Interactions between the monocyte/macrophage and the vascular smooth muscle cell. Stimulation of mitogenesis by a soluble factor and of prostanoid synthesis by cell-cell contact

Smooth Muscle Cell—Macrophage Interactions Leading to Foam Cell Formation in Atherosclerosis: Location, Location, Location

Cholesterol-overloaded cells or “foam cells” in the artery wall are the biochemical hallmark of atherosclerosis, and are responsible for much of the growth, inflammation and susceptibility to rupture of atherosclerotic lesions. While it has previously been thought that macrophages are the main contributor to the foam cell population, recent evidence indicates arterial smooth muscle cells (SMCs) are the source of the majority of foam cells in both human and murine atherosclerosis. This review outlines the timeline, site of appearance and proximity of SMCs and macrophages with lipids in human and mouse atherosclerosis, and likely interactions between SMCs and macrophages that promote foam cell formation and removal by both cell types. An understanding of these SMC-macrophage interactions in foam cell formation and regression is expected to provide new therapeutic targets to reduce the burden of atherosclerosis for the prevention of coronary heart disease, stroke and peripheral vascular disease.

Role of interleukin-1 receptor 1/MyD88 signalling in the development and progression of pulmonary hypertension

Pulmonary artery smooth muscle cell (PA-SMC) proliferation and inflammation are key components of pulmonary arterial hypertension (PAH). Interleukin (IL)-1β binds to IL-1 receptor (R)1, thereby recruiting the molecular adaptor myeloid differentiation primary response protein 88 (MyD88) (involved in IL-1R1 and Toll-like receptor signal transduction) and inducing IL-1, IL-6 and tumour necrosis factor-α synthesis through nuclear factor-κB activation.

We investigated the IL-1R1/MyD88 pathway in the pathogenesis of pulmonary hypertension.

Marked IL-1R1 and MyD88 expression with predominant PA-SMC immunostaining was found in lungs from patients with idiopathic PAH, mice with hypoxia-induced pulmonary hypertension and SM22-5-HTT+mice. Elevations in lung IL-1β, IL-1R1, MyD88 and IL-6 preceded pulmonary hypertension in hypoxic mice. IL-1R1−/−, MyD88−/−and control mice given the IL-1R1 antagonist anakinra were protected similarly against hypoxic pulmonary hypertension and perivascular macrophage recruitment. Anakinra reversed pulmonary hypertension partially in SM22-5-HTT+mice and markedly in monocrotaline-treated rats. IL-1β-mediated stimulation of mouse PA-SMC growth was abolished by anakinra and absent in IL-1R1−/−and MyD88−/−mice. Gene deletion confined to the myeloid lineage (M.lys-Cre MyD88fl/flmice) decreased pulmonary hypertension severityversuscontrols, suggesting IL-1β-mediated effects on PA-SMCs and macrophages. The growth-promoting effect of media conditioned by M1 or M2 macrophages from M.lys-Cre MyD88fl/flmice was attenuated.

Pulmonary vessel remodelling and inflammation during pulmonary hypertension require IL-1R1/MyD88 signalling. Targeting the IL-1β/IL-1R1 pathway may hold promise for treating human PAH.

Modulation of atherogenesis by dietary gamma-linolenic acid

Data from our in vitro studies indicate that macrophages isolated from mice fed GLA-enriched diets inhibit vascular SMC proliferation via a PGE1-cAMP dependent mechanism. Since SMC proliferation is one of the main events implicated in the pathogenesis of atherosclerosis (Ross, 1993), this anti-proliferative effect observed by dietary GLA is noteworthy. In vivo studies have established that dietary GLA is capable of retarding the atherosclerotic lesion formation in ApoE knock out mice, an animal model that develops atherosclerosis similar to humans (Reddick, 1994). We propose that dietary GLA has the potential to inhibit SMC proliferation leading to retardation of atherosclerotic lesion formation, and therefore favorable modulation of the atherogenic process.

Macrophages influence gap junctional intercellular communication between smooth muscle cells in a co-culture model

The present study demonstrates for the first time that cells cultured on pore membrane inserts (macrophages) modulate gap junctional intercellular communication (GJIC) between a second cell type (smooth muscle cells (SMC)) co-cultured in Transwell-COL cell culture chambers. Unstimulated J774A.1 murine macrophages reduced GJIC between human SMC. Stimulation of J774A.1 cells by lipopolysaccharide (LPS) or interferon-γ abrogated this modulation of GJIC. Unstimulated human monocyte-macrophages did not affect GJIC between human SMC. Upon stimulation of these monocyte-macrophages with LPS, a substantial increase in GJIC between co-cultured SMC was observed. Thus, activation of macrophages alters their interaction with co-cultured SMC. Since these results were obtained in an indirect co-culture system in which direct cell-cell contact is prevented, it is hypothesized that soluble factors released by macrophages may be involved in this modulation of GJIC between SMC. The possible nature of the responsible soluble factors is discussed in the context of atherosclerosis.

Macrophage products inhibit human aortic smooth muscle cell proliferation and alter 1 alpha (I) procollagen expression

While the role of the foam cell in early atherogenesis has been well characterized, much less is known about the interaction between infiltrating macrophages and medial smooth muscle cells (SMC) in chronic atherosclerosis. Our purpose was to determine the effects of soluble macrophage mediators on normal human aortic SMC proliferation and matrix expression. Human aortic SMC in subconfluent culture were exposed to supernatants from activated lipopolysaccharide (LPS) and nonactivated macrophages. SMC proliferation and type-I procollagen expression were determined. Both activated and nonactivated macrophage supernatants exhibited a potent growth inhibitory effect which became apparent at 48 hours. While nonactivated macrophage supernatant had no effect on procollagen expression, activated supernatant inhibited its expression. (33%; p < 0.05) These findings are consistent with the loss of medial SMC and matrix proteins associated with chronic atherosclerosis.

Retinal pigment epithelial cell proliferation: potentiation by monocytes and serum

BACKGROUND

The development of proliferative vitreoretinopathy (PVR) results often from a breakdown of the blood-retina barrier and the intraocular accumulation of serum proteins and leukocytes, particularly monocytes, that then come into contact with retinal pigment epithelial (RPE) cells. To examine the effect of these two factors on RPE proliferation, which is characteristic of PVR, we used a coculture system of blood monocytes and human RPE cells.

METHODS

RPE cells were incubated with a variable number of monocytes at different serum concentrations and assayed for proliferation by [3H]-thymidine incorporation and cell counting. To assess cell-cell communication. RPE cells were labeled with 2', 7' -bis(carboxyethyl)-5(and 6) carboxyfluorescein acetoxy-methyl ester, and the dye transfer to monocytes was analyzed using an UV microscope.

RESULTS

Monocytes (P < 0.0004) and serum (P < 0.0001), each on its own, significantly stimulated RPE cell growth, and these two variables were interrelated (P < 0.0001), showing a potentiating synergism. In serum-free medium, monocytes increased proliferation to just above control levels, whereas the same number of monocytes in 5% serum increased the [3H]-thymidine incorporation 3.8 times. This effect was greatly reduced by prevention of direct cell contact by means of placement of a well insert, which also lessened the monocyte-induced proliferation in both serum-free and serum-containing medium. Furthermore, the transfer of the intracellular dye from RPE cells to cocultured monocytes indicates that RPE cells transferred parts of their cytoplasm to monocytes.

CONCLUSION

These observations underline the importance of protein leakage through a damaged blood-ocular barrier and of direct contact of monocytes/macrophages with RPE cells, as well as their reciprocal potentiating effect on RPE cell proliferation. Thus, early stabilization of the blood-ocular barrier, which would preclude or reduce protein leakage and invasion of inflammatory cells into the eye, could be a target for pharmacologic prevention of PVR.

Correlation between the presence of an immature smooth muscle cell population in tunica media and the development of atherosclerotic lesion. A study on different-sized rabbit arteries from cholesterol-fed and Watanabe heritable hyperlipemic rabbits

Mapping the distribution of an immature smooth muscle cell (SMC) subpopulation in large- and small-sized arterial vessels was carried out in normocholesterolemic rabbits and compared with the mapping atherosclerotic lesions in endogenously (Watanabe heritable hyperlipemic, WHHL) and exogenously derived (cholesterol-fed, CT) hypercholesterolemic rabbits. This cell subset is identified by a specific myosin isoform content and displays an intermediate degree of differentiation between fetal- and adult-type SMC. Monoclonal anti-myosin antibodies, immunofluorescence procedures, and different arterial segments of a rabbit vessel tree, i.e. from aorta to dental pulp (common carotid, external carotid, lingual, facial, maxillary, inferior alveolar arteries, and dental branches of alveolar arteries) were studied. WHHL of different ages (3 to 12 months), and two different concentrations of CT (2% and 0.2%) in the diet for 3 and 12 months, respectively, were used. The results of the present study indicate that: (1) using a diet with a higher percentage of CT (rabbits fed 2% CT-diet for 3 months) there is maximum expansion of atherosclerotic lesions from the aorta up to the maxillary artery; (2) localization of atherosclerotic lesions with a lower CT content in the diet is dependent on the duration of feeding and may involve the aorta up to the external carotid artery; (3) the development of the atherosclerotic lesion in hypercholesterolemic rabbit is strictly related to the appearance of an intermediate SMC subtype; (4) atherosclerotic lesions occur only in those arterial sites which, in corresponding normocholesterolemic rabbit, contain intermediate-type SMC; and (5) no differences can be found in the distribution of SMC subpopulations present in the lesions from WHHL, CT-fed animals, or at various arterial levels, whereas some discrepancies can be shown in aortic atherogenesis.

On radiation damage to normal tissues and its treatment. II. Anti-inflammatory drugs

In addition to transiently inhibiting cell cycle progression and sterilizing those cells capable of proliferation, irradiation disturbs the homeostasis effected by endogenous mediators of intercellular communication (humoral component of tissue response to radiation). Changes in the mediator levels may modulate radiation effects either by assisting a return to normality (e.g., through a rise in H-type cell lineage-specific growth factors) or by aggravating the damage. The latter mode is illustrated with reports on changes in eicosanoid levels after irradiation and on results of empirical treatment of radiation injuries with anti-inflammatory drugs. Prodromal, acute and chronic effects of radiation are accompanied by excessive production of eicosanoids (prostaglandins, prostacyclin, thromboxanes and leukotrienes). These endogenous mediators of inflammatory reactions may be responsible for the vasodilatation, vasoconstriction, increased microvascular permeability, thrombosis and chemotaxis observed after radiation exposure. Glucocorticoids inhibit eicosanoid synthesis primarily by interfering with phospholipase A2 whilst non-steroidal anti-inflammatory drugs prevent prostaglandin/thromboxane synthesis by inhibiting cyclooxygenase. When administered after irradiation on empirical grounds, drugs belonging to both groups tend to attenuate a range of prodromal, acute and chronic effects of radiation in man and animals. Taken together, these two sets of observations are highly suggestive of a contribution of humoral factors to the adverse responses of normal tissues and organs to radiation. A full account of radiation damage should therefore consist of complementary descriptions of cellular and humoral events. Further studies on anti-inflammatory drug treatment of radiation damage to normal organs are justified and desirable.