الفهرس 
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Abstract
Heat shock proteins (HSPs) are highly conserved molecular chaperones with divergent roles in various cellular processes. The HSPs are classified according to their molecular size. The HSPs prevent nonspecific cellular aggregation of proteins by maintaining their native folding energetics. The disruption of this vital cellular process, driven by the aberrant expression of HSPs, is implicated in the progression of several different carcinomas. Many HSPs are also actively involved in promoting the proliferation and differentiation of tumor cells, contributing to their metastatic phenotype.
HSP90, HSP70 and HSP27 are the three most well investigated strain-inducible HSPs. Unusually high levels of the HSPs expression and/or activity are present in the majority of cancer cells, and they are further accelerated by a number of death-related stimuli. They are potent anti-apoptotic proteins that interact with key apoptosis factors and inhibit this cell death process at various stages.
The study aims to investigate the role of heat shock proteins 27, 40, 70 and 90 in inducing radioresistance by preventing apoptosis induced by radiation in colon cancer cells, and to explore the possible role of HSP inhibition in overcoming radioresistance.
The present study is an in vitro study employing the human colorectal cancer cell lines Caco-2 and HCT-116, each of which was divide into four groups:
group 1: Cells were exposed to radiation (at doses 2, 4, 6, 8 and 10 Gy).
group 2: Cells were treated by quercetin as a HSPs inhibitor.
group 3: Cells were exposed to a combination of HSPs inhibitor and radiation (at doses 2, 4, 6, 8 and 10 Gy).
group 4: Treatment cells were grown without HSP inhibitor or radiation.
The human colon cancer Caco-2 and HCT-116 cell line were cultured. HSPs inhibition was done using Quercetin at 15 and 75 μM concentrations. 24h after treatment by quercetin, exposure to radiation was carried out using X-ray from linear accelerator 6 MV at 2, 4, 6, 8 and 10 Gy. Real-time PCR was used to detect the expression of the following: HSF1, HSPs (27, 40, 70 and 90), P21, Caspase-3 and STAT-3. Cell viability was measured by MTT assay.
Our results showed that:
• In HCT cells, exposure to radiation resulted in a significant inhibition in HSF-1 expression at all radiation doses, while in Caco cells, a mild increase in HSF1 expression was noticed after irradiation. When treated by both quercetine and radiation, HCT cells showed HSF1 inhibition at all radiation doses, while Caco cells showed HSF1 up regulation at all radiation doses. The up-regulation in Caco cells was dependent on quercetine dose.
• In HCT cells, expression of HSPs 27 and 70 were significantly decreased in response to radiation alone or with quercetin. In Caco cells HSPs 27 and 70 showed a slight increase in response to radiation, while they were highly up-regulated when treated by radiation and quercetin. This up-regulation increased with increasing quercetin concentration.
• HSP90 expression was down-regulated in response to radiation in both cell lines. When treated by radiation and quercetin, HCT cells showed inhibition 15μM, but at 75μM the HSP90 expression was restored. In Caco cells treatment by radiation and quercetin caused HSP90 induction in a manner that was dependent on quercetin concentartion.
• In HCT cells, all three apoptotic markers; STAT3, caspase-3 and p21, followed the same pattern of expression through all treatment conditions. Exposure to radiation caused a significant and sustained down-regulation of all apoptotic markers through all radiation doses. Upon treatment by 15μM quercetin with radiation, the inhibition of all three parameters was still dominant; however, treatment by 75μM quercetin with radiation restored the expression of all markers; STAT3, caspase-3 and p21, especially at higher radiation doses.
• In Caco cells too, all three apoptotic markers followed the same pattern of expression through all treatment conditions. Exposure to radiation caused a significant decrease in expression of all apoptotic markers, STAT3, caspase-3 and p21, which was dose-dependent in STAT3 and p21. Upon treatment by 15μM quercetin with radiation, a small but significant up-regulation of all three parameters was seen; however, treatment by 75μM quercetin with radiation caused a higher up-regulation of all parameters.
• Upon irradiation, HCT cells showed less mean percent survival than Caco cells. However, concomitant treatment by radiation and 12.5M quercetin did not have a significant effect on viability of either HCT or Caco cells. Exposure
to radiation in presence of 75M quercetin significantly reduced cell viability of HCT cells, but it had an even greater effect on viability of Caco cells, where percent survival levels decreased to values comparable to those of HCT cells.