Growth in high serum concentrations leads to rapid deadaptation of cells previously adapted to growth in an extremely low concentration of serum

Nitroprusside induces melanoma ferroptosis with serum supplementation and prolongs survival under serum depletion or hypoxia

BACKGROUND

When proliferating tumor cells expand to areas distant from vascular sites, poor diffusion of oxygen and nutrients occur, generating a restrictive hypoxic gradient in which susceptible tumor cells die. The heterogeneous population surviving hypoxia and metabolic starvation include de-differentiated cancer stem cells (CSC), capable of self-renewing tumor-initiating cells (TICs), or those that divide asymmetrically to produce non-tumor-initiating differentiated (NTI-D) cell progeny. Under such restrictive conditions, both populations slowly proliferate, entering quiescence or senescence, when exiting from cell cycle progression. This may drive chemoresistance and tumor recurrence, since most anti-cancer treatments target rapidly proliferating cells.

PURPOSE

Since persistent or additional stress may increase NTI-D cells conversion to TICs, we investigated whether nutrient depletion or hypoxia influence expression of tyrosinase, a crucial enzyme for melanin synthesis, and B16 melanoma survival, when exposed to iron-dependent cell death oxidative stress produced by the Fenton reaction, resembling ferroptosis.

RESULTS

-a) proliferating B16 melanoma with 10% serum-supplementation (10%S) normoxically express hypoxia inducible factor 1α (HIF1α) but lose tyrosinase, in contrast to those transiently exposed to (SF) serum-free medium, in which both HIF1α and tyrosinase are co-expressed; b) in contrast to the resistance to SNP toxicity in (SF) cells with higher tyrosinase expression, those in (10%S) are killed by iron from nitroprusside/ferricyanide (SNP) irrespective of exogenous H₂O₂, in a reaction antagonized by the anti-oxidant and MEK inhibitor UO126; c) Moreover, under transient serum depletion, SNP cooperates with hypoxia (1.5% oxygen), prolonging B16 melanoma (SF) survival; d) the hypoxia mimetic CoCl₂ inhibits proliferation-associated cyclin A, irrespective of SNP, in (10%S) cells or in transiently serum-depleted (SF) cells. However, only in the latter cells, CoCl₂ but not SNP, induce loss of HIF1α and apoptosis-associated PARP cleavage; e) longer term adaptation to survive serum depletion, generates (SS) cells resistant to SNP toxicity, which aerobically co-express HIF1α and tyrosinase. In SS B16 melanoma, exogenous non-toxic 100 μM H₂O₂ super-induces the ratio of tyrosinase to HIF1α. However, co-treatment of SS-B16 cells with SNP plus exogenous H₂O_2, partly increases PARP cleavage by reciprocally decreasing tyrosinase expression.

SIGNIFICANCE

- These results suggest that a phenotypic plasticity in response to depletion of nutrients and/or oxygen, helps decide whether melanoma cells undergo either death by ferroptosis, or resistance to it, when challenged by the same exogenous oxidative stress (iron ± H₂O₂).

Phenotypic selection as the biological mode of epigenetic conversion and reversion in cell transformation

Exposure of certain cell lines to methylcholanthrene, X-rays, or physiological growth constraint leads to preneoplastic transformation in all or most of the treated cells. After attaining confluence, a fraction in those cells progress to full transformation, as evidenced by their ability to form discrete foci distinguishable from the surrounding cells by virtue of their higher density. Transformation induced by suspension in agar, an even stronger growth-selective condition than confluence, is reminiscent of all but the final differentiated stage of a normal developmental process, epithelial-mesenchymal transition. Changes associated with transformation are not restricted to focus-forming cells, as the permissiveness for focus formation provided by confluent cells surrounding transformed foci is greater than that of nonselected cells. The neoplastic process can also be reversed in culture. Transformed cells passaged at low density in high serum revert to normal morphology and growth behavior in vitro and lose the capacity for tumor formation in vivo. We propose that transformation and its reversal are driven by a process of phenotypic selection that involves entire heterogeneous populations of cells responding to microenvironmental changes. Because of the involvement of whole cell populations, we view this process as fundamentally adaptive and epigenetic in nature.

Improved cellular response of ion modified poly(lactic acid-co-glycolic acid) substrates for mouse fibroblast cells

In this report, the effects of argon (Ar) ion irradiation on poly(lactic acid-co-glycolic acid) (PLGA) substrates on biocompatibility were studied. PLGA scaffold substrates were prepared by spin coating glass surfaces with PLGA dissolved in anhydrous chloroform. Previously, we showed that surface modifications of PLGA films using ion irradiation modulate the inherent hydrophobicity of PLGA surface. Here we show that with increasing ion dose (1×10(12) to 1×10(14) ions/cm(2)), hydrophobicity and surface roughness decreased. Biocompatibility for NIH3T3 mouse fibroblast cells was increased by argon irradiation of PLGA substrates. On unirradiated PLGA films, fibroblasts had a longer doubling time and cell densities were 52% lower than controls after 48 h in vitro. Argon irradiated PLGA substrates supported growth rates similar to control. Despite differences in cell cycle kinetics, there was no detectible cytotoxicity observed on any substrate. This demonstrates that argon ion irradiation can be used to tune the surface microstructure and generate substrates that are more compatible for the cell growth and proliferation.

Isolation, characterization and recombinant protein expression in Veggie-CHO: A serum-free CHO host cell line

The dihydrofolate reductase-deficient Chinese hamster ovary cell line, DXB11-CHO, commonly used as a host cell for the production of recombinant proteins requires 7.5% serum-supplementation for optimal growth. Regulatory issues surrounding the use of serum in clinical production processes and the direct and indirect costs of using serum in large-scale production and recovery processes have triggered efforts to derive serum-independent host cell lines. We have successfully isolated a serum-free host that we named Veggie- CHO. Veggie-CHO was generated by adapting DXB11-CHO cells to growth in serum-free media in the absence of exogenous growth factors such as Transferrin and Insulin-like growth factor, which we have previously shown to be essential for growth and viability of DXB11- CHO cells. Veggie-CHO cells have been shown to maintain an average doubling time of 22 hr in continuous growth cultures over a period of three months and have retained the dihydrofolate reductase -deficient phenotype of their parental DXB11-CHO cells. These properties and the stability of its serum-free phenotype have allowed the use of Veggie- CHO as host cells for transfection and amplified expression of recombinant proteins. We describe the derivation a serum-free recombinant cell line with an average doubling time of 20 hr and specific productivity of 2.5 Units recombinant Flt-3L protein per 10e6 cells per day.

Degrees and kinds of selection in spontaneous neoplastic transformation: an operational analysis

Spontaneous neoplastic transformation develops within days in the NIH 3T3 line of cells through differential inhibition of their proliferation under contact inhibition. A small fraction of the population continues to multiply after saturation density is reached and is selected to progressively increase saturation density in successive rounds of confluence. The degree of selection at confluence depends on the extent of proliferation of some cells in a heterogeneous population. The development of transformed foci is an extension of the same selective process that increases saturation density. The expression of the foci is enhanced with increases in the saturation density of the surrounding cells. Transformation is also induced by moderately reducing the concentration of calf serum in the medium during low-density passages, which allows selection of cells that require less growth factor. Further stepwise reductions in serum increase the degree of transformation. Contact inhibition and reduction in serum concentration select for the same phenotype of cell that increases saturation density and generates transformed foci. There is mounting evidence that selection is a major factor in the development of common epithelial tumors of humans, but it extends over decades rather than days, and the in vivo microenvironment selects from more stable populations of cells than those in culture. The many progressive levels of increased saturation density and transformed focus formation suggest that a very large number of genes participate in neoplastic development. The operational model of variation and selection presented here may aid in understanding chemical carcinogenesis and cancer recurrence after chemotherapy.

A free-radical hypothesis for the instability and evolution of genotype and phenotype in vitro

It has been known for several decades that cultured murine cells undergo a defined series of changes, i.e., an in vitro evolution, which includes crisis, spontaneous transformation ('immortalization'), aneuploidy, and spontaneous neoplastic transformation. These changes have been shown to be caused by the in vitro environment rather than an inherent instability of the murine phenotype or genotype. Serum amine oxidases were recently identified as a predominant cause of crisis. These enzymes generate hydrogen peroxide from polyamine substrates that enter the extracellular milieu. This finding implicates free-radical toxicity as the underlying cause of in vitro evolution. We propose an oxyradical hypothesis to explain each of the stages of in vitro evolution and discuss its significance for cytotechnology and long-term cultivation of mammalian cell types.