Caveolin-1 Facilitates Mechanosensitive Protein Kinase B (Akt) Signaling In Vitro and In Vivo

Abstract

Abstract-Mechanotransduction represents an integral part of vascular homeostasis and contributes to vascular lesion formation. Previously, we demonstrated a mechanosensitive activation of phosphoinositide 3-kinase (PI3-K)/protein kinase B (Akt) resulting in p27 Kip1 transcriptional downregulation and cell cycle entry of vascular smooth muscle cells (VSMC). In this study, we further elucidated the signaling from outside-in toward PI3-K/Akt in vitro and in an in vivo model of elevated tensile force. When VSMC were subjected to cyclic stretch (0.5 Hz at 125% resting length), PI3-K, Akt, and Src kinases were found activated. Disrupting caveolar structures with ␤-cyclodextrin or transfection of VSMC with caveolin-1 antisense oligonucleotides (ODN) prevented PI3-K and Akt activation and cell cycle entry. Furthermore, PI3-K and Akt were resistant to activation when Src kinases were inhibited pharmacologically or by overexpression of a kinase-dead c-Src mutant. ␣ V ␤ 3 integrins were identified to colocalize with PI3-K/caveolin-1 complexes, and blockade of ␣ V ␤ 3 integrins prevented Akt activation. The central role of caveolin-1 in mechanotransduction was further examined in an in vivo model of elevated tensile force. Interposition of wild-type (WT) jugular veins into WT carotid arteries resulted in a rapid Akt activation within the veins that was almost abolished when veins of caveolin-1 knockout (KO) mice were used. Furthermore, late neointima formation within the KO veins was significantly reduced. Our study provides evidence that PI3-K/Akt is critically involved in mechanotransduction of VSMC in vitro and within the vasculature in vivo. Furthermore, caveolin-1 is essential for the integrin-mediated activation of PI3-K/Akt. (Circ Res. 2005;96:635-642.) Key Words: remodeling Ⅲ muscle, smooth Ⅲ signal transduction Ⅲ stress Ⅲ vasculature I t is currently recognized that the machinery governing the cell cycle regulates multiple cellular functions in the cardiovascular system, thereby maintaining the homeostasis of the vasculature and allowing its adaptation to acute and chronic changes. Besides organizing cellular proliferation, the cell cycle is involved in migration, apoptosis, and hypertrophy. 1 One of the major constituents of the blood vessel wall responsible for the maintenance of vessel structures and functions are vascular smooth muscle cells (VSMCs). In the vasculature, VSMCs are constantly exposed to alternating mechanical forces. Under normal tensile stress, VSMCs are relatively insensitive to mitogens. During altered mechanical stress (eg, high blood pressure), however, VSMCs upregulate protein synthesis in response to growth factors, dedifferentiate, and increase their proliferative rate, resulting in medial hypertrophy and intimal hyperplasia. 2 Whereas the commonly accepted "response to injury" hypothesis suggests that growth factors are locally released, thereby initiating cell cycle entry and progression of vascular cells, the signaling pathways arising solely from mechanical force have just partially c...

Differential dependence of stretch and shear stress signaling on caveolin-1 in the vascular wall

“…In jugular veins exposed to arterial pressure and flow in vivo for 15 min, ERK1/2 and Akt phosphorylation were dramatically increased in WT mice but not KO mice (23). In this model, the venous endothelium will be affected by a greatly increased shear stress, and the vessel wall is extensively stretched by the elevated pressure, triggering a repair response, as shown by the subsequent neointima formation in the transplanted vein (23).…”

“…In this model, the venous endothelium will be affected by a greatly increased shear stress, and the vessel wall is extensively stretched by the elevated pressure, triggering a repair response, as shown by the subsequent neointima formation in the transplanted vein (23). Neointima formation can be reduced by external protection of the transplanted vessel, which suggests that excessive mechanical stretch is an important stimulus for neointimal growth (14).…”

“…Both caveolae and caveolin have been implicated in flowinduced endothelial NO production via the regulation of endothelial NOS activity by PI3K-Akt signaling (21,23,27). In addition, studies have shown that NO promotes Akt phosphorylation via cGMP production in vascular smooth muscle cells (26), which could provide a link between shear stress on the endothelium and Akt phosphorylation in smooth muscle.…”

“…The mechanisms that regulate mechanically induced signaling events in the vasculature are, however, not yet fully understood (15). Recent studies have implicated lipid-rich membrane caveolae in mechanosensing (1,8,23,27). Caveolae are 50-to 100-nm flaskshaped invaginations of the cell membrane that are found in numerous cell types including endothelial cells, adipocytes, and striated and smooth muscle cells.…”

Combined expression of caveolin-1 and an activated AKT/mTOR pathway predicts reduced disease-free survival in clinically confined renal cell carcinoma

“…Although the relationship on disease progression was not evaluated, the authors suggested that the presence of caveolin-1 positively affects AKT activation. Support for the clinical relevance of caveolin-1/AKT interaction and its impact on the downstream targets such as mTOR comes from a series of in vitro experiments that directly demonstrate that caveolin-1 has the capacity to facilitate AKT signalling in different cell types (Li et al, 2003;Sedding et al, 2005;Zhang et al, 2007). From the perspective of human cancer progression, Li et al (2003) showed that caveolin-1 overexpression was able to maintain activated AKT levels in human prostate cancer cells via inhibition of the tumour suppressor enzymes PP1 and PP2A, which dephosphorylate AKT at both Ser473 and The308.…”