| Research Abstract |
A small hepatocyte can clonally proliferate for more than 3 months when the cells are cultured in DMEM supplemented with 10% FBS,10mM nicotinamide, 1mM ascorbic acid 2-phosphate, 10ng/mI EGF and 1% DMSO.The cell divides to form a colony and the number of the cells reaches to more than 100 cells within 20 days. With time in culture, cells with a large cytoplasm appear within a colony. They have many mitochondria and large peroxisomes with a crystaline nucleoid, which are typical mature heatocytes. Immunoreactivity to Cx32 and well-developed bile canaliculus structures are often observed between cells. Thus, we suggest that small hepatocytes may be considered as "committed progenitor cells" that can further differentiate into mature hepatocytes.
We established a culture system in which, by adding 2% DMSO to the culture medium after the hepatocytes proliferate, the cells are able to recover differentiated functions such as albumin and transferrin secretion. Although expressions of connexin 32, Cx 26, and tryptophan 2,3-dioxygenase, thought to a highly differentiated function of mature hepatocytes, have never been maintained or re-induced in primary hepatocytes cultured for long period of time by using the conventional culture methods, those mPNA expressions were gradually restored with time after the addition of 2% DMSO and maintained for a month in this culture system.
We showed the role of the liver-enriched transcription factors in the transition during which proliferating hepatocytes become quiescent. We found that hepatic differentiation requires not only inhibition of DNA synthesis but also induction of appropriate transcription facors. Thus, expression of HNF3gamma, C/EBPalpha, and C/EBPbeta may be necessary for hepatocytes to acquire highly differentiated functions in addition to coexpression of certain amounts of transcripts of HNF1alpha, HNF1beta, HNF3alpha, HNF3beta, and HNF4 as well as suppression of C/EBPdelta.
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