitotic state of myotubes.

Endothelial activation by angiotensin II through NFκB and p38 pathways: Involvement of NFκB-inducible kinase (NIK), free oxygen radicals, and selective inhibition by aspirin†

Angiotensin‐II (AII), the dominant effector of the renin–angiotensin system, is involved in the pathogenesis of cardiovascular diseases, such as atherosclerosis. Upregulation of the adhesion molecules VCAM‐1, ICAM‐1, and E‐selectin in endothelial cells by inflammatory cytokines through nuclear factor kappa B (NFκB) activation is implicated in formation and progression of atherosclerotic plaque. Here we show that AII induces NFκB‐dependent transcription in primary endothelial cell lines, leading to the upregulation of ICAM‐1 and VCAM‐1 expression. NFκB activation by AII is mediated by the NFκB‐inducing kinase (NIK), a common mediator of NFκB activation by inflammatory cytokines, such as TNF‐α. However, NFκB stimulation by AII differs from that of TNF‐α since a TNF‐receptor associated factor 2 (TRAF‐2) dominant negative mutant does not prevent AII‐mediated NFκB activation. In analogy with TNF‐α‐dependent activation of NFκB, treatment with either the anti‐oxidant N‐acetyl cysteine (NAC) or the cyclooxygenase (COX) inhibitor acetyl salicylic acid (aspirin), but not indometacin, prevents the induction of NFκB‐dependent transcription by AII. Thus, production of reactive oxygen species, aspirin (asp)‐sensitive enzymes of the arachidonate metabolism, and NIK are common transducers of AII‐ and TNF‐dependent pathways to NFκB. AII also activates the inflammatory p38 kinase in endothelial cells, an effect inhibited by exposure to either NAC or asp. Pharmacological interference of the p38 pathway, with the inhibitor SB 202190, prevented AII‐mediated activation of the NFκB target V‐CAM, without affecting degradation of IκBα. These results support a pro‐inflammatory effect of the vasoactive peptide AII in endothelial cells, through at least two pathways—NFκB and p38—both of which are sensitive to asp and antioxidants.

Critical Role Played by Cyclin D3 in the MyoD-Mediated Arrest of Cell Cycle during Myoblast Differentiation

ABSTRACT During the terminal differentiation of skeletal myoblasts, the activities of myogenic factors regulate not only tissue-specific gene expressions but also the exit from the cell cycle. The induction of cell cycle inhibitors such as p21 and pRb has been shown to play a prominent role in the growth arrest of differentiating myoblasts. Here we report that, at the onset of differentiation, activation by MyoD of the Rb, p21, and cyclin D3 genes occurs in the absence of new protein synthesis and with the requirement of the p300 transcriptional coactivator. In differentiated myocytes, cyclin D3 also becomes stabilized and is found nearly totally complexed with unphosphorylated pRb. The detection of complexes containing cyclin D3, cdk4, p21, and PCNA suggests that cdk4, along with PCNA, may get sequestered into high-order structures held together by pRb and cyclin D3. Cyclin D3 up-regulation and stabilization is inhibited by adenovirus E1A, and this correlates with the ability of E1A to promote pRb phosphorylation; conversely, the overexpression of cyclin D3 in differentiated myotubes counteracts the E1A-mediated reactivation of DNA synthesis. These results indicate that cyclin D3 critically contributes to the irreversible exit of differentiating myoblasts from the cell cycle.

Reactive oxygen intermediates mediate angiotensin II-induced c-Jun.c-Fos heterodimer DNA binding activity and proliferative hypertrophic responses in myogenic cells

Angiotensin II (Ang-II) receptor engagement activates many immediate early response genes in both vascular smooth muscle cells and cardiomyocytes whether a hyperplastic or hypertrophic response is taking place. Although the signaling pathways stimulated by Ang-II in different cell lines have been widely characterized, the correlation between the generation of different second messengers and specific physiological responses remains relatively unexplored. In this study, we report how in both C2C12 quiescent myoblasts and differentiated myotubes Ang-II significantly stimulates AP1-driven transcription and c-Jun•c-Fos heterodimer DNA binding activity. Using a set of different protein kinase inhibitors, we could demonstrate that Ang-II-induced increase in AP1 binding is not mediated by the cAMP-dependent pathway and that both protein kinase C and tyrosine kinases are involved. The observation that in quiescent myoblasts Ang-II increase of AP1 binding and induction of DNA synthesis and, in differentiated myotubes, Ang-II stimulation of protein synthesis are abolished by the cysteine-derivative and glutathione precursor N-acetyl-L-cysteine strongly suggests a role for reactive oxygen intermediates in the intracellular transduction of Ang-II signals for immediate early gene induction, cell proliferation, and hypertrophic responses.

Reactive Oxygen Intermediates (ROIs) Are Involved in the Intracellular Transduction of Angiotensin II Signal in C2C12 Cells

Increasing evidence suggests that angiotensin II may act as a growth factor for several muscle cell types. Angiotensin II stimulation activates many immediate early response genes like c-Fos, c-Jun, c-Myc and Egr-1 in both vascular smooth muscle cells and cardiomyocytes, independently of whether a hyperplastic or hypertrophic response is taking place. In this study we report that angiotensin II significantly stimulates AP1-driven transcription in mouse skeletal muscle cells C2C12 stably transfected with a TRE-tk-CAT plasmid in a dose-dependent manner (peak stimulation at 10(-5) M of angiotensin II). Moreover, angiotensin II increases the binding of the AP1 complex to its DNA target in both quiescent C2C12 myoblasts and in differentiated C2C12 myotubes. Most of the TRE-bound complexes in both unstimulated and angiotensin II-treated cells consist of c-jun/c-fos heterodimers. Using a set of different protein kinase inhibitors, including HA1004, H7, tyrphostin, genistein and staurosporine, we could demonstrate that the angiotensin II-induced AP1 binding increase is not mediated by the cAMP-dependent pathway and that protein kinase C and tyrosine kinases are involved. Treatment of C2C12 cells with H2O2 induces a dose-dependent increase in c-jun/c-fos heterodimer binding, specifically reverted by the cysteine derivative and glutathione precursor N-acetyl-L-cysteine (NAC). The observation that the induction by angiotensin II of both the AP1 DNA binding activity and DNA synthesis in quiescent C2C12 myoblasts is abolished by NAC strongly suggests a role for reactive oxygen intermediates (ROIs) in the intracellular transduction of angiotensin II signals for immediate early gene induction and for cell proliferation.

The AP1 Transcription Factor as a Model to Study the Modulation of Intracellular Signalling Pathways by the Hepatitis B Virus Transactivator pX

The Hepatitis B Virus (HBV) X protein (pX) is capable of activating transcription regulated by viral and cellular promoters containing binding sites for different transcription factors, including AP-1. In this study we have analyzed the mechanisms of AP-1 induction by pX. The activation of API dependent transcription in HepG2 cells was associated with an increase in the AP-1 DNA-binding activity, that was not dependent on an increase in the overall amount of c-Fos and c-Jun proteins in the cells or on dimers formation between pX and the two proteins, thus suggesting the involvement of post-translational modifications of the transcription factor. The increased API binding observed in presence of pX was unaffected by the protein-kinase C (pKC) inhibitors Calphostin C and Sphingosine, and by the protein-kinase A (pKA) inhibitor HA1004, while it was almost completely blocked by Staurosporine, a potent and relatively non-specific protein-kinases inhibitor, suggesting that pKC- and pKA-independent phosphorylation events might play a role in the phenomenon.