Toll-Like Receptor Activation Promotes Multiple Myeloma Cell Growth and Survival By Suppression Of Endoplasmic Reticulum Stress Factor CHOP

Multiple Myeloma (MM) patients are vulnerable to infections, which remain a major cause of death, including early death. During infection, human immune cells sense the presence of invading pathogens through the toll-like receptor family (TLR) of receptors. TLRs detect microbes to activate transcriptional programs that orchestrate adaptive responses to specific insults. This means that they induce the endoplasmic reticulum (ER) unfolded protein response (UPR) to accommodate essential protein translation. If the UPR fails to resolve the protein-folding defect, due to severe and prolonged ER stress, apoptosis is activated. However, the timing sequence of the prolonged ER stress has probably implications for ER stress-induced apoptosis that might help cells to adapt under these conditions rather than driving them to apoptosis. Studies have shown that prolonged ER stress occurs in response to microbes and specifically when cells are exposed to lipopolysaccharide (LPS), a TLR4 activator. The prolonged stress, possibly arising from a massive increase in protein synthesis, has shown to suppress CHOP, an apoptosis biomarker, in ER-stressed macrophages, while low levels of CHOP expression promotes B cell survival. Expression and function of TLRs in MM has recently become the focus of several studies and although the regulatory role of TLRs in MM plasma cells has been reported, the underlying molecular mechanisms remain unclear. It has been shown that human myeloma cell lines (HMCL) and primary myeloma cells express high levels of TLRs and specifically of TLR-4 and TLR-9. The aim of our study was to investigate TLR4 signaling in myeloma cells and to explore possible implications with endoplasmic reticulum unfolded protein response as a potential mechanism of drug resistance.

We initially investigated whether TLR-4 is expressed in human myeloma cell lines and primary myeloma cells and we found that TLR-4 mRNA is expressed at increased levels (2-10 fold) both in HMCLs and primary cells. To test the hypothesis that TLR-4 signaling may suppress CHOP expression during sustained UPR response, two myeloma cell lines, H929 and U266, were pre-treated with low dose LPS (1 ng/mL) and then subjected to ER stress conditions by treatment with tunicamycin (TM). LPS pre-treatment significantly decreased CHOP mRNA expression after 24 hours. Despite the marked suppression of CHOP, LPS pre-treatment of these myeloma cell lines did not suppress ATF4 mRNA levels which also were not altered by TM treatment. LPS pre-treatment did not also suppress XBP-1 splicing compared to the control ER-stressed cells. To test the specificity of these effects, the same set of experiments where performed on other cancer tissues such as ovarian cancer cell lines. Interestingly, although LPS pre-treatment increased TLR-4 mRNA expression in SKOV3 ovarian cancer cell line, CHOP mRNA levels remained intact prior and after treatment while TM treatment did not make any difference in CHOP mRNA expression. These results suggest the relevance of exploring this pathway in tissues such as plasma cells which are highly dependent on the UPR as a repair mechanism. Pre-treated LPS and TM samples of HMCLs were also subjected to Annexin-PI staining to determine the amount of apoptosis. As expected, pre-treated LPS myeloma cells which were exposed to TM had 30% lower Annexin-FITC stained cells compared to the TM-stressed cells only. These data suggest that blockage of CHOP by TLR4 ligands may promote the growth and survival of MM cells. We then examined the impact of therapy with bortezomib on TLR4 and CHOP mRNA expression in primary tumors cells which were collected before and at day 7 after bortezomib-based therapy from 6 myeloma patients. In 5 out of 6 cases TLR-4 expression was significantly up-regulated and was accompanied with a coupled down-regulation of CHOP mRNA expression.

In conclusion, our data suggest that the TLR-4 signaling pathway might provide a translational control pathway which enables cells to carry out essential protein synthesis and avoid CHOP-induced apoptosis. Further exploration of this pathway is needed to establish its role as a potential mechanism of drug resistance.