Mechanotransduction in Cardiac Hypertrophy

186A

AfH-APRIL 1996-VOL. 9, NO.4, PART 2

ASH XI ABSTRAcrS

Friday, May 17, Mercury Ballroom, 1:30 pm Theme I: Mechanisms of Mechanical Injury to the Vessel Wall

Friday, May 17, Mercury Ballroom, 1:30 pm Theme I: Mechanisms of Mechanical Injury to the Vessel Wall

Effects of Mechanical Forces on Gene Expression and Signal Transduction in Vascular Endothelial Cells. Shu Chien, Departments of Bioengineering and Medicine, University of California, San Diego, La Iolla, CA 92093

ENDOTHELIAL RESPONSES TO F1..0W IN VIVO AND IN VITRO

Hemodynamic forces, including shear stress and pressure, can modify the expression of several genes by endothelial cells. Cultured human umbilical vein endothelial cells increased its monocyte chemotactic protein-l (MCP-l) mRNA by 2 to 3- fold in response to a shear stress of 16 dyneslcm2. Functional analysis on the promoter region of the MCP-l gene by sequential deletion and site-specific mutagenesis showed that the shear-inducibility is dependent on the presence of a cis-element TGAcrCC, which is a TPA responsive element (TRE). Transactivation assays indicate that activating protein-I (AP-l), which is known to be the nuclear binding protein for TRE, plays an important role in the shear activation of MCP-l. Studies on molecular signaling mechanisms have shown that Ras and the downstream phosphorylation cascades involving the Nterminal Iun kinase (INK) play significant roles in the signaling. Co-transfection of dominant negative mutants or catalytically inactive mutants of Ha-Ras, mitogen-activated protein kinase kinase kinase (MEKK) and INK blocked the shear-activated INK, c-jun, and TRE responses. In contrast, dominant negative mutants of Raf-l, ERK-l, and ERK-2 had little effects. Thus, fluid shear stress activates primarily the Ras-MEKK-INK pathway in inducing endothelial MCPI gene expression. Studies on other genes in our laboratories and by others indicate that interplays among different cis-elements and various signaling pathways may orchestrate gene expression in response to mechanical, and chemical stimuli in health and disease. Quantitative analyses of the kinetics and temporal relations among the various signaling events in response to hemody~am~c forces, including pressure, are needed for the elucldauon of the molecular basis of gene regulation in endothelial cells. Key Words:

endothelial cells, gene expression, monocyte chemotactic protein-I, shear stress, signal transduction

A MECHANISM FOR HETEROGENEOUS

Peter F. Davies and Kenneth A. Barbee Department of Pathology. The University of Chicago, USA Exposure of endothelium to a nominally uniform flow field in vivo and in vitro frequently results in a heterogeneous distribution of individual cell responses. Extremes in response levels are often noted in neighboring cells. Such variations are important for the spatial interpretation of vascular responses to flow and for an understanding of mechanotransduclJon mechanisms at the level of Single cells. We propose that variations of local forces defined by the cell surface geometry contribute to these differences. Atomic force microscopy measurements of cell surface topography in living endothelium both in vitro and in situ combined wilh computational fluid dynamics demonstrated large cell-to-cell variations in the distribution of forces throughout the surface of individual cells of lhe monolayer was also found to vary considerably and to be defined by the surface geometry. We conclude that the endothelial 3-dimenslonal surface geometry defines the detailed distribution of shear stresses and gradients at the single cell level. and that there are large variations 10 force magnitude and distnbution between neighboring cells. The measurements suppon a topographic basis for differential endothelial responses to flow observed in vivo and in vitro. Included 10 these studies are the first preliminary measurements of the livlOg endothelial cell surface in an intact artery, an approach that should allow detailed analysis of endolhelial topographies 10 different regional flow fields in vivo. Supported by NHLBI.

Key Words:

Endolhelium; hemodynamics; shear stress; mechanotransduclJon; atomic force microscopy

Friday, May 17, Mercury Ballroom, 1:30 pm Theme I: Mechanisms of Mechanical Injury to the Vessel Wall MECHANOTRANSDUCTION IN CARDIAC HYPERTROPHY Seigo Izumo and Jun-ichi Sadoshima, Cardiovascular Research Center. University of Michigan Medical Center, Ann Arbor, MI48109. USA Hypertrophy is a fundamental adaptive process employed by post-mitotic cardiac muscle in response to mechanical load. Usmg an in vitro model of stretch-induced cardiac hypertrophy, we examined signal transduction pathway of load-induced hypertrophy of cultured neonatal cardiac myocytes. We found that cell stretch rapIdly activates a plethora of second messengers, including tyrosine kmases. p21 "', mitogen-activated protem kinase. pp90"". 70kDa ribosomal S6 kinase, as well a.~ phospholipa.'lCs C, 0 and AI' In contrast, cAMP pathway was not significantly activated by stretch. The signals generated by these second messengers appear to converge mto activation of nuclear transcription factors and mcrea.<>ed prodUClJon of multiple growth factors. Among them. autocnne secrelJon of angiotensin II occurs rapidly and play a dorrnnant role in mediating load-induced cardiac hypertrophy in ,·itro. Treatment of cardiac myocytes with Angiotensm II causes the activation of a similar set of second messengers includmg tyrosine kinases and p21". Angiotensm II treatment induces a number of growth factors Including endothelin-l and TGFj3 and causes hypertrophy of cardiac myocytes. These results suggest that mechanotransductlon in cardiac myocytes involves autocnne growth response.

KeyWords:

Hypertrophy. Stretch, Angiotensin