3.4. Transcriptional regulation of metabolism

Transcription factor c-Myc, a highly unstable protein [63, 77, 78], acts as a heterodimer with Max, recruiting coactivators through CACGTG-binding motifs to promotors of target genes involved in the regulation of cell growth, proliferation, metabolism, and apoptosis. Interestingly, c-Myc appears to be targeted by diverse PTMs and proteolytic mechanisms. UPS-dependent degradation of c-Myc depends on several E3 enzymes, F-Box proteins, and antagonizing pathways [79]. Using gene set enrichment analysis, PA28γ was shown to reprogram energy metabolism via the c-Myc-glycolysis axis, affecting glucose utilization and lactate production in cancer cells. PSME3 gene silencing reduced c-MYC levels, whereas the increased levels of PA28γ in cancer cells increased Myc. High levels of PA28γ protein in the cytosol and nucleus repress UPS-mediated c-Myc decay and reprogram pancreatic tissue and pancreatic cancer cell lines metabolically [80]. These results were surprising, since overexpression of PA28γ in Hela cells promoted degradation of c-Myc, while depletion of PA28γ markedly increased the protein stability of c-Myc [51]. These contradictory observations indicate contextual variability of c-Myc turnover in pancreatic tumors versus other models and underline the requirement for studies focusing on cross regulation of UIPP and UPS.

might be of relevance for cancer therapy, since PA28γ depletion sensitizes tumors to treatment

Proteasome Activator 28γ: Impact on Survival Signaling and Apoptosis

http://dx.doi.org/10.5772/intechopen.74731

81

The stoichiometry of the key regulators of UPS and UIPP, namely, 19S and 11S regulators, is of particular interest to further understand how cross talk of these two proteolytic systems is affected by PA28γ. Since each biochemical process is restricted kinetically by its rate limiting step, the assembly of proteasomal complexes should be a matter of serious debate. Only few studies so far covered this question due to methodological limitations. Therefore, a recent proteomic approach is remarkable, elaborating the stoichiometry of 20S proteasome-associated regulators in detail [86, 87]. Using formaldehyde cross-linking in combinations with 20Scomplex directed affinity purification on MCP21-coupled sepharose, and applying mass spectrometric quantification, proteomic analysis compared the stoichiometry of 20S proteasome complexes in nine different cell lines. Interestingly, 19S and 20S particles reached 1:1 ratio, but PA28γ-associated proteasomes occurred as minor species, occupying about 1–0.2% of proteasomes [86]. All investigated cellular systems had in common that about 20–40% of 20S proteasomes persisted in a regulator-unbound state, establishing a pool of proteasomes directly available for UIPP tasks or for dynamic behavior in the case of expressional alterations

As central pathways of protein degradation, UPS and UIPP participate in regulating the delicate balance of pro- and anti-apoptotic proteins and cell cycle regulators [88, 89]. The contribution of proteasomal degradation in controlling crucial steps of the mitochondrial pathway of apoptosis has been reviewed comprehensively [89]. Here, we are focusing on the PA28γ-mediated mechanisms, affecting the balance between pro- and anti-apoptotic regula-

Initial studies on PA28γ-deficient mice suggested a role for PA28γ as a regulator of cell proliferation and body growth [90]. Lack of PA28γ did not affect expression of other PSME family members such as PA28α or PA28β and resulted in smaller body size. Entry into S phase was impeded and number of G1 cells increased. MEFs depleted in PA28γ revealed increased

Recently, we demonstrated a correlation between cellular PA28γ levels and the sensitivity of cells toward apoptosis in different cellular contexts, thereby confirming a role of proteasome activator PA28γ as an anti-apoptotic regulator [46]. We investigated the anti-apoptotic role of PA28γ upon UV-C stimulation in B8 mouse fibroblasts stably overexpressing the PA28γencoding PSME3 gene and upon butyrate-induced apoptosis in human HT29 adenocarcinoma cells with silenced PSME3 genes. Interestingly, our results demonstrate that PA28γ has a strong influence on different apoptotic hallmarks, especially the levels of transcriptionally active

with hexokinase inhibitor 2-desoxy-glucose [85].

in the activator population [86].

tory proteins (Figure 2).

3.6.2. Discovery of anti-apoptotic properties of PA28γ

spontaneous apoptosis during logarithmic growth.

phosphorylated p53, BclXL, and active effector caspases (Figure 2) [46].

3.6. Cross talk of UIPP and UPS in the regulation of apoptosis

3.6.1. Stoichiometry of 20S proteasomes and regulators in cellular systems
