Identification of biological functions and gene networks regulated by heat stress in U937 human lymphoma cells

Abstract

Abstract. although cancer cells exposed to temperatures >42.5˚C undergo cell death as the temperature rises, exposure of up to 42.5˚C induces slight or no cytotoxicity. The temperature of 42.5˚C is, therefore, well known to be the inflection point of hyperthermia. To better understand the molecular mechanisms underlying cellular responses to heat stress at temperatures higher and lower than the inflection point, we carried out global scale microarray and computational gene expression analyses. Human leukemia U937 cells were incubated at 42˚C or 44˚C for 15 min and cultured at 37˚C for 0-6 h. Apoptosis accompanied by the activation of caspase-3 and DNA fragmentation was only observed in cells treated with heat stress at 44˚C, but not at 42˚C. Although a large number of genes were differentially expressed by a factor of 2.0 or greater, we found substantial differences with respect to the biological functions and gene networks of the genes differentially expressed at the two temperatures examined. Interestingly, we identified temperature-specific gene networks that were considered to be mainly associated with cell death or cellular compromise and cellular function and maintenance at 44˚C or 42˚C, respectively, by using the Ingenuity pathway analysis tools. These findings provide the molecular basis for a further understanding of the mechanisms of the biological changes that are responsive to heat stress in human lymphoma cells.

Induction of Oxidative Stress by Hyperthermia and Enhancement of Hyperthermia-Induced Apoptosis by Oxidative Stress Modification

“…When U937 cells were exposed to HT (44 C, 10 min; 15 % apoptosis level), 39 upregulated and 3 downregulated genes were identified. From these genes and our previous findings [18,19,45], we found a gene network containing many HSPs, BCL2-associated athanogene 3 (BAG3), protein kinases c-JUN N-terminal kinase (JNK; MAPK8), nuclear factor, erythroid 2-like 2 (NFE2L2; NRF2) and caspase-3 (CASP3) [18] (Fig. 2.3a).…”

“…It is already well known that the JNK pathway is involved in apoptosis evoked by HT [50]. In U937 cells, activations of CASP3 and JNK have been observed under apoptotic conditions [18,19,45]. Interestingly, functional category analysis predicted activation of an NRF2-mediated antioxidant response in HT-treated U937 cells [45].…”

“…In mammals, the expression of HSPs, HSPA (heat shock 70 kDa proteins) family members, such as HSPA1A/B and HSPA6, DNAJ (DNAJ (Hsp40) heat shock proteins) family members, such as DNAJA1 and DNAJB1, or BAG3 is mainly regulated by HSF1 [48,49], and these proteins function as cytoprotective and anti-apoptotic molecules against various stressors, particularly HT [1,48,49]. The induction of HSPs appears to be a common response to HT as described in many previous microarray studies [18,44,45]. Sequential activation of caspases plays a central role in the execution phase of apoptosis.…”

“…In U937 cells, activations of CASP3 and JNK have been observed under apoptotic conditions [18,19,45]. Interestingly, functional category analysis predicted activation of an NRF2-mediated antioxidant response in HT-treated U937 cells [45]. NRF2 is an emerging regulator of cellular resistance to oxidants, including ROS.…”

“…Moreover, bioinformatics analysis now allows for the mapping of gene expression data into relevant pathway maps based on their functional annotation and known molecular interactions. Using either microarray or bioinformatics technology, many genes and gene networks responsive to HT have been identified [18,19,44,45]. Moreover, proteomics techniques have revealed valuable information about the HT response in cancer cells [46,47].…”