生物论文代写:The End Replication Problem Of Cellular Senescence

ROS generation:

In the process of creation mammalian cells , antioxidant enzymes has developed to cooperate and remove ROS generation , which is generated under normal physiology conditions by aerobic metabolism. However, during these processes many enzymes are involved to enhance the reaction such as; catalase (CAT), glutathione reductase (GR), manganes superoxide dismutase (MnSOD), glutathione peroxidise (GPx), and copper/zinc superoxide dismutase (Cu/ZnSOD) (Beckman et al., 1998). The activation by Cu/ZnSOD and MnSOD lead to change the superoxide anions to H2O2. Then, the activation by GPx or CAT changes the H2O2 to water( Lee HC and Wei YH,2007). To protect a suitable redox status of the cells and to maintain the sulfhydry groups of proteins, GSH is used. GPx is used to generate oxidized glutathione, which is then reduced by GR to GSH. However, the association between proteins and anti-oxidant enzymes, together with other small-molecular -weight antioxidants, may remove ROS and free radicals under normal physiologic condition (Beckman et al.,1998). Whereas, the over generation of ROS may lead to destroy the antioxidant system and lead to DNA damage. Studies show that, most of the antioxidant enzymes are decreased with age expect MnSOD which is raise up to 65 years and then start to reduce (Lu et al., 1999). From these finding we can suggest that, oxidative stress increased by the decrease of antioxidant capacity and disorders in the expression of free radicals scavenging enzymes, which may lead to increase cells damage in the aging process.

Oxidative Damage:

DNA damage by oxidative agents may lead to changes the purine and pyrimidine bases sequences, breaks down of the single and double-strand of DNA, and reacts with other molecules. So many changes of nuclear DNA and mtDNA of tissue cells are raised with the age (Beckman et al., 1998). Thereafter, features of human mtDNA become highly influenced to cause cell injury. However, during these reaction several changes has happened such as, the reaction take place near to site of ROS generation from the respiratory chain, lack of prevention by histones, and limitation to repair DNA damage (Ames et al., 1993). It has been shown that, the progression of DNA polymerase is blocked by the damaged nucleotides, which is causing of decreased amplification of the target sequence (Yakes et al,. 1997).

Loss of telomere integrity:

Telomerase is an enzyme whose synthesised Telomeric DNA and this enzyme play a key role in detecting the starting of replicative senescence. However, telomeric DNA become shorter in case of absence or low expressed of telomerase during cell division. This phenomenon called telomere integrity, which is triggering senescence by trigger DNA damage checkpoints and then stop cell multiplication ( Kurz et al., 2004) ) . As mention above, the significant progressive oxidative damage of DNA resulting from the frequent contact of cellular contents to oxidative stress is one of the important processes that related to aging and age-related pathologies. However, studies on fibroblasts shows that oxidative stress may activate the growth of replicative senescence ( Kurz et al., 2004 ). This process called stress-induces premature senescence (Toussaint et al., 2000). Several studies show that, the starting point of senescence has been caused by accelerated telomere attrition, most likely resulting from the formation of single strand breaks in the telomere (von Zglinicki, 2002). Some other studies illustrated that; telomere damage may not attributed by stress-induced premature senescence (Chen et al., 2001; Toussaint et al., 2002). However, it is not clear why these differences are their between the studies. It may be of cell type, the type or level of oxidative used in the studies, the level of antioxidant protection and may be for some other experimental differences.

The G1/S control in cellular senescence:

pRb is a vital protector of cell cycle progression in eukaryotes. Ther are several factors that are regulate the activity of pRb, such as acetylation, ubiqitination and phosphorylation. However, the activity of pRb may creat a block on G1 progression that is developed by phosphorylation (Sherr and McCormick, 2002). Furthermore, a series of cyclin -dependent kinases (CDKs), CDK2, CDK4 and CDK6, play a vital function in the phosphorylation of pRb (Rowland and Bernards, 2006). When CDKs phosphorylated pRb , pRb lose its ability to join E2F/Dp transcription factor complexes, which may result in entry into S-phase of the cell cycle. The action of CDKs is inactivated in senescent cells due to release of CDK inhibitors. There are two main CDK inhibitors that are released by senescent cells such as, p21Cip1/Waf1/Sdi1 and P16NK4a. However, p21Cip1/Waf1/Sdi1 work to links between p53 pathway and Rb-pathway to provide a tight security network in the direction of tumour suppression (Gil, Peters G, 2006). Several studies indicated that, the regulation of p21Cip1/Waf1/Sdi1 expression has a vital role in processes like cellular senescence and DNA damage-induced cell cycle, which may prevent the cells to become carcinoma cell (Sherr and Roberts, 1999). p16INK4a is wok to inhibit the activation of D-type CDKs, CDK4 and CDK6. When the p16INK4a is binding to CDK4 and CDK6, it stimulates the redistribution of Cip/Kip family CDK inhibitors, P27Kip1 and P21Cip1/Waf1/Sid1, from cyclinD-CD4/6 to cyclinE-CDK2 complexes, causing inactivation of CDK2-kinase. Thereafter, the association between p16INK4a and p21Cip1/Waf1/Sdi1 prevent the phosphorylation of pRb, which result in stable G1 arrest in senescent cells (Gil and Peters, 2006) . Interestingly, the p16INK4a gene is recognized as a tumour suppressor gene because it is often inactivated in a wide range of human cancers. In addition, it may also participate with another tumour suppressor gene called p14ARF. Studies show that the mutation within this region only affects p16INK4a activity but not p14ARF activity. This finding suggests that, p16INK4a /Rb-pathway play a vital function in tumour suppression (Gil and Peters, 2006).

Cytokinetic block:

However, it is known that, the p16INK4a inactivate the pRb pathway, following the inactivation of pRb pathway induce the DNA synthesis but not cell proliferation, this happened if p16INK4a is ectopically released prior to inactivation of pRb in human cells. Whereas, inactivation of pRb is adequate to inactivate the p16INK4a effect, this is happened if pRb pathway is inactivated prior to p16INK4a expression (Gil and Peters, 2006). However, when p16INK4a activated pRb so pRb enhance another pathway which is irreversibly causes cell cycle arrest either in M or G2 phase. Moreover, poly-nucleated cells is gradually increase when the human cells inactivated by pRb and P53 and release high level of p16INK4a (Takahashi et al., 2006). This finding may explain that this mechanism may target cytokinesis. Studies using SVts8 cells indicated that, p16INK4a/ Rb pathway associate with mitogenic signals to stimulate irreversible cytokinetic block during the generation of reactive oxygen species (ROS) (Takahashi et al., 2006). It has been mention before that, the physiological function of the cells required ROS, but overproduction of ROS may lead to apoptosis or cellular senescence. In case of low stress, mitogenic signals inactivate pRb , which is then stimulate E2F/DP complexes to induce S-phase entry. E2F/DP activation reduces the amount of ROS (Takahashi et al., 2006). These finding shows that, the E2F/DP activity manage this reaction in proliferating normal human cells, even mitogenic signals have the power to generate ROS. Whereas, in case of high cellular stress, p16INK4a/Rb -pathway inactivate E2F/DP. So in this case, mitogenic signals stimulate the ROS production by activating PKCδ, which is a significant downstream mediator of the ROS signalling pathway.

Interestingly, when ROS and PKCδ activate ROS signalling pathway lead to the over generation of ROS, which may create a positive feedback loop to mainatain ROS- PKCδ signalling. In human senescent cells, maintained activation of ROS- PKCδ signaling permanently blocks cytokinesis by decreasing WARTS and a mitotic exit network (MEN) kinase, which are required for cytokinesis (Takahashi et al., 2006). This may lead to increase the level of p16INK4a and create an autonomous stimulation of ROS- PKCδ signalling, result in an irreversible block to cytokinesis in human senescent cells. However, this mechanism may act as a fail-safe mechanism, practically in case of the sudden inactivation of p53 and pRb in human senescent cells (Ramsey and Sharpless, 2006).