Tumor cell autocrine motility factor

Biochemical mechanisms of tumor invasion and metastases

Cancer invasion and metastases is a complex multistep process. In order for a tumor cell to successfully traverse all the steps of this process and initiate a metastatic colony, it must express the right combination of gene products. Such gene products may include proteins which regulate cell interaction with the basement membrane and cell motility. Tumor cells attach to the basement membrane glycoprotein laminin via the cell surface laminin receptor. The human laminin receptor was purified and molecularly cloned. The level of laminin receptor mRNA is a variety of human carcinoma cells correlated with the number of laminin receptors on the cell surface of these cells. Following attachment to the basement membrane, the tumor cell next secretes proteases which may degrade type IV collagen. A genetic linkage between type IV collagenase secretion and metastases was studied using our new genetic system for inducing metastases employing the ras oncogene. Following attachment and local proteolysis, the third step of invasion is tumor cell motility. We have isolated a tumor cell autocrine motility factor (AMF). This factor is secreted by the tumor cells and binds to a cell surface receptor resulting in a profound (greater than 100x) stimulation of cell locomotion. AMF may play a major role in the autonomous invasive behavior of tumor cells.

Factors regulating basement membrane invasion by tumor cells

Basement membranes serve as significant barriers to the passage of tumor cells but ones which metastatic cells can pass. This involves the production of a cascade of proteases leading to the activation of a specific collagenase that degrades the unique collagen network in basement membrane. Breast cancer cells, when estrogen dependent, show a requirement for estrogen for invasive activity. However, when these cells progress to an estrogen independent state and increased malignancy, they express an invasive phenotype constitutively. Studies with various anti-estrogens suggest that these responses are mediated via the estrogen receptor. Anti-estrogens lacking agonist activity suppress invasiveness as well as growth of the breast cancer cells.

Association of SIBA treatment and a Met-depleted diet inhibits in vitro growth and in vivo metastatic spread of experimental tumor cell lines

We have used 5'-deoxy-5'-S isobutyl-thioadenosine (SIBA), an analog of S-adenosylhomocysteine, alone or in association with a methionine-depleted diet in order to obtain an antitumoral effect in two different tumor models: a transplantable rat rhabdomyosarcoma (RMS-J1) induced by i.m. injection of nickel and the well-known Lewis lung carcinoma (3LL) of C57BL/6 mice. Since SIBA has been reported to inhibit the methyl group transfer from methionine to S-adenosylhomocysteine, among other activities, its association with a reduction of methyl donors, achieved by methionine depletion of the diet (in vivo) or the culture medium (in vitro), should logically lead to an additive effect. In vitro, 3LL and RMS-J1 were sensitive to the cytotoxic effect of SIBA and were methionine-dependent for their proliferation. Fibroblast proliferation was not affected by these two treatments alone or in association. In vivo, either SIBA treatment or a low methionine diet led to a significant decrease in the metastatic character of these two tumors; however, local tumor growth was not significantly affected. The median number of 3LL metastases counted in the lungs was reduced from 100 to 18 by SIBA treatment, and to 27 by the low methionine diet. No additive effect could be detected when the treatments were given simultaneously. RMS-J1-bearing rats treated with SIBA and fed a low Met diet underwent primary tumor excision. The median numbers of lung metastatic nodules were 27, 26, 14 and 8 for the control, SIBA-treated rats, methionine-deprived rats and rats receiving the combined therapy. Expressed as percentages 20 per cent were cured, 23 per cent showed a low number of lung metastases (P less than 10), whereas all the rats in the control group developed more than 10 pulmonary nodules. No cytotoxic effect could be observed on the treated rats. The role of SIBA and methionine depletion, as agents interfering with transmethylation processes, in regard to the control of tumor development, namely metastatic invasiveness, is discussed.

Tumor promoter-induced membrane-bound protein kinase C regulates hematogenous metastasis

A strong correlation was found between the basal levels of membrane-bound protein kinase C and the ability of B16 melanoma cell sublines (F10, F1, and BL6) to metastasize to the lung after intravenous injection. By treating with tumor-promoting phorbol esters for 1 hr, the low-metastasizing F1 cells exhibited both translocation of protein kinase C from cytosol to plasma membrane and an increase in metastasis to a level comparable to the (untreated) highly metastatic subline F10. Prolonged treatment of melanoma sublines with phorbol 12-myristate 13-acetate for 24 hr resulted in both inactivation of protein kinase C activity and loss of their metastasizing capabilities. Under conditions that induced only the activation of protein kinase C but not its membrane association, no increase in metastasis occurred, suggesting that activation of protein kinase C alone is insufficient to promote metastasis and that its membrane association is also necessary. Exposure of B16 melanoma sublines to phorbol esters for 1 hr had (i) no effect on the growth and morphology of these cells in vivo and in vitro and (ii) a short-term effect (approximately equal to 5 hr) on membrane association of protein kinase C. Nonetheless, in this period, the membrane-bound protein kinase C, probably by influencing cell-surface and cell-attachment properties, increased the retention of circulating melanoma cells in the lung, which eventually led to an increased number of metastatic nodules. The membrane-bound protein kinase C activity also correlated with metastatic ability in rapidly growing cells, growth-arrested cells, and cells growing in a low-Ca2+ medium. The results strongly suggest that the membrane-bound protein kinase C influences hematogenous metastasis of tumor cells and show that tumor promoters like phorbol esters have an additional role in promoting hematogenous spread of cancer in the body.

Cytogenetic evidence of gene amplification as a mechanism for tumor cell invasion

In order to study the process by which human melanoma cells achieve invasion of basement membranes, a modification of the Membrane Invasion Culture System was developed to allow the in vitro collection of human melanoma cell populations that had invaded acellular human amniotic membranes. A significant increase in the number of double-minute chromosomes (DMs) was observed in metaphase nuclei of A375P human melanoma cells which had passed through two amniotic membranes (A375P-2) over that of control cells. Eighteen percent of the first monolayer of A375P-2 cells contained 1-89 DMs/cell, whereas 3-8.3% of the control A375P cells contained 1-10 DMs/cell. There was a rapid loss of DMs in A375P-2 cells as a function of passage number. After 25 days in tissue culture, the incidence of DMs had essentially dropped below the control range. These data indicate that an unstable gene amplification event may be part of the process by which melanoma cells execute invasion through basement membranes.

Polarized secretion of a platelet-derived growth factor-like chemotactic factor by endothelial cells in vitro

Cultured bovine aortic endothelial cells secrete a potent migration-stimulating factor for vascular smooth muscle cells (SMCs) and adventitial fibroblasts. Vascular pericytes are 20-fold less responsive, and endothelial cells themselves do not respond at all. Checkerboard analysis of SMC migration in a micro-chemotaxis chamber assay shows that the factor is chemotactic. Chemotactic activity for SMCs and adventitial fibroblasts is specifically inhibited by antibodies against platelet-derived growth factor. Endothelial cells cultured on nitrocellulose filters secrete the platelet-derived growth factor-like factor almost exclusively into the basal compartment. We suggest that this factor plays an important role in the recruitment of vascular wall cells during the morphogenesis of blood vessels and pathological conditions, such as atherosclerosis.

Cytokine-induced pseudopodial protrusion is coupled to tumour cell migration

Pseudopodia protrusion is a prominent feature of actively motile cells in vitro and invading tumour cells in vivo; however, the function and regulation of pseudopodia are poorly understood. Tumour autocrine motility factor (AMF) represents a new class of cytokines which are secreted by tumour cells and embryonic cells and induce random motility in the producer cells or in heterologous cells with appropriate receptors. Here we report that a major effect of this factor is to induce the extension of cell pseudopodia before cell translocation. Using a new method to quantify and isolate pseudopodia, we find that human breast carcinoma cell AMF (at concentrations of 1 nM or below) stimulates random pseudopodia formation in a dose-dependent and time-dependent manner. Anti-AMF antibodies inhibit pseudopodia protrusion and cell motility, showing the importance of pseudopodia formation during locomotion. AMF-stimulated motility and pseudopodia formation occur on a wide variety of adhesive substrata which suggests that certain intrinsic motility events are independent of the attachment mechanism. Induced pseudopodia show a prominent axial actin network in the electron microscope. The number of laminin receptor and fibronectin RGD recognition sites is increased by a factor of 20 in the induced pseudopodia when compared to the average distribution in unstimulated cells. Exploratory pseudopodia regulated by cell-derived motility factors contain receptors for matrix proteins and could serve as 'senseorgans' essential to the process of cell locomotion.

Endocytosis of particulate and soluble IgG immune complexes: differential effects of cytoskeletal modulating agents

We determined the ability of cytoskeletal modulating agents to affect endocytosis of particulate and soluble immune complexes. Either immunoglobulin G-opsonized 51Cr-labelled erythrocytes (E) or 125I-labelled soluble IgG anti-dinitrophenyl immune complexes (IC) were added to adherent murine peritoneal macrophages and the extent of internalization was measured. Either intracellular 51Cr radioactivity or trichloroacetic acid soluble 125I radioactivity was monitored as an indication of uptake by the cell. In addition to cytochalasin B and colchicine, the phospholipid methylation inhibitors, 3-deazaadenosine and 3-deaza(+/-)aristeromycin, were used. Whereas incubation of macrophages with colchicine alone resulted in 50% inhibition of uptake of E-IgG, there was no effect on the degradation of soluble IC. Both cytochalasin B and colchicine produced a 95% inhibition of E-IgG uptake by macrophages, but these drugs only minimally inhibited (17%) the degradation of soluble immune complexes. Both inhibitors of methylation produced a 50% decrease in phospholipid methylation in treated cells. However, only 3-deazaadenosine inhibited phagocytosis (50% of control for E-IgG and 75% of control for soluble IC). These data suggest that an intact cytoskeleton is necessary for the uptake of a particulate immune complex and much less important for the internalization and degradation of these model soluble immune complexes. In addition, inhibition of phospholipid methylation reactions alone do not impare the uptake and degradation of either a soluble or a particulate immune complex.