Stjepan Uldrijan

The plasmamembrane calmodulin–dependent calcium pump a major regulator of nitric oxide synthase I

The plasma membrane calcium/calmodulin-dependent calcium ATPase (PMCA) (Shull, G.E., and J. Greeb. 1988. J. Biol. Chem. 263:8646–8657; Verma, A.K., A.G. Filoteo, D.R. Stanford, E.D. Wieben, J.T. Penniston, E.E. Strehler, R. Fischer, R. Heim, G. Vogel, S. Mathews, et al. 1988. J. Biol. Chem. 263:14152–14159; Carafoli, E. 1997. Basic Res. Cardiol. 92:59–61) has been proposed to be a regulator of calcium homeostasis and signal transduction networks of the cell. However, little is known about its precise mechanisms of action. Knock-out of (mainly neuronal) isoform 2 of the enzyme resulted in hearing loss and balance deficits due to severe inner ear defects, affecting formation and maintenance of otoconia (Kozel, P.J., R.A. Friedman, L.C. Erway, E.N. Yamoah, L.H. Liu, T. Riddle, J.J. Duffy, T. Doetschman, M.L. Miller, E.L. Cardell, and G.E. Shull. 1998. J. Biol. Chem. 273:18693–18696). Here we demonstrate that PMCA 4b is a negative regulator of nitric oxide synthase I (NOS-I, nNOS) in HEK293 embryonic kidney and neuro-2a neuroblastoma cell models. Binding of PMCA 4b to NOS-I was mediated by interaction of the COOH-terminal amino acids of PMCA 4b and the PDZ domain of NOS-I (PDZ: PSD 95/Dlg/ZO-1 protein domain). Increasing expression of wild-type PMCA 4b (but not PMCA mutants unable to bind PDZ domains or devoid of Ca2+-transporting activity) dramatically downregulated NO synthesis from wild-type NOS-I. A NOS-I mutant lacking the PDZ domain was not regulated by PMCA, demonstrating the specific nature of the PMCA–NOS-I interaction. Elucidation of PMCA as an interaction partner and major regulator of NOS-I provides evidence for a new dimension of integration between calcium and NO signaling pathways.

An essential function of the extreme C‐terminus of MDM2 can be provided by MDMX

MDM2 (HDM2) is a ubiquitin ligase that can target the p53 tumor suppressor protein for degradation. The RING domain is essential for the E3 activity of MDM2, and we show here that the extreme C‐terminal tail of MDM2 is also critical for efficient E3 activity. Loss of E3 function in MDM2 mutants deleted of the C‐terminal tail correlated with a failure of these mutants to oligomerize with MDM2, or with the related protein MDMX (HDMX). However, MDM2 containing point mutations within the C‐terminus that inactivated E3 function retained the ability to oligomerize with the wild‐type MDM2 RING domain and MDMX, and our results indicate that oligomers containing both wild‐type MDM2 and a C‐terminal mutant protein retain E3 function both in auto‐degradation and degradation of p53. Interestingly, the E3 activity of C‐terminal point mutants of MDM2 can also be supported by interaction with wild‐type MDMX, suggesting that MDMX can directly contribute to E3 function.

The plasmamembrane calmodulin-dependent calciumpump: a major regulator of nitric oxide synthase I

The plasma membrane calcium/calmodulin-dependent calcium ATPase (PMCA) (Shull, G.E., and J. Greeb. 1988. J. Biol. Chem. 263:8646–8657; Verma, A.K., A.G. Filoteo, D.R. Stanford, E.D. Wieben, J.T. Penniston, E.E. Strehler, R. Fischer, R. Heim, G. Vogel, S. Mathews, et al. 1988. J. Biol. Chem. 263:14152–14159; Carafoli, E. 1997. Basic Res. Cardiol. 92:59–61) has been proposed to be a regulator of calcium homeostasis and signal transduction networks of the cell. However, little is known about its precise mechanisms of action. Knock-out of (mainly neuronal) isoform 2 of the enzyme resulted in hearing loss and balance deficits due to severe inner ear defects, affecting formation and maintenance of otoconia (Kozel, P.J., R.A. Friedman, L.C. Erway, E.N. Yamoah, L.H. Liu, T. Riddle, J.J. Duffy, T. Doetschman, M.L. Miller, E.L. Cardell, and G.E. Shull. 1998. J. Biol. Chem. 273:18693–18696). Here we demonstrate that PMCA 4b is a negative regulator of nitric oxide synthase I (NOS-I, nNOS) in HEK293 embryonic kidney and neuro-2a neuroblastoma cell models. Binding of PMCA 4b to NOS-I was mediated by interaction of the COOH-terminal amino acids of PMCA 4b and the PDZ domain of NOS-I (PDZ: PSD 95/Dlg/ZO-1 protein domain). Increasing expression of wild-type PMCA 4b (but not PMCA mutants unable to bind PDZ domains or devoid of Ca2+-transporting activity) dramatically downregulated NO synthesis from wild-type NOS-I. A NOS-I mutant lacking the PDZ domain was not regulated by PMCA, demonstrating the specific nature of the PMCA–NOS-I interaction. Elucidation of PMCA as an interaction partner and major regulator of NOS-I provides evidence for a new dimension of integration between calcium and NO signaling pathways.

Cyclin-dependent kinase inhibitors as anticancer drugs.

Poor therapeutic outcomes and serious side effects, together with acquired resistance to multiple drugs, are common problems of current cancer therapies. Therefore, there is an urgent need for new cancer-targeted drugs, which has led (inter alia) to the development of molecules that can specifically inhibit cyclin-dependent kinases (CDKs). In addition to their cell cycle regulatory functions, CDKs, especially CDK7 and CDK9, play important roles in the regulation of RNA polymerase II-mediated transcription. Here, we report on progress in the preclinical development of CDK inhibitors and their anticancer activities. Special attention is paid to the action mechanisms of the pan-specific CDK inhibitors flavopiridol and roscovitine, which have already entered phase II clinical trials as treatments for various tumours. The links between their ability to inhibit transcription and sensitisation of some types of cancer to apoptosis, mechanisms leading to p53 activation, and their synergistic cooperation with common DNA damaging drugs are also discussed. It has been demonstrated that drug-resistant cancer cells can arise during therapeutic application of small molecule protein kinase inhibitors. Clinical resistance to CDK inhibitors has not yet been described, but by comparing CDKs to other kinases, and CDK inhibitors to other clinically used protein kinase inhibitors, we also discuss possible mechanisms that could lead to resistance to CDK inhibitors.

Potent induction of wild-type p53-dependent transcription in tumour cells by a synthetic inhibitor of cyclin-dependent kinases

Abstract. Activation of the p53 tumour suppressor protein by distinct forms of stress leads to inhibition of cellular proliferation by inducing cell cycle arrest or apoptosis. The cyclin-dependent kinase inhibitor roscovitine has been shown to induce nuclear accumulation of wild-type p53 in human untransformed and tumour-derived cells. We analyzed the response of different human tumour cell lines to roscovitine treatment with respect to their p53 status. Striking induction of wild-type p53 protein and dramatic enhancement of p53-dependent transcription, coinciding with p21WAF1 induction, was observed in wild-type, but not mutant, p53-bearing tumour cells after treatment with roscovitine. The transcriptional activity of p53 was substantially higher in roscovitine-treated cells than in cells irradiated with ultraviolet C or ionizing radiation, even though all these agents induced a similar amount of p53 accumulation. These results highlight the therapeutic potential of roscovitine as an anticancer drug, especially in tumours retaining a functional wild-type p53 pathway.

Benzo[c]phenanthridine alkaloids exhibit strong anti-proliferative activity in malignant melanoma cells regardless of their p53 status

BACKGROUND Search for new substances with antiproliferative activity towards melanoma cells is important since malignant melanoma is notoriously resistant to conventional chemotherapy. Benzo[c]phenanthridine alkaloids (BAs) are natural products with significant anti-proliferative activities, therefore they are considered as agents promising for cancer therapy. OBJECTIVES The effects of five BAs (sanguinarine, chelerythrine, chelidonine, sanguilutine, and chelilutine) on human malignant melanoma cell lines were compared. The study focused on BAs effects on DNA, anti-apoptotic and p53 protein levels; and the involvement of p53 in cellular responses to alkaloids treatment. METHODS Melanoma cell lines, two wild types and two with dysfunctional p53 derived from one of them were used. The mechanism of anti-proliferative and pro-apoptotic effects and the effect on DNA was investigated using MTT assay, flow cytometry, Western blot analysis, fluorescence and electron microscopy. RESULTS All tested alkaloids exhibit strong anti-proliferative activity. CHL, CHE and SA induced apoptosis, which was probably mediated by decreasing levels of anti-apoptotic proteins (Bcl-xL, Mcl-1, XIAP) and was accompanied by mitochondrial membrane potential decrease as well as caspase-3 and PARP cleavage. Although all alkaloids caused DNA damage, which was demonstrated by induction of H2AX phosphorylation, none of the tested alkaloids stabilised p53 and their toxicity in cells with non-functional p53 was comparable to wild type cells. CONCLUSION Despite the profound similarity of BAs molecular structures, it is clear that the mechanism of cell death induction is different for each alkaloid. Our results indicate that BAs could be effective in malignant melanoma treatment, including tumours which have lost wild type p53.

Necroptosis modulated by autophagy is a predominant form of melanoma cell death induced by sanguilutine

Abstract We show that the plant quaternary benzo[c]phenanthridine alkaloid sanguilutine (SL) is a strong inducer of caspase-independent non-apoptotic death in human melanoma cells. Necrostatin-1, a specific inhibitor of necroptosis, completely reversed the cytotoxic effect of SL, suggesting that necroptosis was a predominant type of cell death induced by SL in these cells. In addition, we showed that SL can trigger an autophagic response, as confirmed by GFP-LC3 puncta formation and LC3-II accumulation. Interestingly, we observed a significant decrease in the viability of melanoma cells treated with combination of autophagy inhibitors (3-methyladenine, bafilomycin-A1 and LY294002) and SL. Our results further indicated that autophagy may serve as a pro-survival mechanism, delaying the induction of necroptosis in melanoma cells. The ability of SL to induce caspase-independent non-apoptotic cell death (necroptosis) suggests its possible therapeutic potential in the treatment of apoptosis-resistant melanoma tumours. Furthermore, SL might serve as a useful tool for studying the mechanisms of necroptosis and autophagy induction and the interplay between these two processes.

The melanoma-associated antigen 1 (MAGEA1) protein stimulates the E3 ubiquitin-ligase activity of TRIM31 within a TRIM31-MAGEA1-NSE4 complex

The MAGE (Melanoma-associated antigen) protein family members are structurally related to each other by a MAGE-homology domain comprised of 2 winged helix motifs WH/A and WH/B. This family specifically evolved in placental mammals although single homologs designated NSE3 (non-SMC element) exist in most eukaryotes. NSE3, together with its partner proteins NSE1 and NSE4 form a tight subcomplex of the structural maintenance of chromosomes SMC5–6 complex. Previously, we showed that interactions of the WH/B motif of the MAGE proteins with their NSE4/EID partners are evolutionarily conserved (including the MAGEA1-NSE4 interaction). In contrast, the interaction of the WH/A motif of NSE3 with NSE1 diverged in the MAGE paralogs. We hypothesized that the MAGE paralogs acquired new RING-finger-containing partners through their evolution and form MAGE complexes reminiscent of NSE1-NSE3-NSE4 trimers. In this work, we employed the yeast 2-hybrid system to screen a human RING-finger protein library against several MAGE baits. We identified a number of potential MAGE-RING interactions and confirmed several of them (MDM4, PCGF6, RNF166, TRAF6, TRIM8, TRIM31, TRIM41) in co-immunoprecipitation experiments. Among these MAGE-RING pairs, we chose to examine MAGEA1-TRIM31 in detail and showed that both WH/A and WH/B motifs of MAGEA1 bind to the coiled-coil domain of TRIM31 and that MAGEA1 interaction stimulates TRIM31 ubiquitin-ligase activity. In addition, TRIM31 directly binds to NSE4, suggesting the existence of a TRIM31-MAGEA1-NSE4 complex reminiscent of the NSE1-NSE3-NSE4 trimer. These results suggest that MAGEA1 functions as a co-factor of TRIM31 ubiquitin-ligase and that the TRIM31-MAGEA1-NSE4 complex may have evolved from an ancestral NSE1-NSE3-NSE4 complex.

Mutational analysis of Mdm2 C-terminal tail suggests an evolutionarily conserved role of its length in Mdm2 activity toward p53 and indicates structural differences between Mdm2 homodimers and Mdm2/MdmX heterodimers

Mdm2 can mediate p53 ubiquitylation and degradation either in the form of the Mdm2 homodimer or Mdm2/MdmX heterodimer. The ubiquitin ligase activity of these complexes resides mainly in their respective RING finger domains and also requires adjacent C-terminal tails. So far, structural studies have failed to show significant differences between Mdm2 RING homodimers and Mdm2/MdmX RING heterodimers. Here, we report that not only the primary amino acid sequence, but also the length of the C-terminal tail of Mdm2 is highly conserved through evolution and plays an important role in Mdm2 activity toward p53. Mdm2 mutants with extended C termini do not ubiquitylate p53 despite being capable of forming Mdm2 homodimers through both RING-acidic domain and RING-RING interactions. All extended mutants also retained the ability to interact with MdmX, and this interaction led to reactivation of their E3 ubiquitin ligase activity. In contrast, only a subset of extended Mdm2 mutants was activated by the interaction with Mdm2 RING domain, suggesting that Mdm2 homodimers and Mdm2/MdmX heterodimers may not be structurally and functionally fully equivalent.

Induction of cancer cell stemness by depletion of macrohistone H2A1 in hepatocellular carcinoma

Hepatocellular carcinomas (HCC) contain a subpopulation of cancer stem cells (CSCs), which exhibit stem cell–like features and are responsible for tumor relapse, metastasis, and chemoresistance. The development of effective treatments for HCC will depend on a molecular‐level understanding of the specific pathways driving CSC emergence and stemness. MacroH2A1 is a variant of the histone H2A and an epigenetic regulator of stem‐cell function, where it promotes differentiation and, conversely, acts as a barrier to somatic‐cell reprogramming. Here, we focused on the role played by the histone variant macroH2A1 as a potential epigenetic factor promoting CSC differentiation. In human HCC sections we uncovered a significant correlation between low frequencies of macroH2A1 staining and advanced, aggressive HCC subtypes with poorly differentiated tumor phenotypes. Using HCC cell lines, we found that short hairpin RNA–mediated macroH2A1 knockdown induces acquisition of CSC‐like features, including the growth of significantly larger and less differentiated tumors when injected into nude mice. MacroH2A1‐depleted HCC cells also exhibited reduced proliferation, resistance to chemotherapeutic agents, and stem‐like metabolic changes consistent with enhanced hypoxic responses and increased glycolysis. The loss of macroH2A1 increased expression of a panel of stemness‐associated genes and drove hyperactivation of the nuclear factor kappa B p65 pathway. Blocking phosphorylation of nuclear factor kappa B p65 on Ser536 inhibited the emergence of CSC‐like features in HCC cells knocked down for macroH2A1. Conclusion: The absence of histone variant macroH2A1 confers a CSC‐like phenotype to HCC cells in vitro and in vivo that depends on Ser536 phosphorylation of nuclear factor kappa B p65; this pathway may hold valuable targets for the development of CSC‐focused treatments for HCC. (Hepatology 2018;67:636‐650).