Cellular membrane alterations in neoplasia: a review and a unifying hypothesis

Targeting and cytotoxicity of SapC-DOPS nanovesicles in pancreatic cancer

Only a small number of promising drugs target pancreatic cancer, which is the fourth leading cause of cancer deaths with a 5-year survival of less than 5%. Our goal is to develop a new biotherapeutic agent in which a lysosomal protein (saposin C, SapC) and a phospholipid (dioleoylphosphatidylserine, DOPS) are assembled into nanovesicles (SapC-DOPS) for treating pancreatic cancer. A distinguishing feature of SapC-DOPS nanovesicles is their high affinity for phosphatidylserine (PS) rich microdomains, which are abnormally exposed on the membrane surface of human pancreatic tumor cells. To evaluate the role of external cell PS, in vitro assays were used to correlate PS exposure and the cytotoxic effect of SapC-DOPS in human tumor and nontumorigenic pancreatic cells. Next, pancreatic tumor xenografts (orthotopic and subcutaneous models) were used for tumor targeting and therapeutic efficacy studies with systemic SapC-DOPS treatment. We observed that the nanovesicles selectively killed human pancreatic cancer cells in vitro by inducing apoptotic death, whereas untransformed cells remained unaffected. This in vitro cytotoxic effect correlated to the surface exposure level of PS on the tumor cells. Using xenografts, animals treated with SapC-DOPS showed clear survival benefits and their tumors shrank or disappeared. Furthermore, using a double-tracking method in live mice, we showed that the nanovesicles were specifically targeted to orthotopically-implanted, bioluminescent pancreatic tumors. These data suggest that the acidic phospholipid PS is a biomarker for pancreatic cancer that can be effectively targeted for therapy utilizing cancer-selective SapC-DOPS nanovesicles. This study provides convincing evidence in support of developing a new therapeutic approach to pancreatic cancer.

Cancer-selective targeting and cytotoxicity by liposomal-coupled lysosomal saposin C protein

PURPOSE

Saposin C is a multifunctional protein known to activate lysosomal enzymes and induce membrane fusion in an acidic environment. Excessive accumulation of lipid-coupled saposin C in lysosomes is cytotoxic. Because neoplasms generate an acidic microenvironment, caused by leakage of lysosomal enzymes and hypoxia, we hypothesized that saposin C may be an effective anticancer agent. We investigated the antitumor efficacy and systemic biodistribution of nanovesicles comprised of saposin C coupled with dioleoylphosphatidylserine in preclinical cancer models.

EXPERIMENTAL DESIGN

Neuroblastoma, malignant peripheral nerve sheath tumor and, breast cancer cells were treated with saposin C-dioleoylphosphatidylserine nanovesicles and assessed for cell viability, ceramide elevation, caspase activation, and apoptosis. Fluorescently labeled saposin C-dioleoylphosphatidylserine was i.v. injected to determine in vivo tumor-targeting specificity. Antitumor activity and toxicity profile of saposin C-dioleoylphosphatidylserine were evaluated in xenograft models.

RESULTS

Saposin C-dioleoylphosphatidylserine nanovesicles, with a mean diameter of approximately 190 nm, showed specific tumor-targeting activity shown through in vivo imaging. Following i.v. administration, saposin C-dioleoylphosphatidylserine nanovesicles preferentially accumulated in tumor vessels and cells in tumor-bearing mice. Saposin C-dioleoylphosphatidylserine induced apoptosis in multiple cancer cell types while sparing normal cells and tissues. The mechanism of saposin C-dioleoylphosphatidylserine induction of apoptosis was determined to be in part through elevation of intracellular ceramides, followed by caspase activation. In in vivo models, saposin C-dioleoylphosphatidylserine nanovesicles significantly inhibited growth of preclinical xenografts of neuroblastoma and malignant peripheral nerve sheath tumor. I.v. dosing of saposin C-dioleoylphosphatidylserine showed no toxic effects in nontumor tissues.

CONCLUSIONS

Saposin C-dioleoylphosphatidylserine nanovesicles offer promise as a novel, nontoxic, cancer-targeted, antitumor agent for treating a broad range of cancers.

Abnormal matrix recognition by Morris hepatomas correlates with low glucagon binding capacity

Liver biomatrix contains a group of connective tissue components needed for attachment, survival, and maintenance of liver-specific functions of adult rat hepatocytes in culture. An acidic extract of liver biomatrix that contains a group of glycoproteins can replace intact biomatrix in promoting attachment and survival of hepatocytes. However, except for albumin synthesis, liver-specific functions have not been tested. Acidic extracts of biomatrices prepared from heart, kidney, lung, and spleen (heterologous) contain a similar group of glycoproteins, but differ with respect to liver glycoproteins in their capacity to sustain hepatocyte binding. Normal hepatocytes attach poorly to heterologous glycoprotein extracts, although regenerating and tumoral hepatocytes attach to liver glycoproteins and adhere equally well or with greater efficiency to heterologous glycoprotein extracts. The increased efficiency of hepatocytes to attach to kidney biomatrix-derived glycoproteins showed a linear correlation with the decreased glucagon binding capacity of their isolated plasma membranes. An epithelioid cell-line derived from kidney (MDCK) attached with higher efficiency to kidney than to liver glycoproteins. These results suggest that biomatrices may contain specific glycoproteins needed for attachment and survival of their epithelial cells. This specificity is lost during the proliferative state of regenerating and tumoral hepatocytes and could be important in the general mechanism of tumor dissemination and metastases.

Comparison of phospholipid profiles of primary adenocarcinoma in the lung and other organs

Phospholipid profiles, particularly molecular structure of phosphatidylcholine, of human primary lung adenocarcinoma were compared with those of the histologically same type of carcinoma from other organs in order to search for a possibility that differentiates between primary and metastatic tumors in the lung. The saturated class, mainly containing palmitic acid at both positions, accounted for 20.8% of phosphatidylcholine in lung adenocarcinoma, whereas it accounted for only 6-10.7% in the adenocarcinoma of other organs. Adenocarcinoma of organs other than the lung had specific characteristics of unsaturated molecular classes of phosphatidylcholine for each organ. Stomach tumor had high contents of dienes and tetraenes with particularly high proportion of arachidonic acid at the 2-position. Breast tumor had a high content of monoenes, containing palmitoleic acid at the 2-position. Adenocarcinomas of rectum, colon and thyroid contained more dienes compared to lung adenocarcinoma.

Membrane anomalies of neoplastic cells

Neoplastic cells exhibit numerous membrane anomalies. Those involving the plasma membrane have attracted the greatest attention, although ample evidence indicates that the membranes of mitochondria, endoplasmic reticulum and lysosomes are also profoundly implicated. The information on these topics is briefly reviewed and it is concluded that of the multiple membrane anomalies observed, those responsible for high aerobic lactate production, abnormal plasma membrane transport and release of hydrolytic enzymes may figure prominently in malignant behaviour, i.e. invasiveness and metastasis. It is proposed that the membrane polymorphism of neoplastic cells can be explained in terms of the Changeux membrane lattice hypothesis. In particular it is suggested that the concerted behaviour of tumor cell membranes might deviate from normal due to one or more of the following processes: (a) insertion of a new protein (or lipid); (b) alteration of existing proteins (or lipids); (c) change in the proportion of phospholipid; (d) change in the proportion of glycolipid; (e) change in the proportion of cholesterol; (f) change in the steady-state of membrane ligands. The validity of this proposal is evaluated in terms of recent advances in membrane molecular biology.

Biosynthesis of plasma membrane components by SV40-virus-transformed 3T3 mouse cells temperature sensitive for expression of some transformed cell properties

We have studied the plasma membranes of an SV40-transformed 3T3 cell line temperature sensitive for the transformed growth phenotype (ts H6-15 cells), and have found that they vary little as a function of temperature of cultivation. Analysis by polyacrylamide gel electrophoresis was performed on plasma membranes prepared from ts H6-15 cells cultured at the permissive (32 degrees C) and non-permissive (39 degrees C) temperatures and radioactively-labelled in several ways. No significant differences were seen when the electrophoretic patterns of polypeptides of the plasma membranes of ts H6-15 cells, grown through 3-4 generations in medium containing radioactive leucine (32 degrees C and 39 degrees C temperatures) were compared. Plasma membranes derived from cells similarly grown in medium with radioactive glucosamine indicated that extensive alterations in the intrinsic glycopeptides occurred in association with alteration in growth phenotype. A shift towards decreased synthesis of large molecular weight (congruent to 100 000-160 000) glycopeptides occurred in cells grown at the temperature of non-transformed growth (39 degrees C). A decrease in amount of a 120 000 molecular weight glycopeptide at 39 degrees C was the most prominent of these alterations. We have studied the surface exposure of polypeptides and glycopeptides of intact cells grown at 32 and 39 degrees C, using lactoperoxidase-catalyzed iodination, NaBH4 reduction of galactose oxidase-treated cells, and metabolic-labelling with glucosamine of trypsin-sensitive molecules. We found no major qualitative differences between whole cell extracts or between plasma membrane preparations of cells cultivated at the permissive and non-permissive temperatures. Of special interest was the observation that the formation and surface exposure of a trypsin-sensitive, 240 000 molecular weight polypeptide appeared not to be ts in ts H6-15 cells. The significance of these observations will be discussed.

Glycolipid glycosyl transferases of a hamster cell line in culture. I. Kinetic constants, substrate and donor nucleotide sugar specificities

The properties of enzymes catalysing the transfer of a galactose from UDP-galactose to exogenous ceramide monohexoside and ceramide di-hexoside derived from the Syrian hamster cell line NIL 2 were studied. The products of these enzymes were characterized by chemical and enzymatic methods. Kinetic analyses showed that the enzymes are susceptible to inhibition and activation by a number of substrate analogues. The kinetic and inhibition constants, glycolipid substrate specificity and nucleotide sugar donor specificity have been studied.