The Juicy Deets about The Teats
To be, or not to be, that is the question. It’s also a question which bacteria are not given the chance to ask. For years it has been known that breast milk contains secretory antibodies, lipids, proteins, carbohydrates and many other compounds – some of which have not been identified. Even though the protein I’ll be writing about today has not recently been identified, a greater understanding of its function has recently been discovered. In this post, I want to share some really interesting discoveries about an antimicrobial and anti-tumor protein-fatty acid complex found in breast milk.
While Hamlet is defeated by his enemy, there’s a new kind of Hamlet who will not be defeated by his. It is better known as Human α-lactalbumin made lethal to tumor cells. HAMLET’s function is not limited to tumor cells as it has been recently found to act as a single bactericidal compound or one component of a bactericidal combination. HAMELT is an α-lactalbumin protein, a protein involved in lactose production, that is stabilized when it interacts with the human milk lipids oleic or linoleic acid in physiological conditions like an infant’s stomach. It is the most abundant protein in breast milk, but before it interacts with these fatty acids it is in a partially folded, inactive state. As a single bactericidal chemical, HAMLET is particularly effective in preventing growth of the bacteria Streptococcus pneumonia, and somewhat effective against Haemophilus influenza and Moraxella catarrhalis. These three bacteria, especially S. pneumonia, can cause serious infections. In fact, S. pneumonia is the leading cause of death for children under the age of five. As such, it is especially important to be aware of potential antibiotics available to fight off infections. Most antibiotics have a minimum inhibitory concentration (MIC), which is the minimum concentration of an antibiotic required to stop bacterial growth overnight. HAMLET is not as powerful as a typical antibiotic because when it is used alone it requires a higher concentration to inhibit the growth of bacteria compared to the MIC of antibiotics like penicillin, erythromycin and gentamicin. However, this complex is truly a superhero when it is combined with other antibiotics: it reduces the MIC of antibiotics that is needed to inhibit bacterial growth – up to a 300-fold decrease in some cases of antibiotic resistant bacteria! One recent study showed this by combining penicillin, erythromycin, or gentamicin with low levels of HAMLET to demonstrate that the antibiotic-HAMLET combination works better to stop bacterial growth than either the antibiotic or HAMLET alone. By using low levels of HAMLET – specifically, subinhibitory concentrations that were less than the MIC – they showed that HAMLET does not contribute by stopping bacterial growth, but rather by helping the antibiotics stop bacterial growth.
The researchers also showed that specific concentrations of antibiotics and HAMLET could be used to optimize the antibacterial effect of the combination. But it gets better. The specific concentrations of HAMLET and an antibiotic were also shown to sensitize antibiotic-resistant bacteria to the antibiotic-HAMLET combination. In other words, when HAMLET is present, one can use a lower antibiotic concentration to stop bacterial growth even for antibiotic-resistant bacteria compared to the regular antibiotic dosage when the antibiotic acts alone. Way to be, HAMLET!
This may not seem relevant to the development of an infant, but here’s why I think it’s important to know about HAMLET. Many chemicals can be passed on through breast milk to the infant, including antibiotics. While most people taking antibiotics are taking them to combat “bad” bacteria, they may also prevent the growth of the “good” bacteria in a baby’s developing gut micobiome. This is because the presence of both HAMLET and an antibiotic may result in a powerful antibiotic combination that could possibly fight off those good bacteria. Another reason that it’s important to know about is if the baby is given antibiotics to fight off an infection. Perhaps the enhanced antibacterial activity may have an effect on the infant’s recovery.
The tests mentioned above were done outside of the body, but the researches also did tests in live mice. They looked at the effect of the antibiotic-HAMLET combination to kill bacteria growing in the upper part of the throat and biofilms (which grow on liquid-bathed, solid surfaces like teeth). In both situations, the antibiotic-HAMLET combination decreased bacterial growth better than either compound could by itself. If you need a recap of the many ways HAMLET works, look at Figure 1 for a detailed web chart.
Figure 2 – Different combination used to test HAMLET’s antibiotic activity and the effect on the bacteria.
This protein clearly helps to inhibit bacterial growth, but only to a certain extent. Currently, the mechanism by which the HAMLET-oleic acid complex inhibits bacteria is not entirely known. To learn about HAMLET’s mode of action, the researchers performed a test in which they studied the uptake of antibiotic binding to the bacteria in the presence of low levels of HAMLET. It showed that for some antibiotics, HAMLET increased the bacterial uptake of that antibiotic. Another test showed that HAMLET uses an influx of calcium and kinase activity (which involves a protein adding a phosphate group to a target protein) to increase antibiotic activity. Using HAMLET or certain antibiotics alone typically results in lysis – a response which causes the cell to burst. When HAMLET is combined with antibiotics, however, lysis does not occur and the bacteria dies by another mechanism. If you’re wondering how this is important for breast milk and infants, take note that lysis can result in an immune response like inflammation. If a baby is taking antibiotics and lysis does not occur, maybe it can help remove speed up the infant’s recovery since the baby’s immune system does not respond like it would with cell lysis.
HAMLET is just as interesting if you look at it from an anti-tumor perspective. For several years, studies have shown that HAMLET selectively targets certain types of cancer cells while ignoring healthy cells. Perhaps this function helps keep cellular division in line and ensures tumors do not develop in the rapidly growing baby. How, exactly, they only target malignant cells is unknown, but researchers think it might have to do with an important cell-signaling protein called p53. This protein is highly involved in cell death by apoptosis, which is exactly how HAMLET kills tumor cells. In cancer cells, p53 is often mutated which possibly provides a more favourable binding site for HAMLET. As for the selective targeting, it might have to do with the reductive state of the cancer cells. Because cancer cells grow much faster than healthy cells, they have lower oxygen levels – this means they are reductive. Despite the mechanism being different than that of HAMLET’s antibiotic activity, the result is the same: cell death. See Figure for the multiple roles HAMLET can play and where it all begins.
Unlike Hamlet who contemplates life versus death, bacteria and tumor cells have no choice in the matter when fighting HAMLET – they will surely die. Considering that α-lactalbumin can be used for lactose production in milk and then recycled to be used as an antibacterial and anti-tumor protein just shows versatile proteins can be! This new research showing the numerous applications of HAMLET’s antibiotic activity are providing us with more information about the great immunological benefits of breast milk.
Until next time,
Maria
If you want to read the papers I looked at, here are the links:
The human milk protein-lipid complex HAMLET sensitizes bacterial pathogens to traditional antimicrobial agents.
http://www.ncbi.nlm.nih.gov/pubmed/22905269
Human alpha-lactalbumin made lethal to tumor cells (HAMLET) kills human glioblastoma cells in brain xenografts by an apoptosis-like mechanism and prolongs survival
http://www.ncbi.nlm.nih.gov/pubmed/15026350
