Role of miRNA in liver cell proliferation during hepatic regeneration and cancer

The extraordinary ability of liver cells to proliferate underlies the remarkable regenerative capacity of this organ, and is also responsible for the high incidence of the primary hepatic cancers. Understanding of the factors and the mechanisms that govern both normal and cancerous liver cell proliferation is of extreme importance.;The work presented here examines the role of microRNA at the onset of hepatic regeneration after partial hepatectomy (PHx) in mice, a procedure that elicits a strong proliferate response accompanied with prominent rearrangements of extracellular matrix (ECM). Gelatinase activity in regenerating tissues paralleled the tissue levels of miR-181b all through out the time course and was a mirror image of expression of TIMP-3 - a tissue inhibitor of matrix metalloproteinases (MMPs). Consistent with the in vivo observations, a set of in vitro experiments demonstrated that miR-181b directly controls TIMP-3 levels and activity, as evident from gelatinase activities of MMPs. Additionally, miR-181 was shown to modulate hepatocyte proliferation in vitro. The results of this study implicate microRNA in regulation of ECM remodeling and hepatocyte proliferation during hepatic regeneration.;Chapter 4 comprises a study on exosomes as a means of intracellular communication. This study identifies a cholangiocarcinoma cell line, KMCH that is capable of exosome secretion, and characterizes microscopic appearance, expression of exosomal markers and effects that these exosomes have on recipient cells.;The observations of this study led to the conclusion that the tumor derived exosomes can selectively suppress growth of normal recipient cells versus cancer cells by inducing cell stress evident from p38MAPK activation and triggering cell cycle aberrations that lead to impediment in cell proliferation of nonmalignant recipient biliary cells when compared to malignant cholangiocytes. Additionally, examining exosomal content yielded identification of several microRNA species, which may enhance the above effects.;Chapter 5 outlines a regulatory mechanism underlying correlative associations between upregulation IL-6 and DNMT driven tumor suppressor gene silencing - the two most studied events in cholangiocarcinoma. The experimental data in this study demonstrated that DNMT-1 is directly regulated by miR-148, while IL-6 enhances recruitment of MITF to the miR-148 promoter thus explaining the mechanisms of downregulation of this antiproliferative microRNA in malignant cholangiocytes. Moreover inhibiting STAT3, a conventional downstream effecter of IL-6 activity, leads to reduction in immunodetection of MITF, yet again implicating IL-6 in regulation of MITF.;Taken together these findings outline a regulatory mechanism that links IL-6 to epigenetic tumor suppressor genes silencing commonly observed in cholangiocarcinoma and brings out MITF and miR-148 as new important factors in cholangiocarcinoma development.;Collectively, the results of these studies enhance our understanding of liver physiology specifically in such important aspects of it as regulation of cell proliferation during hepatic regeneration or cholangiocarcinoma. We have characterized and elucidated the effect of tumor derived exosomes on cholangiocytes proliferation. These studies, if continued, may lead to a development of novel more precise therapies for selective modulating liver cell proliferation both in liver regeneration and cancer.