For my final post I wanted to focus on the topic that our cancer project centers around: the use of the anti-diabetic drug metformin as a potential chemopreventative for pancreatic cancer. Many papers, including the paper I have chosen to analyze, focus on the link between pancreatic cancer (PC) and diabetes mellitus. It is because of this link that researchers believe the answer to decreasing the vast number of new cases of PC developing in the world is in a drug used to treat diabetes mellitus. Pancreatic cancer, with the most common type being pancreatic ductal adenocarcinoma (PDAC), is characterized by uncontrolled cell growth in the tissues of the pancreas (Ref. 1). According to various epidemiologic studies, there is an increased risk of developing PC associated with diabetes mellitus (Ref. 2). Diabetes mellitus is characterized by an abnormality in the body’s ability to either produce or use insulin efficiently, known as insulin resistance (Ref. 1). Both of these situations result in chronic hyperglycemia in the blood stream. PC is one of the deadliest forms of cancer with more than 38,460 deaths in a single year (of the 42,220 total new cases) (Ref. 2). There is a desperate need for a preventative drug or an effective treatment to inhibit the cell cycles in tumor cells or prevent the hyperactivation of certain pathways.
It has been estimated that approximately 80% of patients with pancreatic cancer have some form of diabetes (Ref. 2). Metformin (1,1-dimethylbiguanide) is reported to be one of the most commonly prescribed drugs for type 2 diabetes mellitus (T2DM) (Ref. 2). Already, these two statements have me thinking. If such a large percentage of PC patients have diabetes and Metformin is already the most common drug used to treat diabetes, why are our incidence levels of pancreatic cancer still so high? If Metformin really had such an impact and could be considered as a chemopreventative for pancreatic cancer, the number of cases of pancreatic cancer should be decreasing, at least since the point of the integration of Metfomin into the T2DM community. Instead, we still see a huge number of pancreatic cancer cases every year. Perhaps the dosages prescribed for treatment of diabetes is not enough to have an effect on pancreatic cancer cells, which would account for the lack of impact the seemingly popular drug has on pancreatic cancer cases. If this is the case, then I believe the possibility of using this drug as a chemopreventative is still a valid option, otherwise the statements simply do not line up.
One of the significant pathways involved in unrestricted growth of tumor cells is the hyperactivation of the mammalian target of rapamycin (mTOR) pathway. According to researchers, “In vitro and preclinical animal models confirmed that metformin induces AMPK activation, inhibits the Akt/mTOR pathway, and also eliminates cancer stem cells (CSCs) and inhibits tumor growth” (Ref. 2). While the AMPK activation and elimination of cancer stem cells is vital to halting the uncontrolled division of pancreatic, and other, cancer cells, I am choosing to just focus on the Akt/mTOR pathway, with an emphasis on mTOR. mTOR in part controls the ATP:AMP ratio which determines energy available to the cell and plays an integral role in regulating the cell’s division (mitosis and all of the replication proteins/machinery needed requires a lot of energy).
The study conducted in this research article consisted of investigating the effects of metformin on PanIN and their progression to PDAC in p48Cre/+.LSL-KrasG12D/+ mice. PanIN stands for pancreatic intraepithelial neoplasia, which is essentially tumor growth of the pancreatic tissue (Ref. 2). This investigation was carried out in part (which is what I am choosing to focus on) by examining the mRNA expression of mTOR via quantitative “real time” PCR (Ref. 2). The results of this analysis can be seen in the following figure (I apologize for the poor quality when the photo was enlarged).
Figure 4a. Effect of metformin on the expression of mRNA for mTOR as determined with quantitative real-time PCR. Column labeled at 0 on the x-axis represents the p48Cre/+.LSL-KrasG12D/+ mice while the 1000 and 2000 represent dosages of metformin (ppm) given to transgenic mice.
According to the figure, the mice who were fed metformin-supplemented diets showed a significantly lower relative expression of mTOR mRNA. The column representing mice who were not fed metformin reaches approx. 1.0 on the y-axis scale. The columns representing mice who were fed diets with 1000ppm and 2000ppm supplements of metformin reached approx. 0.5 and 0.55 respectively. This shows nearly a two-fold decrease between mice without and mice with metformin in their diets. The slight difference between the two dosages is not significant enough (see p-values) to say that a larger dose of metformin may lead to an increase in mRNA expression again of mTOR, however toxicity of drugs is always important to keep in mind.
A quick note that I wanted to bring up is the diabetic state of the mice is not defined in this paper. Whether that be a detail that is inherent in the type of mice used, I do not know. This bit of information would be important to know when analyzing the results of these tests. What is the health state of the mouse before give metformin or not given metformin? We should be cautious of the accuracy of the results and their relevancy to human patients in that we do not have knowledge of how severe the pancreatic tumor is, if there is metastasis in the tumors, do the mice have a form of diabetes or was this controlled, etc. In another study in the paper, it is clear that PanIN lesions are currently present and that the metformin seemingly halts the further progression however is this true across all of the studies done in this particular paper? These are all questions I came up with while reading the paper.
In conclusion, I find that the data supporting the idea that metformin helps prevent pancreatic cancer hopeful yet inconclusive at this early of a stage. They certainly point towards metformin having a positive impact and decreasing expression of the mTOR pathway, which plays a vital role in regulating “cell growth, cell proliferation, cell motility, cell survival, protein synthesis, and transcription,” all functions crucial to cancer development (Ref. 3). The mTOR pathway communicates with upstream pathways, including insulin and growth factors such as IGF-1 and -2, and in normal cells, regulates proper cell growth and senses the nutrient, oxygen, and energy levels of a cell (Ref. 3). In PC cells, this normal regulation is overridden by the cancer’s objectives: cell growth. While metformin seems to decrease the mTOR expression, we do not know if this new level is sufficient to sustain normal cell cycles and growth, or if it will result in toxicity for the healthy cells. The mTOR pathway is vital to living, normal cells; take it away and how can you keep all cells in the body from halting their proliferation? I would like to see more research and data on this exact topic and look forward to delving deeper when investigating for our wiki project. There is simply not enough data, or else the data is not quite presented in a clear enough manner to convince me of metformin being the next PC “wonder drug”, however the preliminary research is promising and starts us in the right direction.
- "Diabetes Mellitus: Types, Symptoms, Causes, Treatments." WebMD. WebMD, Web. 17 May 2014. <http://www.webmd.com/diabetes/types-of-diabetes-mellitus>.
- Mohammed, Altaf, Vaneena B. Janakiram, and Misty Brewer. "Antidiabetic Drug Metformin Prevents Progression of Pancreatic Cancer by Targeting in Part Cancer Stem Cells and mTOR Signaling." Translational Oncology 6.6 (2013): 649-659. Print
- "PI3K/AKT/mTOR pathway." Wikipedia. Web. 19 May 2014. <http://en.wikipedia.org/wiki/pi3k/akt/mtor_pathway>.