To many cancer biologists, the in vitro culturing of cancer cells and even tumors is so commonplace that we often don’t think twice about it. What does it take to grow cancer in a dish? As it turns out, not too much: an incubator set to body temperature (37C), some atmospheric control (cells need to breathe, after all), and a culture medium rich with all of the molecules that cells need to grow—sugars, vitamins, salts, various micronutrients, and a dash of baby cow blood. Wait, what?
You heard me right—while there are many different variations of culture media, one of the most ubiquitous media additives is called fetal bovine serum (FBS)—that’s science-talk for the cell-free liquid component of blood collected from baby cows. “The practice of including FBS in culture medias was born from early media development efforts, which were largely ‘trial-and-error’,” notes Dr. Lucas Sullivan, an associate professor in the Human Biology Division at Fred Hutch. “Basically, early developers of tissue culture systems found that—in addition to defined combinations of sugars and salts—cells needed various animal extracts to grow. These extracts worked, so they caught on, and FBS in particular gained popularity to the present day, where it is a nearly ubiquitous media additive.”
Okay, so apart from seeming a little Frankenstein-esque, what’s wrong with adding a little baby cow blood extract to cell cultures? “The requirement for FBS in tissue culture is not ideal for several reasons,” notes Dr. Oliver Newsom, a former PhD student in the Sullivan lab and first-author of a recent publication investigating this practice. “FBS is expensive to purchase, and there are ethical concerns associated with its harvesting. The fact that it comes from an animal also means that it’s not defined—the exact chemical composition of FBS is unknown and actually varies from lot to lot, which means that different batches of FBS might affect the behavior of otherwise identical cell cultures, affecting the reproducibility of experiments. Finally, media containing FBS may not accurately reflect what cancer cells experience in a (human) patient, which could hamper efforts to translate findings from culture to more in vivo scenarios.”
So, FBS is bad for our wallets, our ethics, and our science—can we get rid of it? “We perhaps could, if we first understand why it is essential for cultured cells,” says Sullivan. One common reason given for the necessity of FBS is that it’s rich in various growth hormones, but Dr. Sullivan is unconvinced of this argument. “One of the hallmarks of cancer cells—that is, one of the things that differentiates cancer cells from normal cells—is that they are either insensitive to growth hormones or able to produce their own, enabling their runaway proliferation. Thus, instead of fulfilling this primarily ‘signaling’ role, we hypothesized that FBS fulfills one or more metabolic demands for proliferation.” This hypothesis started a years-long effort, spearheaded by Dr. Newsom, to figure out what exactly it was about FBS that supported cancer cell growth in culture.
In line with their hypothesis of serum filling a metabolic role, Newsom and Sullivan found that cancer cells grown in serum-limiting conditions grew normally for some time before their proliferation stalled, suggesting that they were consuming some component(s) of FBS that became growth-limiting once depleted. Using some prior knowledge about what FBS contains and a considerable amount of trial-and-error, Newsom was able to narrow down these crucial components to two: lipids, and trace metals. While either factor modestly improved the proliferation of cells cultured in serum-free conditions on their own, supplementing both to culture media enabled the long-term, exponential proliferation of cancer cells in the complete absence of FBS!
While the collection of necessary trace metals was fairly small, serum contains hundreds of different types of lipids of diverse structure and function. To determine which of these lipid classes were consumed by proliferating cancer cells, Newsom used mass spectrometry to quantify the abundance of nearly 200 lipid types in (serum-containing) culture media over several days of cell growth—hoping to find specific lipid species which were depleted over time. Surprisingly, this analysis revealed that cells don’t indiscriminately consume media lipids. Instead, they preferentially consume fatty acids and lysolipids—two relatively simple lipids that serve as precursors for cells to synthesize more complex lipid species. “The fact that cancer cells depend on serum for lipids—and specifically for these simpler lipid classes—is puzzling, considering that cells are generally able to synthesize these lipids themselves,” notes Newsom. Indeed, he found that inhibiting the de novo synthesis of fatty acids had no effect on the proliferation of cells in lipid-containing conditions, while the inhibition of lipid scavenging from the media markedly inhibited growth—the combined blocking of fatty acid synthesis and scavenging had an even stronger effect.
Thus, it appears that fetal bovine serum is indeed necessary for maximal proliferation of cultured cancer cells, but not due to some vague and undefined requirement for growth factors or cow-derived ‘life energy.’ Instead, Newsom and Sullivan’s results support a model in which serum provides cells with two defined metabolic precursors—trace metals and precursor lipids—which the cells are either unable to synthesize on their own (in the case of metals), or unable to synthesize at sufficient quantities to meet demand (in the case of lipids).
Are we ready to finally dump the practice of culturing cells in baby cow blood? “To be sure, there are some practical challenges that come with totally removing serum—serum also provides factors that help cells stick to plastic dishes well, for example—but our results provide a proof of concept that serum-free culture of cancer cells is an attainable goal,” says Newsom. As Dr. Sullivan notes, “we are also not claiming that serum-derived growth factors are never important for cultured cells; there are certainly some specialized cases or cell types which depend on serum for other functions as well. Nevertheless, we’re excited about these results, both from the standpoint of reducing our reliance on ethically and financially dubious FBS, but also from a basic science standpoint. By removing the poorly defined ‘serum’ variable, we have a more controlled culture system in which we can better study the effects of environmental conditions on cancer cells. We envision this resource could be used to further study mechanisms of lipid scavenging in cancer cells, for example, or to uncover dependencies that would otherwise be masked by supraphysiological levels of growth factors or other molecules in serum-containing media.” Listen up, baby cow blood—your days are numbered!
The spotlighted work was funded by the National Institutes of Health.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium member Dr. Lucas Sullivan contributed to this research.
Newsom, O. J., Zheng, E., & Sullivan, L. B. (2025). Defined media reveals the essential role of lipid scavenging in supporting cancer cell proliferation. Journal of Biological Chemistry, 110693.
Science Spotlight writer David Sokolov is a graduate student in the Sullivan Lab at the Fred Hutch. He studies how cancer cells modify their metabolism to facilitate rapid proliferation and accommodate tumor-driving mitochondrial defects. He's originally from the east coast and has bachelors' and masters' degrees from West Virginia University. Outside of the lab, you'll find him enjoying the outdoors, playing music, or raising composting worms in his front yard.
For the Media