Purpose: Several HSP-70 genes have been cloned and sequenced in human cells. Among these genes, the HSP-70A mRNA and protein levels correlate best with the development and decay of thermotolerance and intrinsic thermal sensitivity. Leukemic and nonleukemic human tumor cells express low levels of the normally heat inducible HSP-70A mRNA in control nonheated cells. Using a competitive quantitative polymerase chain reaction, we have measured the levels of mRNA for this gene and have correlated it with both transient and intrinsic thermal sensitivity of tumor cells. Such studies were also extended to tumor samples obtained from patients.
Methods and materials: In these studies, the plasmid phHSP-70 which contains the entire human HSP-70A gene was modified by the insertion of the T7 promoter at the 5'-end untranslated region as well as the insertion of a 23 bp synthetic linker at the BamH1 site in the promoter region of the HSP-70A gene. The PCR primers were located such that the amplified fragment contained the linker. Using the T7 polymerase, the HSP-70A mRNA was transcribed from this plasmid (phHSP-70L) in vitro. A known amount of HSP-70A mRNA was then added to the total RNA prepared from the cell samples or from the tumor tissues obtained from patients. Using the components of the PCR reaction plus known amounts of HSP-70A mRNA synthesized from phHSP-70L and unknown amounts of total cellular RNA, the samples were amplified and analysed on a denaturing acrylamide gel. The PCR products obtained from phHSP-70L were 23 bp larger than the PCR products obtained from the cell samples due to the addition of the synthetic linker to the HSP-70A gene in phHSP-70L and therefore, the two products could be easily distinguished from each other and quantitated. The alpha-32P-dCTP incorporated in each sample was quantitated by AMBIS Scanner. When the 32P-counts were equal in the known and the unknown samples, the amount of the HSP-70A mRNA was taken to be equal in the known and the unknown sample.
Results: The results show that HSP-70A mRNA levels can be used to predict the survival levels during the development and decay of thermotolerance. In nonleukemic human tumor cell lines, there are as much as 40-50-fold induction of HSP-70A mRNA levels during the peak of thermotolerance. In leukemic cell lines, however, HSP-70A mRNA levels are induced only by three-fold during the same time period. These differences between the levels of HSP-70A mRNA positively correlate with the amount of tolerance development in leukemic and nonleukemic tumor cells. HSP-70A mRNA levels also vary in different tumor cells under nonheated conditions and there is a positive correlation between HSP-70A mRNA levels in nonleukemic human tumor cells and the level of their intrinsic thermal resistance.
Conclusion: HSP-70A mRNA levels can be used to predict the intrinsic thermal sensitivity of nonleukemic human tumor cells.