Abrogation of p53 by its antisense in MCF-7 breast carcinoma cells increases cyclin D1 via activation of Akt and promotion of cell proliferation

Resistin causes G1 arrest in colon cancer cells through upregulation of SOCS3

Resistin, a proinflammatory cytokine, is elevated in a number of pathological disorders, including cancer. The serum resistin level in colon cancer patients is elevated and correlates with tumor grade. However, the implications of increased resistin on colon cancer cells remain unclear. In the present study, we find that resistin binds to TLR4 on colon cancer cell membrane and initiates TLR4-MyD88-dependent activation of ERK. In addition, the upregulation of SOCS3 by ERK downregulates the JAK2/TAT3 pathway and causes the arrest of cells in G1 phase. Interestingly, we observe that resistin-exposed cells survive 5-fluorouracil treatment because of a decrease in drug uptake due to the arrest of cells in G1 phase.

Constitutively activated ERK sensitizes cancer cells to doxorubicin: Involvement of p53-EGFR-ERK pathway

The tumour suppressor gene p53 is mutated in approximately 50% of the human cancers. p53 is involved in genotoxic stress-induced cellular responses. The role of EGFR and ERK in DNA-damage-induced apoptosis is well known. We investigated the involvement of activation of ERK signalling as a consequence of non-functional p53, in sensitivity of cells to doxorubicin. We performed cell survival assays in cancer cell lines with varying p53 status: MCF-7 (wild-type p53, WTp53), MDA MB-468 (mutant p53, MUTp53), H1299 (absence of p53, NULLp53) and an isogenic cell line MCF-7As (WTp53 abrogated). Our results indicate that enhanced chemosensitivity of cells lacking wild-type p53 function is because of elevated levels of EGFR which activates ERK. Additionally, we noted that independent of p53 status, pERK contributes to doxorubicin-induced cell death.

Strategy to enhance efficacy of doxorubicin in solid tumor cells by methyl-β-cyclodextrin: Involvement of p53 and Fas receptor ligand complex

Doxorubicin (DOX) is one of the preferred drugs for treating breast and liver cancers. However, its clinical application is limited due to severe side effects and the accompanying drug resistance. In this context, we investigated the effect on therapeutic efficacy of DOX by cholesterol depleting agent methyl-β-cyclodextrin (MCD), and explored the involvement of p53. MCD sensitizes MCF-7 and Hepa1–6 cells to DOX, Combination of MCD and marginal dose of DOX reduces the cell viability, and promoted apoptosis through induction of pro-apoptotic protein, Bax, activation of caspase-8 and caspase-7, down regulation of anti-apoptotic protein Bcl-2 and finally promoting PARP cleavage. Mechanistically, sensitization to DOX by MCD was due to the induction of FasR/FasL pathway through p53 activation. Furthermore, inhibition of p53 by pharmacological inhibitor pifithrin-α (PFT-α) or its specific siRNA attenuated p53 function and down-regulated FasR/FasL, thereby preventing cell death. Animal experiments were performed using C57BL/6J mouse isografted with Hepa1–6 cells. Tumor growth was retarded and survival increased in mice administered MCD together with DOX to as compared to either agent alone. Collectively, these results suggest that MCD enhances the sensitivity to DOX for which wild type p53 is an important determinant.

Inherent and acquired resistance to paclitaxel in hepatocellular carcinoma: molecular events involved

Hepatocellular carcinoma (HCC) is a primary malignancy of the liver and is a major cause of cancer related deaths worldwide. Only 10 to 20% of HCC can be surgically excised. Therefore, chemotherapeutic intervention and treatment is essential for achieving favorable prognosis. However, therapeutic outcome of chemotherapy is generally poor owing to inherent resistance of cancer cells to the treatment or due to development of acquired resistance. To differentiate and delineate the molecular events, we developed drug resistant Hep3B cells (DRC) by treating cells with the increasing concentration of paclitaxel. We also developed a unique single cell clone of Hep3B cells (SCC) by selecting single cell colonies and screening them for resistant phenotype. Interestingly, both DRC and SCC were resistant to paclitaxel in comparison to parental Hep3B cells. We analyzed the contributory factors that may be involved in the development of resistance. As expected, level of P-glycoprotein (P-gp) was elevated in DRC. In addition, Caveolin-1 (Cav-1), Fatty acid synthase (FASN) and Cytochrome P450 (CYP450) protein levels were elevated in DRC whereas in SCC, FASN and CYP450 levels were elevated. Downregulation of these molecules by respective siRNAs and/or by specific pharmacological inhibitors resensitized cells to paclitaxel. Interestingly, these drug resistant cells were also less sensitive to vinblastine, doxorubicin and methotrexate with the exception of cisplatin. Our results suggested that differential levels of P-gp, Cav-1 and FASN play a major role in acquired resistant phenotype whereas FASN level was associated with the presentation of inherent resistant phenotype in HCC.

Cdk5 phosphorylates non-genotoxically overexpressed p53 following inhibition of PP2A to induce cell cycle arrest/apoptosis and inhibits tumor progression

Background

p53 is the most studied tumor suppressor and its overexpression may or may not cause cell death depending upon the genetic background of the cells. p53 is degraded by human papillomavirus (HPV) E6 protein in cervical carcinoma. Several stress activated kinases are known to phosphorylate p53 and, among them cyclin dependent kinase 5 (Cdk5) is one of the kinase studied in neuronal cell system. Recently, the involvement of Cdk5 in phosphorylating p53 has been shown in certain cancer types. Phosphorylation at specific serine residues in p53 is essential for it to cause cell growth inhibition. Activation of p53 under non stress conditions is poorly understood. Therefore, the activation of p53 and detection of upstream kinases that phosphorylate non-genotoxically overexpressed p53 will be of therapeutic importance for cancer treatment.

Results

To determine the non-genotoxic effect of p53; Tet-On system was utilized and p53 inducible HPV-positive HeLa cells were developed. p53 overexpression in HPV-positive cells did not induce cell cycle arrest or apoptosis. However, we demonstrate that overexpressed p53 can be activated to upregulate p21 and Bax which causes G2 arrest and apoptosis, by inhibiting protein phosphatase 2A. Additionally, we report that the upstream kinase cyclin dependent kinase 5 interacts with p53 to phosphorylate it at Serine20 and Serine46 residues thereby promoting its recruitment on p21 and bax promoters. Upregulation and translocation of Bax causes apoptosis through intrinsic mitochondrial pathway. Interestingly, overexpressed activated p53 specifically inhibits cell-growth and causes regression in vivo tumor growth as well.

Conclusion

Present study details the mechanism of activation of p53 and puts forth the possibility of p53 gene therapy to work in HPV positive cervical carcinoma.

Survival advantage of AMPK activation to androgen-independent prostate cancer cells during energy stress

Androgen-independent prostate cancer usually develops as a relapse following androgen ablation therapy. Removing androgen systemically causes vascular degeneration and nutrient depletion of the prostate tumor tissue. The fact that the malignancy later evolves to androgen-independence suggests that some cancer cells are able to survive the challenge of energy/nutrient deprivation. AMP-activated protein kinase (AMPK) is an important manager of energy stress. The present study was designed to investigate the role of AMPK in contributing to the survival of the androgen-independent phenotype. Most of the experiments were carried out in the androgen-dependent LNCaP cells and the androgen-independent C4-2 cells. These two cell lines have the same genetic background, since the C4-2 line is derived from the LNCaP line. Glucose deprivation (GD) was instituted to model energy stress encountered by these cells. The key findings are as follows. First, the activation of AMPK by GD was much stronger in C4-2 cells than in LNCaP cells, and the robustness of AMPK activation was correlated favorably with cell viability. Second, the response of AMPK was specific to energy deficiency rather than to amino acid deficiency. The activation of AMPK by GD was functional, as demonstrated by appropriate phosphorylation changes of mTOR and mTOR downstream substrates. Third, blocking AMPK activation by chemical inhibitor or dominant negative AMPK led to increased apoptotic cell death. The observation that similar results were found in other androgen-independent prostate cancer cell lines, including CW22Rv1 abd VCaP, provided further assurance that AMPK is a facilitator on the road to androgen-independence of prostate cancer cells.

Hyperthermia-associated carboplatin resistance: differential role of p53, HSF1 and Hsp70 in hepatoma cells

Due to substantial technical improvements, clinical application of heat as a co-adjuvant in cancer treatment is acquiring new interest. The effect of hyperthermia on hepatoma cell lines Hep3B (p53 defective) and HepG2 (p53 wild type) when investigated led to an interesting observation that Hep3B cells are more susceptible to heat stress than HepG2 cells. In addition, heat-induced carboplatin resistance was observed in HepG2 cells only. To investigate the reasons, heat shock response was explored and it was observed that heat stress augmented heat shock protein 70 (Hsp70) expression levels in HepG2 and not in Hep3B cells. Furthermore, in HepG2 cells, induced Hsp70 is regulated by both p53 and heat shock transcription factor 1 (HSF1) wherein HSF1 levels are modulated by p53. The data implies that Hep3B are more susceptible to death upon heat stress than HepG2 cells because of non-induction of Hsp70. In addition, it was observed that inhibition of heat-induced p53/HSF1 diminishes Hsp70 levels, thereby restoring the sensitivity of heat-stressed HepG2 cells to carboplatin-triggered cell death. Collectively, the present study establishes interplay of p53, HSF1, and Hsp70 upon heat stress in HepG2 cells and also defines novel strategies to overcome constraints of utility of hyperthermia in cancer therapy through p53/HSF1-targeted therapeutic intervention.

Mitomycin C induces bystander killing in homogeneous and heterogeneous hepatoma cellular models

Background

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide that is particularly refractory to chemotherapy. Several studies have proposed combination chemotherapy regimen for HCC treatment. However, these therapies are not effective in regressing tumor and prolonging survival of patient's suffering from HCC. Therefore, the development of more effective therapeutic tools and new strategies for the treatment of HCC are urgently needed. Over the last decade much attention has been focused on "bystander effect" as a possible therapeutic strategy for the treatment of certain human tumors. Interest in this therapeutic approach originated from numerous reports describing the radiation induced bystander effect. However, the knowledge about chemotherapy induced bystander effect is still limited. Hence, chemotherapy induced bystander phenomenon in hepatoma cells was explored by utilizing Mitomycin C (MMC).

Results

MMC induced bystander killing was observed only in hepatoma cells and it did not occur in cervical cancer cells. MMC induced bystander killing was transferable via medium. It occurred in co-cultured cells indicating the involvement of secreted as well as membrane bound factors. FasL and TRAIL were detected in the conditioned medium from treated cells. In medium transfer experiment, pre-treatment with EDTA (a broad range protease inhibitor) diminished MMC induced bystander killing. Following drug exposure, expression of Fas and TRAIL receptors increased and treatment with neutralizing antibodies against FasL and TRAIL inhibited bystander killing.

Conclusion

Our results highlight the therapeutic importance of MMC in the treatment of HCC and implicate role of membrane bound and secreted forms of FasL and TRAIL in MMC induced bystander killing.

Prx1 enhances androgen receptor function in prostate cancer cells by increasing receptor affinity to dihydrotestosterone

Androgen receptor (AR) signaling plays a critical role in the development and progression of prostate cancer. It has been reported previously that peroxiredoxin-1 (Prx1), a member of a novel family of peroxidases, interacts physically with AR to enhance AR transactivation of target genes. In the present study, we evaluated the biological significance of Prx1 in modulating dihydrotestosterone (DHT)-stimulated growth and AR target gene expression of prostate cancer cells. We also investigated the mechanism by which Prx1 might potentiate AR signaling. The contribution of Prx1 was assessed mainly by using the approach of stable Prx1 knockdown. The major observations are as follows: (a) A low level of Prx1 desensitizes cells to growth stimulation and AR target gene induction by DHT, such that exposure to a higher level of DHT is required to reach the same magnitude of response when Prx1 is depressed; (b) Prx1 increases the affinity of AR to DHT and decreases the rate of DHT dissociation from the occupied receptor; (c) Prx1 enhances the NH2 terminus and COOH terminus interaction of AR; a stronger N-C interaction is consistent with a more robust AR activation signal by keeping DHT tight in the ligand-binding pocket; (d) the stimulatory effects of Prx1 on AR ligand binding affinity and AR N-C interaction are manifested regardless of a wild-type or mutant AR. The above findings led us to believe that Prx1 may be a therapeutic target in blocking the transition of prostate cancer from an androgen-dependent to an androgen-refractory phenotype.