Role of Janus Kinase/Signal Transducer and Activator of Transcription and Mitogen-activated Protein Kinase Cascades in Angiotensin II- and Platelet-derived Growth Factor-induced Vascular Smooth Muscle Cell Proliferation

Abstract

In vascular smooth muscle cells, the induction of early growth response genes involves the Janus kinase (JAK)/ signal transducer and activators of transcription (STAT) and the Ras/Raf-1/mitogen-activated protein kinase cascades. In the present study, we found that electroporation of antibodies against MEK1 or ERK1 abolished vascular smooth muscle cell proliferation in response to either platelet-derived growth factor or angiotensin II. However, anti-STAT1 or -STAT3 antibody electroporation abolished proliferative responses only to angiotensin II and not to platelet-derived growth factor. AG-490, a specific inhibitor of the JAK2 tyrosine kinase, prevented proliferation of vascular smooth muscle cells, complex formation between JAK2 and Raf-1, the tyrosine phosphorylation of Raf-1, and the activation of ERK1 in response to either angiotensin II or platelet-derived growth factor. However, AG-490 had no effect on angiotensin II-or platelet-derived growth factor-induced Ras/Raf-1 complex formation. Our results indicate that: 1) STAT proteins play an essential role in angiotensin II-induced vascular smooth muscle cell proliferation, 2) JAK2 plays an essential role in the tyrosine phosphorylation of Raf-1, and 3) convergent mitogenic signaling cascades involving the cytosolic kinases JAK2, MEK1, and ERK1 mediate vascular smooth muscle cell proliferation in response to both growth factor and G protein-coupled receptors.Previous work by our laboratory (1-6) on cultured rat aortic vascular smooth muscle cells (VSMC) 1 and phenotypically similar glomerular mesangial cells has shown that protein tyrosine phosphorylation plays a critical role in angiotensin II (Ang II)-mediated intracellular signaling cascades. This is true despite the fact that G protein-coupled receptors in general and the Ang II AT 1 receptor in particular possess no intrinsic tyrosine kinase activity. It is also now recognized that Ang II can act not only as a vasoactive peptide but also as a growth factor. In particular, Ang II has been shown to stimulate proliferative and hypertrophic growth in VSMC, glomerular mesangial cells, cardiac fibroblasts, and myocytes via AT 1 receptor binding (4, 7-9). Like classic growth factors (e.g. platelet-derived growth factor (PDGF) and epidermal growth factor) and some cytokines (e.g. interferons and interleukins) (4, 8 -10), Ang II is also capable of stimulating a rapid increase in the mRNA levels of c-fos, an early growth response gene implicated in VSMC proliferation (4,7,8). However, the Ang II-stimulated intracellular signaling cascades responsible for c-fos induction and therefore proliferation in VSMC have not been well defined.One candidate mitogenic signaling cascade involves the activation of the small GTP-binding protein, Ras, which is traditionally mediated via classic growth factor receptors (4). Ras activation promotes the formation of a membrane-bound complex with Raf-1 (a serine/threonine protein kinase). Subsequent tyrosine phosphorylation of Raf-1 leads to its activation and the sequential stimulati...

Regulation of cell proliferation by G proteins

“…G proteins have been also implicated in the tyrosinephosphorylation and subsequent activation of the STAT-family of transcription factors (Bhat et al, 1994;Marrero et al, 1997). It has been observed that the stimulation of the AT 1 -Angiotensin II receptor that primarily couples through G q results in the phosphorylation of STAT-1 and STAT-3 through a JAK2-mediated mechanism.…”

Blocking the tissue renin-angiotensin system: the future cornerstone of therapy

“…49 Significantly, recent research indicates that angiotensin II also mediates proliferative effects through the Jak/STAT signalling pathway, which leads to the transcription activation of early growthresponse genes. 50,51 While much less is known about the functional properties of the AT 2 receptor, it would appear that it mediates the effects of angiotensin II on growth control and cell differentiation, as well as neuronal differentiation and apoptosis [52][53][54][55][56] (Table 2). The AT 2 -receptor-mediated effects down-modulate the actions by the AT 1 receptor, resulting in decreased cellular proliferation, modulation of AT 1 -mediated vasopressin release in certain experimental conditions and a decreased vasoconstrictor response.…”

Receptors and their classification: focus on angiotensin II and the AT2 receptor

“…64 Also, antibodies against STAT proteins in vascular smooth muscle cells inhibit the Ang II mediated proliferative response. 65 A physical association between Jak2 kinase and a part of the intracellular carboxy tail of the AT 1 receptor has been shown as well. 66 Very recently it has been shown that Ang II mediates the Jak/STAT pathway under pathological conditions as well.…”