James R. Smith, Ph.D.
Department of Pathology, UTHSCSA

(210) 567-5067
smithjr@uthscsa.edu

My laboratory is currently studying the epigenetic aspects of in vitro cellular aging.

Normal cells derived from humans and other animals exhibit a limited division potential in culture. After achieving a certain number of population doublings, depending on the species and age of the donor, the cells stop dividing. However, the cells do not die but remain viable for up to three years. This phenomenon is widely considered aging at the cellular level. As cells age in culture their telomeres become shorter (because they do not express the enzyme telomerase). When the telomeres become "critically" short, the cells are triggered to enter the senescence state. However, these cells can be "immortalized" by expression of the enzyme telomerase. Telomere shortening has emerged as a major candidate for a counting mechanism that determines how many time a cell can divide. However, the way that short telomeres can effect the onset of senescence is not understood.

The Cdk inhibitor p21 is necessary for cells to enter the normal senescence state. However, pathway by which short telomeres induce p21 and, therefore, senescence has not been elucidated. Nor have the processes that result in significant changes in gene expression during cellular senescence. Therefore, it is highly likely that other processes, in addition to telomere shortening, are involved in cellular aging.

As cells age in culture, they lose methyl-cytosine from their genomic DNA. Changes in DNA methylation can have profound effects on the pattern of gene expression. Interestingly, it has recently been reported that simply decreasing DNA methyltransferase (DNA MeTase) activity causes induction of p21, without changes in DNA methylation. Therefore, the observed changes in MeTase activity and/or DNA methylation could account for the changes in gene expression seen with cellular aging. A recent observation in our laboratory, that cells expressing hTERT maintain DNA MeTase provides a possible explanation for the prevention, by hTERT, of changes in gene expression that normally occur with cellular aging. Our current studies are designed to: 1) elucidate the mechanism by which telomerase or telomere length regulate DNA MeTase activity and DNA methylation, 2) determine if there is a causal link between decreasing DNA MeTase activity and DNA methylation and cellular aging and 3) distinguish the changes in gene expression that occur during cellular aging and provide the trigger for cellular senescence from the myriad changes that occur as a result of p21 induction and growth arrest.

PUBLICATIONS:
Young JI, Smith JR. DNA Methyltransferase Inhibition in Normal Human Fibroblasts Induces a p21-dependent Cell Cycle Withdrawal. J Biol Chem. 2001 Jun 1;276(22):19610-6.

Thomas M, Popnikolov NK, Scott C, Smith JR, Hornsby PJ. Contrasting Roles of p57(KIP2) and p21(WAF1/CIP1/SDI1) in Transplanted Human and Bovine Adrenocortical Cells. Exp Cell Res. 2001 May 15;266(1):106-13.

Ran, Q, Wadhwa, R, Kawai, R, Kaul, S.C., Sifers, R.N., Bick, R.J., Smith, J.R. and Pereira-Smith, O.M. Extramitochondrila localization of mortalin/mthsp70/PBP74/GRP75. Bichem. Biophys. Res. Comm. 275:174-179, 2000.