No abstract

In order for cancer cells to successfully colonize a metastatic site, they must detach from the primary tumor using extracellular matrix-degrading proteases, intravasate and survive in the circulation, evade the immune response, and extravasate the vasculature to invade the target tissue parenchyma, where metastatic foci are established. Though many of the steps of metastasis are widely studied, the precise cellular interactions and molecular alterations associated with extravasation are unknown, and further study is needed to elucidate the mechanisms inherent to this process. Studies of leukocytes localized to inflamed tissue during the immune response may be used to elucidate the process of cancer extravasation, since leukocyte diapedesis through the vasculature involves critical adhesive interactions with endothelial cells, and both leukocytes and cancer cells express similar surface receptors capable of binding endothelial adhesion molecules. Thus, leukocyte extravasation during the inflammatory response has provided a model for transendothelial migration (TEM) of cancer cells. Leukocyte extravasation is characterized by a process whereby rolling mediated by cytokine-activated endothelial selectins is followed by firmer adhesions with beta1 and beta2 integrin subunits to an activated endothelium and subsequent diapedesis, which most likely involves activation of Rho GTPases, regulators of cytoskeletal rearrangements and motility. It is controversial whether such selectin-mediated rolling is necessary for TEM of cancer cells. However, it has been established that similar stable adhesions between tumor and endothelial cells precede cancer cell transmigration through the endothelium. Additionally, there is support for the preferential attachment of tumor cells to the endothelium and, accordingly, site-specific metastasis of cancer cells. Rho GTPases are critical to TEM of cancer cells as well, and some progress has been made in understanding the specific roles of the Rho GTPase family, though much is still unknown. As the mechanisms of cancer TEM are elucidated, new approaches to study and target metastasis may be utilized and developed.

Integrin alpha(v)beta(3) is involved in varied cell biological activities, including angiogenesis, cell adhesion, and migration on several extracellular matrix components. Although alpha(v)beta(3) is not typically expressed in epithelial cells, it is expressed in macrophages, activated leukocytes, cytokine-stimulated endothelial cells, osteoclasts, and certain invasive tumors. Interestingly, the adhesion and migration of breast cancer cells on bone matrix are mediated, in part, by alpha(v)beta(3). Similar to breast cancer cells, prostate cancer cells preferentially metastasize to the bone. The biological events that mediate this metastatic pattern of prostate cancer are not well defined. This review discusses the role alpha(v)beta(3) plays in prostate cancer progression, with specific emphasis on bone metastasis and on alpha(v)beta(3) signaling in prostate cancer cells. The data suggest that alpha(v)beta(3), in part, facilitates prostate cancer metastasis to bone by mediating prostate cancer cell adhesion to and migration on osteopontin and vitronectin, which are common proteins in the bone microenvironment. These biological events require the activation of focal adhesion kinase and the subsequent activation of PI-3 kinase/Akt signaling pathway.

We directly manipulated the levels of PtdIns, PtdInsP and PtdInsP2 in digitonin-treated adrenal chromaffin cells with a bacterial phospholipase C (PLC) from Bacillus thuringiensis and by removal of ATP. The PtdIns-PLC acted intracellularly to cause a large decrease in [3H]inositol- or [32P]phosphate-labelled PtdIns, but did not directly hydrolyse PtdInsP or PtdInsP2. [3H]PtdInsP and [3H]PtdInsP2 levels declined markedly, probably because of the action of phosphatases in the absence of synthesis. Removal of ATP also caused marked decreases in [3H]PtdInsP and [3H]PtdInsP2. The decrease in polyphosphoinositide levels by PtdIns-PLC treatment or ATP removal was reflected by the inhibition of the production of inositol phosphates upon subsequent activation of the endogenous PLC by Ca2(+)-dependent catecholamine secretion from permeabilized cells was strongly inhibited by PtdIns-PLC treatment and by ATP removal. Ca2(+)-dependent secretion was similarly correlated with the sum of PtdInsP and PtdInsP2 when the level of these lipids was changed by either manipulation. PtdIns-PLC inhibited only the ATP-dependent component of secretion and did not affect ATP-dependent secretion. Both PtdIns-PLC and ATP removal inhibited the late slow phase of secretion, but had little effect on the initial rapid phase. Although we found a tight correlation between polyphosphoinositide levels and secretion, endogenous phospholipase C activity (stimulated by Ca2+, guanine nucleotides and related agents) was not correlated with secretion. Additional experiments indicated that neither the products of the PtdIns-PLC reaction (diacylglycerol and InsP1) nor the inability to generate products by subsequent activation of the endogenous PLC is likely to account for the inhibition of secretion. Incubation of permeabilized cells with neomycin in the absence of ATP maintained the level of polyphosphoinositides and more than doubled subsequent Ca2(+)-dependent secretion. The data suggest that: (1) Ca2(+)-dependent secretion has a requirement for the presence of inositol phospholipids; (2) the enhancement of secretion by ATP results in part from increased polyphosphoinositide levels; and (3) the role for inositol phospholipids in secretion revealed in these experiments is independent of their being substrates for the generation of diacylglycerol and InsP3.

The binding affinities of polyanions for bovine serum albumin in NaCl solutions from I ) 0.01-0.6 M, were evaluated on the basis of the pH at the point of incipient binding, converting each such pH c value into a critical protein charge Z c . Analogous values of critical charge for mixed micelles were obtained as the cationic surfactant mole fraction Y c . The data were well fitted as Y c or Z c ) KI a , and values of K and a were considered as a function of normalized polymer charge densities (τ), charge mobility, and chain stiffness. Binding increased with chain flexibility and charge mobility, as expected from simulations and theory. Complex effects of τ were related to intrapolyanion repulsions within micelle-bound loops (seen in the simulations) or negative protein domainpolyanion repulsions. The linearity of Z c with I at I < 0.3 M was explained by using protein electrostatic images, showing that Z c at I < 0.3 M depends on a single positive "patch"; the appearance of multiple positive domains I > 0.3 M (lower pH c ) disrupts this simple behavior.