Spontaneous transient reversion of tumor cells: A source of dormant metastasis?

Background - It is known that tumor cell heterogeneity is responsible for tumor progression, including metastatic progression, but tumor cells can also sometimes revert in vitro to a non tumor phenotype and later convert to a transformed phenotype again. Probably, many tumors, including secondary lesions, can contain quiescent reverted cells. The question is whether these reverted cells can be a source of dormant metastasis as described in experimental and clinical settings. Methods - A high cloning efficiency method allowed us to obtain a revertant clone collection, which was examined using the following methods: soft agar assay, density-dependent inhibition method, tumorigenicity in vivo, Northern blot, Western immunoprecipitation blot, etc. Results - We have shown that many murine tumors, including several spontaneous sarcomas, the methylcholantrene-induced sarcoma B6-4, and the JB6 and PDV skin carcinomas, can revert to a non tumor phenotype. There are two types of non tumor revertant clones: one which cannot grow in vivo because of a change in its immunological properties and the other which cannot grow because of a change in its growth characteristics. The latter type also could not grow in soft agar, acquired a dependence on peptide growth factors, and exhibited a reduced expression of c-jun. It is very important to know whether such revertants can revert back to a transformed phenotype at different times. When inoculated in syngeneic animals, these transformed cells showed recurrence in 2-5 months, similar to that dormant metastasis. Conclusions - Spontaneous reversions of tumor cells may play an important role in the dormant metastatic process. The cause of this frequent spontaneous transient reversions appears to be of an epigenic nature and remains to be clarified

Spontaneous reversion of tumor cells as a source of dormant metastases

In the present paper we have shown that JB6 and PDV murine skin carcinoma cells, as well as previously described sarcoma B6-4 cells, can revert to a nontumor phenotype. Revertant carcinoma clones could not grow in soft agar conditions and like sarcoma revertants acquired dependence on peptide growth factors, and exhibited a reduced expression of c-jun. Spontaneous revertants were shown to be instable. They could revert back to a transformed phenotype in 1-5 months of in vitro passaging. Being inoculated in syngeneic animals, these transformed cells show a recurrence in 2-5 months, similar to that of a dormant metastasis. Thus, dormant revertant cells are believed to be included in many tumors of different origin. So, spontaneous reversions of tumor cells may play an important role in the dormant metastatic process. The cause of these frequent spontaneous transient reversions and revertant instability appears to be of epigenetic nature. Causes and mechanisms of cell transformations and reversions remain to be clarified

Effect of the lipophilic spin-labeled inhibitor of cytochrome P-450 on the activity of the microsomal system

It was shown that the lipophilic nitroxyl radical--2-hexyl-2,3,5,5-tetramethyl-4-(3-iodo-2-oxopropyliden)-im idazolidine- 1-oxyl, an affinity modified of rat liver microsomal cytochrome P-450, interacts with various forms of cytochrome P-450 as substrate type I, and it inhibits the oxidation of substrates specific for these forms. During its intravenous injection with egg phosphatidylcholine liposomes the radical is partly bound to liver microsomes, which is accompanied by a decrease of the oxygenase activity of microsomal preparations (by 30-50%) as well as by prolongation of the soporific effect of hexabarbital (2-3-fold)

Internalization of growth factor-receptor complexes under the influence of antibodies initiates cell apoptosis in vitro

Antibodies against growth factors like IGF1, IGF2, aFGF bFGF and, to a certain extent, TGFα and EGF were shown to cause apoptosis of normal and tumorigenic cells while apoptotic cell death could be prevented neither by single growth factors nor by serum. Antibodies to growth factors caused apoptosis by interacting with growth factors bound to their receptors on the cell surface. The phenomenon is likely to be associated with active internalization of growth factor receptors loaded with ligands. Apparently these activated receptors are essential for cell survival and their disappearance from the cell surface initiates apoptosis

Affinity modification of the membrane-bound microsomal cytochrome P-450 by substrate lipophilic analogs

The following lipophilic spin-labeled cytochrome P-450 analogs were synthesized: 2-octyl-4-(3-iodine-2-oxopropylidene)-2,3,5,5-tetramethylimidaz olidine-1-oxyl (RIII), 2-nonyl-4-(3-iodine-2-oxopropylidene)-2,3,5,5-tetramethylimidaz olidine-1-oxyl (RIV), 2-hepta-decyl-4-(3-iodine-2-oxopropylidene)-2,3,5,5-tetramethyl imidazolidine-1- oxyl (RV). The distribution coefficients, k, in water--lipid and water--octanol systems as well as the theoretical estimates of k for these and previously synthesized analogs, i.e., 4-(3-iodine-2-oxo-propylidenyl)-2,2,3,5,5-pentamethylimidazolid ine-1-oxyl (RI) and 2-hexyl-4-(3-iodine-2-oxopropylidene)-2,3,5,5-tetramethylimidaz olidine- 1-oxyl (RII) were determined. It was shown that RIII and RIV bind as type I substrates to cytochrome P-450 from rat microsomes induced with phenobarbital or 3-methylcholanthrene as well as to those from control rats. Radicals RIII and RIV inhibit the oxidation of aniline, aminopyrine and benzphetamine. RIII-RV strongly inhibit the O-deethylation of 7-etoxyresorufin. The inhibitory activity of the radicals increases in the following order: RV less than RIV less than or equal to RI less than or equal to RIII less than RII. The experimental results suggest that the inhibitory properties are nonmonotonuesly related to the lipophility. The high lipophility of RIII and its strong inhibitory properties permit to render the latter to the class of inhibitors which can be transported by liposome membrane vehicles to the liver, inhibit the in vivo activity of the microsomal system and thus prolong the effects of drugs oxidized by cytochrome P-450

Selective adhesion of tumor cells to the endothelium of the target organ in metastasis

The fragments of cell plasma membranes of two lines of metastatic tumour of A/He mice (lung adenocarcinoma--LA-cells and hepatoma HA-1--HA-cells) were used to prepare "hybrid" vesicles (LA- and HA-liposomes). Incorporation of the fragments of LA-cells into the bilayer vesicle increased the binding of LA-liposomes with lung almost by an order of magnitude, their trapping by liver being noticeably decreased. HA-liposomes were retained in the liver in larger amounts than the model phospholipid liposomes. It is assumed that the specificity of metastatic spreading into an organ is associated with the interaction of the plasma membrane of the tumour cells with the apical membrane of the target organ endothelium