The identification of a serum viability factor for SV3T3 cells as biotin and its possible relationship to the maintenance of Krebs cycle activity

Biotin and choline replace the growth requirement of Madin-Darby canine kidney cells for high-density lipoproteins

MDCK cells maintained on extracellular matrix (ECM)-coated dishes and exposed to Dulbecco's modified Eagle's medium (DME) supplemented with transferrin and either high-density lipoproteins (HDLs) or phosphatidyl choline (PC) liposomes have a growth rate and final cell density similar to those of cultures exposed to serum-supplemented DME. When MDCK cells are exposed to a medium consisting of a mixture (1:1) of DME and F12 medium (D/F), the addition of transferrin (10 micrograms/ml) alone supports cell growth and the presence of HDLs or PC liposomes is no longer required. MDCK cells exposed to D/F medium supplemented with transferrin can be passaged for more than 50 generations in total absence of serum. The F12 components that support growth in the absence of HDLs or PC liposomes are biotin (which is absent in DME) and choline (which is present in insufficient concentration in DME). Supplementation of DME with transferrin, biotin (3.6 ng/ml), and choline (10 micrograms/ml) allows optimal growth of MDCK cells and permits serial propagation through more than 50 generations. The growth requirement of MDCK cells for HDLs or PC liposomes can therefore be replaced by adequate concentrations of biotin and choline. The widely observed fact that a combination of DME/F12 medium is more effective than DME alone in supporting cell growth may be due in part to the lack of biotin and suboptimal choline concentration in DME.

The effect of biotin on cellular functions in HeLa cells

HeLa cells cultured in a biotin-deficient medium show reduced rate of protein synthesis, DNA synthesis and growth. Addition of exogenous biotin to the cells cultured in biotin-deficient medium results in enhanced protein synthesis, DNA synthesis and cell growth. Continuous protein synthesis is required for the increase in DNA synthesis observed upon the addition of exogenous biotin to the cells cultured in biotin-deficient medium. These results suggest that cells cultured in biotin-deficient medium are arrested in the G1 stage of cell cycle and this block is removed upon the addition of biotin to the deficient medium.

Requirement for biotin and the function of biotin in cells in culture

The requirement of biotin in the culture medium has been established for HeLa cells, human fibroblasts, baby hamster kidney cells (BHK) as well as polyoma-transformed BHK cells. Growth, viability, biotin content and the activities of biotin-dependent enzymes were used as criteria. Cells in culture appear to bind and internalized avidin as well as the avidin-biotin complex. Avidin seems to mimic a natural ligand that could be the biotin-binding protein of serum. Additions of serum fatty acids, aspartate or asparagine to the culture medium do not supplant the biotin requirement. Nuclear fractions of cells contain significant biotin without the ability to fix carbon dioxide. A biotin-containing protein has been isolated from rat liver nuclei. The incorporation of amino acids into cellular protein is decreased in biotin-deficient HeLa cells. The proportion of active ribosomes is also decreased in these cells. Supplementation of the medium of deficient cells with biotin results in stimulation of protein synthesis. It is suggested that biotin might subserve a function in cells in addition to its role as the prosthetic group of biotin-enzymes.

Evidence for a transforming growth factor(s) in the serum-free conditioned medium of SV40-transformed 3T3 fibroblasts

Unlike untransformed 3T3 fibroblasts, Simian Virus 40-transformed 3T3 cells (SV3T3) do not require the presence of exogenous polypeptide mitogens in order to become competent for the initiation of DNA synthesis. As an explanation for this state of perpetual competence, growth activity for 3T3 cells was found in the serum-free conditioned medium of SV3T3 cells. This activity, which is abolished by incubation with pepsin, but withstands to a fair degree denaturants (8 M urea and 4 M guanidinium HCl) and a reducing agent (65 mM dithioerythritol), enhances the growth of Swiss and BALB/c 3T3 cells in 1% (v/v) calf serum medium and ensures the complete survival of Swiss 3T3 cells for several days in the total absence of serum. The growth activity appears to be derived from the SV3T3 cells, since it is not found in the non-conditioned medium (and the insulin and transferrin components of this serum-free medium have only weak growth activity alone) and the chances of it being due to residual serum growth factors are slight because of the method of growing the cells and the collection of the conditioned medium. Moreover, in a separate experiment to test the possible involvement of cell attachment factors, rat fibronectin was found not to affect 3T3 growth in the standard growth assay. The conditioned medium growth activity apparently has the biological capabilities of transforming growth factors. After concentration by ammonium sulfate precipitation, it can stimulate DNA synthesis in confluent, quiescent Swiss 3T3 cells, in both the absence and presence of 10% depleted calf serum. It also permits the growth of large colonies of Swiss 3T3 cells in soft agar. As part of a preliminary characterization of the conditioned medium growth activity, gel filtration in 0.15 M NaCl, 0.001 M HCl resolves the activity into high (mol. wt. greater than or equal to 100,000) and low (mol. wt. 5,000-6,000) molecular weight factors. Since re-chromatography of the high molecular weight factor, which may still be in an aggregated form, does not generate the smaller, the latter is not a dissociation product of the former. The precise relationship of these SV3T3-conditioned medium growth factors to other growth factors, including known transforming growth factors, has not yet been determined.