Lymphatics - not just a chylomicron conduit

PURPOSE OF REVIEW

Lymphatics are known to have active, regulated pumping by smooth muscle cells that enhance lymph flow, but whether active regulation of lymphatic pumping contributes significantly to the rate of appearance of chylomicrons (CMs) in the blood circulation (i.e., CM production rate) is not currently known. In this review, we highlight some of the potential mechanisms by which lymphatics may regulate CM production.

RECENT FINDINGS

Recent data from our lab and others are beginning to provide clues that suggest a more active role of lymphatics in regulating CM appearance in the circulation through various mechanisms. Potential contributors include apolipoproteins, glucose, glucagon-like peptide-2, and vascular endothelial growth factor-C, but there are likely to be many more.

SUMMARY

The digested products of dietary fats absorbed by the small intestine are re-esterified and packaged by enterocytes into large, triglyceride-rich CM particles or stored temporarily in intracellular cytoplasmic lipid droplets. Secreted CMs traverse the lamina propria and are transported via lymphatics and then the blood circulation to liver and extrahepatic tissues, where they are stored or metabolized as a rich energy source. Although indirect data suggest a relationship between lymphatic pumping and CM production, this concept requires more experimental evidence before we can be sure that lymphatic pumping contributes significantly to the rate of CM appearance in the blood circulation.

Characterization of the Tumor Microenvironment and Tumor-Stroma Interaction by Non-invasive Preclinical Imaging

Tumors are often characterized by hypoxia, vascular abnormalities, low extracellular pH, increased interstitial fluid pressure, altered choline-phospholipid metabolism, and aerobic glycolysis (Warburg effect). The impact of these tumor characteristics has been investigated extensively in the context of tumor development, progression, and treatment response, resulting in a number of non-invasive imaging biomarkers. More recent evidence suggests that cancer cells undergo metabolic reprograming, beyond aerobic glycolysis, in the course of tumor development and progression. The resulting altered metabolic content in tumors has the ability to affect cell signaling and block cellular differentiation. Additional emerging evidence reveals that the interaction between tumor and stroma cells can alter tumor metabolism (leading to metabolic reprograming) as well as tumor growth and vascular features. This review will summarize previous and current preclinical, non-invasive, multimodal imaging efforts to characterize the tumor microenvironment, including its stromal components and understand tumor-stroma interaction in cancer development, progression, and treatment response.

Recent advances in microscopic techniques for visualizing leukocytes in vivo

Leukocytes are inherently motile and interactive cells. Recent advances in intravital microscopy approaches have enabled a new vista of their behavior within intact tissues in real time. This brief review summarizes the developments enabling the tracking of immune responses in vivo.