Molecular Players in Mural Cell Biology in Tumors

Maturation of the wall involves recruitment of mural cells, development of the surrounding matrix and elastic laminae, and organ-specific specialization of endothelial cells, mural cells, and matrix (such as interendothelial junctions, fenestrations, apical-basal polarity, surface receptors, and foot processes; Figure 3.4). Maturation of the network involves optimal patterning of the network by branching, expanding, and pruning to meet local demands (Figure 3.5). Certain antiangiogenic therapy approaches target these pericytes, and thus interrupt the survival signals and structural support provided by these cells to endothelial cells. Nascent vessels are stabilized by recruiting mural cells and by generating an extracellular matrix. At least four molecular pathways are involved in regulating this process and may constitute valid targets: PDGF-B-PDGFR-P; sphingosine-1-phosphate-1 (S1P1)-endothelial differentiation sphingolipid G-protein-coupled receptor-1 (EDG1); Ang-1-Tie-2; and the TGF-p pathway. PDGF-B is secreted by endothelial cells, presumably in response to VEGF, and facilitates recruitment of mural cells. Although PDGF-B is expressed by a number of cells, including endothelial cells and mural cells, signaling through PDGFR-p, which is expressed on mural cells, is responsible for their proliferation and migration during vascular maturation. Compelling support for this hypothesis comes from studies of Pdgfb knockout mice, which undergo embryonic lethality, lack pericytes in certain vessels, and exhibit microvascular aneurysm.

The similarity between the phenotypes of Pdgfb-Pdgfrb and Edgl knockout mice (failure of mural cells to migrate to blood vessels) indicates that signaling through the EDG1 receptor, which is expressed by mural cells, is another key pathway for mural cell recruitment [4]. EDG1 receptor signaling may occur downstream of PDGF signaling, although this hypothesis has recently been questioned. Alternatively, the lack of EDG1 may alter the endothelial cell matrix production or endothelial-mural cell interaction, and interfere with vessel maturation.

In addition, critical for vessel formation and stabilization are the Tie-2 receptor and its two ligands, Ang-1 and Ang-2. Main sources of Ang-1 and Ang-2 are the mural cells and endothelial cells, respectively. Ang-1 is known to stabilize nascent vessels and make them leak-resistant, presumably by facilitating communication between endothelial cells and mural cells. Notably, in the absence of mural cells, recombinant Ang-1 restored a hierarchical order of the larger vessels, and rescued edema and hemorrhage, in the growing retinal vasculature of mouse neonates [88]. Thus, the mechanism of vessel maturation by Ang-1 is far from clear. The role of Ang-2 appears to be contextual. In the absence of VEGF, Ang-2 acts as an antagonist of Ang-1 and destabilizes vessels, ultimately leading to vessel regression. In the presence of VEGF, Ang-2 facilitates vascular sprouting.

(c) EC tube IEL SMCs BM EEL

Capillary

(c) EC tube IEL SMCs BM EEL

Capillary

Arteriole

Venule

(d) EC tube IEL SMCs EEL FBs EM EEL

(e) Lymphatics

Arteriole

Venule

Endothelial cell (EC) Pericyte (PC) rs Smooth muscle cell (SMC) <?>, Fibroblast (FB)

Internal elastic lemina (IEL) External elastic lemina (EEL) Basement membrane (BM) Lymphatic endothelial cell Extracellular matrix (EM)

Internal elastic lemina (IEL) External elastic lemina (EEL) Basement membrane (BM) Lymphatic endothelial cell Extracellular matrix (EM)

Initial Collecting lymphatics lymphatics Extracellular matrix Anchoring filaments

FIGURE 3.4 (See color insert following page 558.) Wall composition of nascent vs. mature vessels. (a) Nascent vessels consist of a tube of ECs. These mature into the specialized structures of capillaries, arteries, and veins. (b) Capillaries, the most abundant vessels in our body, consist of ECs surrounded by basement membrane and a sparse layer of pericytes embedded within the EC basement membrane. Due to their wall structure and large surface-area-to-volume ratio, these vessels form the main site of exchange of nutrients between blood and tissue. Depending on the organ or tissue, the capillary endothelial layer is continuous (as in muscle), fenestrated (as in kidney or endocrine glands), or discontinuous (as in liver sinusoids). The endothelia of the blood-brain barrier or blood-retina barrier are further specialized to include tight junctions, and are thus impermeable to various molecules. (c) Arterioles and venules have an increased coverage of mural cells compared with capillaries. Precapillary arterioles are completely invested with vascular SMCs, which form their own basement membrane and are circumferentially arranged, closely packed, and tightly associated with the endothelium. Extravasation of macromolecules and cells from the blood stream typically occurs from postcapillary venules. (d) The walls of larger vessels consist of three specialized layers: an intima composed of endothelial cells, a media of SMCs, and an adventitia of fibroblasts, together with matrix and elastic laminae. The advential layer has its own blood supply—known as vasa vasorum—which extends in part into the media. SMCs and elastic laminae contribute to the vessel tone and mediate the control of vessel diameter and blood flow. Additional control of blood flow is provided by arteriovenous shunts, which can divert blood away from a capillary bed when necessary. (e) Lymphatic capillaries lack pericytes. Larger (collecting) lymphatic vessels are invested in a basement membrane and contain valves that permit lymph flow only in one (proximal) direction; the lymphatic capillaries (initial lymphatics) contain microvalves in their walls. The lymphatic endothelial cells are connected to the surrounding connective tissue through anchoring filaments. (Reproduced from Jain, R.K., Nat. Med., 9, 685, 2003. With permission.)

TGF-p, a multifunctional cytokine, promotes vessel maturation by stimulating extracellular matrix production and by inducing differentiation of mesenchymal cells to mural cells [89]. It is expressed in a number of cell types, including endothelial cells and mural cells and, depending on the context and concentration, is both pro- and antiangiogenic [90].

In tumors, the data on the mural cell coverage of vessels are somewhat controversial, with some studies showing a paucity of mural cells and others showing their abundance [19]. Whether this is due to the different tumor types examined or the different molecular markers (such as smooth muscle actin and desmin) used to identify these cells is unknown: also unknown is the origin of the mural cells. One possibility is that fibroblasts at the tumorhost interface are triggered by components of the tumor microenvironment (such as TGF-P) to differentiate first into myofibroblasts, and then into pericyte-like cells. Indeed, these

Normal

Normal

Cell proliferation Migration Apoptosis Specialization

Tumor

Endothelial procursor cells Host vasculature

Myofibroblast

Endothelial procursor cells Host vasculature

Pericyte-like cell

Evolving network Unstable

Abnormal structure Abnormal function inappropriate to location

Pericyte-like cell

Evolving network Unstable

Abnormal structure Abnormal function inappropriate to location

Growth/expansion/ remodeling/regression

Cell proliferation Migration Apoptosis Specialization Necrosis

FIGURE 3.5 (See color insert following page 558.) Steps in network formation and maturation during embryonic (physiological) angiogenesis (a) and tumor (pathological) angiogenesis (b). The nascent vascular network forms from an initial cell plexus by processes of vasculogenesis or angiogenesis. This is regulated by cell-cell and cell-matrix signaling molecules and mechanical forces, as its further growth and expansion of the network (by proliferating and migrating cells) alongside its normal remodeling by cell death (apoptosis). Ordered patterns of growth, organization, and specialization (including the investment of vascular channels by mural cells) produce mature networks of arteries, capillaries, and veins—networks that are structurally and functionally stable and appropriate to organ and location. (b) ECs and mural cells derived from circulating precursor cells or from the host vasculature form networks, which are structurally and functionally abnormal. Continual remodeling by inappropriate patterns of growth and regression (cell apoptosis and necrosis) contributes to the instability of these networks. (Reproduced from Jain, R.K., Nat. Med., 9, 685, 2003. With permission.)

Day 0: Abnormal

FIGURE 3.6 (See color insert following page 558.) Vessel normalization and EC-mural cell interactions in tumors growing in dorsal windows in mice. (a) Normal capillary bed (dorsal skin and striated muscle). (b) Tumor vasculature (human tumor xenograft). (c, d) Anti-VEGFR-2 therapy prunes immature vessels, leading to a progressively "normalized" vasculature by day 1 and day 2. (e) Further treatment leads to a vasculature that is inadequate to sustain tumor growth by day 5. (f) Perivascular cells expressing GFP (under the control of the VEGF promoter) envelope some vessels in the tumor interior. (g) A perivascular cell, presumably a fibroblast, leading the endothelial sprout (arrow). Scale bar (f, g) = 50 m. Images were obtained using a two-photon microscope. (Reproduced from Jain, R.K., Nat. Med., 7, 987, 2001; Jain, R.K., Nat. Med., 9, 685, 2003. With permission.)

FIGURE 3.6 (See color insert following page 558.) Vessel normalization and EC-mural cell interactions in tumors growing in dorsal windows in mice. (a) Normal capillary bed (dorsal skin and striated muscle). (b) Tumor vasculature (human tumor xenograft). (c, d) Anti-VEGFR-2 therapy prunes immature vessels, leading to a progressively "normalized" vasculature by day 1 and day 2. (e) Further treatment leads to a vasculature that is inadequate to sustain tumor growth by day 5. (f) Perivascular cells expressing GFP (under the control of the VEGF promoter) envelope some vessels in the tumor interior. (g) A perivascular cell, presumably a fibroblast, leading the endothelial sprout (arrow). Scale bar (f, g) = 50 m. Images were obtained using a two-photon microscope. (Reproduced from Jain, R.K., Nat. Med., 7, 987, 2001; Jain, R.K., Nat. Med., 9, 685, 2003. With permission.)

pericyte-like cells have been proposed to guide the endothelial sprouts in tumors [14,19]. Both intravital and immunostaining studies show that the tumor-associated pericytes have abnormal morphology and form tenuous contacts with the endothelial cells and the matrix (Figure 3.6). Intravital studies also show that these perivascular cells produce VEGF, which can serve as a survival factor for the endothelial cells and make these vessels leaky [13,14]. These contrasting roles of the tumor-associated pericyte—stabilizing vs. rendering tumor vessels leaky—are enigmatic and need further investigation.

Was this article helpful?

0 0

Post a comment