Clonal expansion of adult rat hepatic stem cell lines by suppression of asymmetric cell kinetics (SACK).

Adult stem cells have potential use for several biomedical applications, including cell replacement therapy, gene therapy, and tissue engineering. However, such applications have been limited due to difficulties encountered in expanding functional adult stem cells. We have developed a new approach to the problem of adult stem cell expansion based on the suppression of asymmetric cell kinetics (SACK). We postulated that asymmetric cell kinetics, required for adult stem cell function, were a major barrier to their expansion in culture. As such, conversion of adult stem cells from asymmetric cell kinetics to symmetric cell kinetics would promote their exponential expansion and longterm propagation in culture. The purine nucleoside xanthosine (Xs), which promotes guanine ribonucleotide biosynthesis, can be used to reversibly convert cells from asymmetric cell kinetics to symmetric cell kinetics. We used Xs supplementation to derive clonal epithelial cell lines from adult rat liver that have properties of adult hepatic stem cells. The properties of two Xs-derived cell lines, Lig-8 and Lig-13, are described in detail and compared to properties of adult rat hepatic cell lines derived without Xs supplementation. The Xs-derived cell lines exhibit Xs-dependent asymmetric cell kinetics and Xs-dependent expression of mature hepatic differentiation markers. Interestingly, Lig-8 cells produce progeny with properties consistent with hepatocyte differentiation, while Lig-13 progeny cells have properties consistent with bile duct epithelium differentiation. A stable adult cholangiocyte stem cell line has not been previously described. Consistent with the principles of their derivation, the SACK-derived hepatic cell lines exhibit neither senescence nor tumorigenic properties, and their differentiation properties are stable after longterm culture. These characteristics of SACK-derived stem cell lines underscore asymmetric cell kinetics as an essential adult stem cell property with potential to be the basis for a general approach to expansion and propagation of diverse adult stem cells.

Functional differentiation of hepatocyte-like spheroid structures from putative liver progenitor cells in three-dimensional peptide scaffolds.

We investigated the ability of a new type of biological material, the self-assembling peptide scaffold, to foster tissue-like function by a putative adult rat hepatocyte progenitor cell line, Lig-8. In conventional adherent petri-dish cultures, Lig-8 cells divide exponentially, express markers for definitive endoderm HNF3 beta and hepatocyte lineage, including CK8 and alpha-fetoprotein, but lack expression of mature hepatocyte markers. However, in the three-dimensional peptide scaffold cultures, Lig-8 exhibits non-exponential cell kinetics, acquires a spheroidal morphology, and produces progeny cells with mature hepatocyte properties. The differentiated progeny cells display expression of transcription factor C/EBP alpha and several other indicators that suggest hepatocyte maturation, including binucleation, up-regulation of albumin, and expression of cytochrome P450s CYP1A1, CYP1A2, and CYP2E1. Moreover, all three cytochrome p450 enzyme activities are induced using 3-methylcholanthrene in these spheroids. These results suggest that a designed biological material may provide a conducive microenvironment in which putative adult progenitor cells differentiate into functional hepatocyte-like spheroid clusters. This bioengineered scaffold system provides a better physiological approach to "progenitor cell differentiation" for future biomedical and pharmaceutics applications.

Cosegregation of chromosomes containing immortal DNA strands in cells that cycle with asymmetric stem cell kinetics.

A long-standing intriguing hypothesis in cancer biology is that adult stem cells avoid mutations from DNA replication errors by a unique pattern of chromosome segregation. At each asymmetric cell division, adult stem cells have been postulated to selectively retain a set of chromosomes that contain old template DNA strands (i.e., "immortal DNA strands"). Using cultured cells that cycle with asymmetric cell kinetics, we confirmed both the existence of immortal DNA strands and the cosegregation of chromosomes that bear them. Our findings also lead us to propose a role for immortal DNA strands in tissue aging as well as cancer.

Breaching the Kinetic Barrier to In Vitro Somatic Stem Cell Propagation.

Here we have reviewed the conventional definitions and fundamental characteristics of the two basic types of stem cells, embryonic stem cells (ESCs) and somatic stem cells (SSCs). By taking into account the often-overlooked asymmetric cell kinetics of SSCs, we consider the evidence that should SSCs retain these growth kinetics in vitro, a natural kinetic barrier to SSC propagation exists. Recent discoveries showing that the tumor suppressor gene p53 can act as a regulator of asymmetric cell kinetics provide a target pathway for in vitro SSC propagation strategies.