Biochemical characterization of nonapoptotic cell death in two human gastrointestinal tumor cell lines.

Abstract

The aim of this study is to analyze different aspects of the cell death processes in HuTu80 and HT29 cell lines to determine if either process is a form of programmed cell death (PCD). In HuTu80 cells, drug-induced cytotoxicity is accompanied by DNA fragmentation from $\approx$50 to 200 kb. In HT29 there is a broader distribution of fragments, ranging from $\approx$50 kb to 1-2 Mb. I compared autodigestion of nuclei from both cell lines, to determine if the differences in intact cells were preserved. At early times, the fragment distributions from nuclei recapitulated the distributions in the corresponding intact cells. Autodigestion in both types of nuclei was more extensive in hypotonic buffer and was activated by acidic conditions and did not require the addition of ATP or divalent cation. To determine if the death process in either cell line has any aspects in common with apoptosis, I prepared clones of each line that expressed human Bcl-2 and measured the sensitivity of these clones to drug exposure. In HuTu80 cells, Bcl-2 abrogated both loss of clonogenicity and DNA fragmentation after exposure to fluorodeoxyuridine (FdUrd) and aphidicolin. In HT29 cells Bcl-2 did not protect against FdUrd- and aphidicolin-induced cytotoxicity, but there was significant protection from FdUrd-induced DNA fragmentation. Bcl-2 did not affect cellular responses to etoposide or doxorubicin. To determine the involvement of wild-type p53 (wtp53) in modulating cell death in HT29 cells (endogenously mutp53), clones of HT29 were produced that expressed a temperature sensitive mutant p53 protein that takes on the wild-type conformation at 32.1$\sp\circ$C. In the clonal lines, in the absence of exogenous stimuli, wtp53 expression induced an up-regulation of both endogenous Bcl-x$\sb{\rm L}$ and Bax, conflicting biochemical modulators of PCD. From these studies I have concluded that (1) the endonucleolytic activity that generates the distinct fragmentation patterns seen in HT29 and HuTu80 cells is most similar to DNase II; (2) Bcl-2 is able to protect HuTu80 cells from drug-induced DNA fragmentation and cytotoxicity, suggesting that the death process in these cells may share a common pathway with apoptosis; and (3) wtp53 expression in HT29 cells is unable to induce down-regulation of endogenous Bcl-x$\sb{\rm L}$, suggesting a possible mechanism for HT29 resistance to PCD induction.