The statistical mechanics of complex signaling networks: nerve growth factor signaling.

The inherent complexity of cellular signaling networks and their importance to a wide range of cellular functions necessitates the development of modeling methods that can be applied toward making predictions and highlighting the appropriate experiments to test our understanding of how these systems are designed and function. We use methods of statistical mechanics to extract useful predictions for complex cellular signaling networks. A key difficulty with signaling models is that, while significant effort is being made to experimentally measure the rate constants for individual steps in these networks, many of the parameters required to describe their behavior remain unknown or at best represent estimates. To establish the usefulness of our approach, we have applied our methods toward modeling the nerve growth factor (NGF)-induced differentiation of neuronal cells. In particular, we study the actions of NGF and mitogenic epidermal growth factor (EGF) in rat pheochromocytoma (PC12) cells. Through a network of intermediate signaling proteins, each of these growth factors stimulates extracellular regulated kinase (Erk) phosphorylation with distinct dynamical profiles. Using our modeling approach, we are able to predict the influence of specific signaling modules in determining the integrated cellular response to the two growth factors. Our methods also raise some interesting insights into the design and possible evolution of cellular systems, highlighting an inherent property of these systems that we call 'sloppiness.'

Biomechanical strain induces class a scavenger receptor expression in human monocyte/macrophages and THP-1 cells: a potential mechanism of increased atherosclerosis in hypertension.

BACKGROUND: Although hypertension is an important risk factor for the development of atherosclerosis, the mechanisms for this interaction are incompletely described. Previous studies have suggested that biomechanical strain regulates macrophage phenotype. We tested the hypothesis that biomechanical strain can induce expression of the class A scavenger receptor (SRA), an important lipoprotein receptor in atherogenesis. METHODS AND RESULTS: Human monocyte/macrophages or THP-1 cells were cultured in a device that imposes uniform biaxial cyclic 1-Hz strains of 0%, 1%, 2%, or 3%, and SRA expression was analyzed. Mechanical strains induced SRA mRNA (3.5+/-0.6-fold at 3% strain for 48 hours, P<0.01) and SRA protein in THP-1 cells in an amplitude-dependent manner. This induction was accompanied by augmented expression of the class B scavenger receptor CD36 (2.8+/-0.3-fold, P<0.001) but not by increased peroxisome proliferator-activated receptor-gamma expression. To evaluate this effect in vivo, apolipoprotein E(-/-) mice were randomly assigned to receive standard chow, a high-cholesterol diet, or a high-cholesterol diet with hypertension induced by angiotensin II infusion for 8 weeks. Immunohistochemistry revealed that among macrophages in atherosclerotic lesions of the aorta, the proportion of macrophages with SRA expression was highest in hypertensive animals on a high-cholesterol diet (43.9+/-0.7%, versus 12.0+/-2.0% for normotensive animals on a high-cholesterol diet and 4.7+/-4.7% for animals on standard chow; P<0.001). CONCLUSIONS: Biomechanical strain induces SRA expression by monocyte/macrophages, suggesting a novel mechanism for promotion of atherosclerosis in hypertensive patients.

Statins alter smooth muscle cell accumulation and collagen content in established atheroma of watanabe heritable hyperlipidemic rabbits.

BACKGROUND: Acute coronary syndromes often result from rupture of vulnerable plaques. The collagen content of plaques probably regulates their stability. This study tested whether HMG-CoA reductase inhibitors (statins) alter interstitial collagen gene expression or matrix metalloproteinase (MMP) levels in rabbit atheroma. METHODS AND RESULTS: We administered equihypocholesterolemic doses of pravastatin (a hydrophilic statin, 50 mg. kg(-1). d(-1), n=9), fluvastatin (a cell-permeant lipophilic statin, 20 mg. kg(-1). d(-1), n=10), or placebo (n=10) to mature Watanabe heritable hyperlipidemic rabbits for 52 weeks. The fluvastatin group achieved a much higher peak plasma concentration (23.7 micromol/L) than did the pravastatin group (1.3 micromol/L) under these conditions. Immunohistochemistry revealed that MMP-1, MMP-3, and MMP-9 expression by macrophages in the intima was lower in both the pravastatin and fluvastatin groups than in the placebo group, whereas there was no difference in macrophage numbers. Numbers of intimal smooth muscle cells (SMCs) (identified by immunohistochemistry) and expression of type I procollagen mRNA (detected by in situ hybridization), however, were significantly higher in the pravastatin group than in the fluvastatin group. Treatment with pravastatin, but not fluvastatin, preserved interstitial collagen content in vivo (detected by picrosirius red polarization). In vitro, fluvastatin, but not pravastatin, decreased numbers of rabbit and human aortic SMCs without altering procollagen I mRNA expression. CONCLUSIONS: This study showed that statins can reduce MMP expression in atheroma and that cell-permeant statins can decrease SMC number and collagen gene expression in vivo.

1,25-Dihydroxy-16-ene-23-yne-vitamin D3 and prostate cancer cell proliferation in vivo.

OBJECTIVES: 1,25-Dihydroxyvitamin D can inhibit the proliferation of prostate cancer cells, but its clinical use is limited by hypercalcemia. We examined the effects of a "noncalcemic" vitamin D analogue, 1,25-Dihydroxy-16-ene-23-yne-cholecalciferol (16-23-D3), on the proliferation of human prostate cancer cells in a mouse model. METHODS: Twenty-four athymic nude mice were inoculated with human prostate carcinoma cells from the PC-3 cell line. Twelve mice (experimental group) received injections of 1.6 micrograms of 16-23-D3 on alternate days over a 22-day period. Twelve mice (control group) received sham injections. Tumor volumes, pathologic findings, and terminal serum calcium levels were compared between groups. RESULTS: The relative increase in tumor volume was significantly lower in the experimental than in the control group in the first interval following treatment (P < 0.01). Mean tumor volumes in the experimental group were approximately 15% smaller than in the control group. Serum calcium levels did not differ between groups. CONCLUSIONS: 16-23-D3 showed modest antiproliferative effects on prostate cancer cells in this model without evidence of drug-induced hypercalcemia. These findings support the concept that vitamin D analogues can inhibit the proliferation of human prostate cancer cells in vivo.