The TP53 tumor suppressor and autophagy in malignant lymphoma

Abstract

The TP53 (p53) protein is a well-known tumor suppressor that plays a crucial role in maintaining genome stability under various cellular stresses. Loss of TP53 causes lymphomagenesis in mouse models and/or promotes tumor progression. However, the prognostic significance of TP53 has been inconsistent in human cancers including malignant lymphoma. In our recent review of TP53 dysfunction in lymphoid malignancies, we discussed the fact that the TP53 function in autophagy may be one of the important mechanisms responsible for the inconsistency of TP53 prognostic value, in that autophagy can promote survival of lymphoma cells by recycling toxic intracellular materials and inhibiting apoptosis. Here we discuss the biological mechanisms of TP53 functional switches from apoptosis to autophagy, and provide a brief summary of how TP53 regulates autophagy through its transcriptional activity on 14 genes of the TP53 pathway.

The prognostic value of TP53 has been inconsistent in several types of cancer including lymphoma, possibly due to five reasons: (1) The lack of direct correlation between TP53 mutations and TP53 overexpression measured by immunohistochemical approaches. (2) The heterogeneity of TP53 mutations. Therefore, TP53 mutations detected by gene sequencing method do not necessarily lead to functional disruption. (3) Lymphoma cells have developed various mechanisms to disrupt TP53 function, other than TP53 mutations alone detected by prognosis studies. (4) The heterogeneity of tumor cells and stromal microenvironment with different treatment regimens present in the same cohort. (5) The diversity of TP53 functions in addition to the methodological issues listed above. Figure 1 illustrates a model for how different TP53 functions contribute to the inconsistency. Functioning in the apoptosis mode, wild-type (WT)-TP53 correlates with better clinical outcomes, whereas the cell cycle arrest and DNA repair function can either suppress or promote survival of lymphoma cells. In comparison, in the autophagy mode, WT-TP53 promotes survival of lymphoma cells, which is associated with worse clinical outcomes. The mechanisms underlying TP53 functional switch to autophagy need to be revealed, in order to modulate TP53 function to promote, not to protect, death of lymphoma cells.

Figure 1. Wild-type TP53 (WT-TP53) can function in three modes (apoptosis, cell cycle arrest and DNA repair, and autophagy) in tumor cells treated with anticancer agents. Apoptosis mode is associated with better clinical outcomes. The function of cell cycle arrest and DNA repair contributes differently to the TP53 prognostic value at low or high dose of anticancer agents. At a lower dose of agents, WT-TP53 is associated with better prognosis, because without functional TP53, tumor cells continue to replicate damaged DNA, whereas tumor cells with WT-TP53 can progress slower with less mutation events, or are permanently arrested when the apoptosis pathway is blocked. At a higher dose of agents, tumor cells with WT-TP53 escape mitotic catastrophe contributing to a worse prognosis. The autophagy mode is generally associated with worse clinical outcomes. The diversity of TP53 function can explain the inconsistency of TP53 prognostic value (better, worse, or not an independent prognostic factor).

The function of TP53 is intricately regulated by various post-translational modifications and regulators as discussed in our review. Since anti-apoptotic autophagy induced by WT-TP53 has mostly been associated with resistance to tyrosine kinase inhibitors, proteasome inhibitors and other drugs that induce endoplasmic reticulum (ER) stress, it is possible that the phosphorylation and ubiquitination status of TP53 and/or TP53 regulators, as well as calcium concentration, can directly or indirectly affect TP53 function to selectively transactivate different sets of genes, or affect TP53 subcellular locations, thereby switching TP53 function to autophagy mode.

It is also possible that the switch to autophagy mode is determined by the available effectors downstream of the TP53 pathway, which favor autophagy induction instead of apoptosis. Autophagy initiates by loading of toxic cytoplasmic cargos into phagophores that are derived from various membrane sources such as the ER or Golgi apparatus, maturing into autophagosomes (requiring the ULK1 complex, the PIK3C3/BECN1 complex, and LC3 recruitment), which fuse with lysosomes to form autolysosomes (Fig. 2). Autophagy can be induced by starvation, unfolded or misfolded proteins, elevated intracellular calcium concentration and reactive oxygen species. Mechanistic target of rapamycin complex 1 (MTORC1) negatively regulates autophagy, by inhibiting ULK1 and phagophore formation, and by activating eukaryotic translation initiation factor 4E (EIF4E) and elongation factor 2 (EEF2), which initiate protein translation. BCL2 and BCL2L1 (BCL-XL) inhibit binding of BECN1 to PIK3C3. This interaction can be blocked by MAPK8 (JNK) and BNIP3. The TP53-induced autophagy has been observed in chronic lymphocytic leukemia with abnormal B-cell receptor signaling, chronic myelogenous leukemia with constitutively active BCR-ABL1 tyrosine kinase, acute lymphoblastic leukemia, 20–30% of which also has the BCR-ABL1 fusion protein, and multiple myeloma with a high level of unfolded or misfolded protein, resulting in a higher basal level of autophagy. The B-cell receptor signaling pathway activates NF-κB (which enhances autophagy by transactivating BECN1) and AKT1 (which inhibits autophagy by activating MTOR). BCR-ABL1 suppresses autophagy through activating AKT1-MTOR. Therefore, it is possible that downstream of the TP53 pathway is directed to autophagy, but not to apoptosis, because the above cellular contexts and/or the addition of inhibitors and calcium mobilizers create optimal conditions for autophagy.

Figure 2. Regulation of autophagy through p53 transcription-dependent activities in lymphocytes. (1) TP53 induces expression of the autophagy gene ULK1. (2) TP53 induces SESN1, which activates AMPK and TSC2, thereby inhibiting the autophagy inhibitor MTORC1. (3) TP53 induces NUPR1, which can inhibit the autophagy negative regulator AKT1 via transactivating ATF4, DDIT3 and TRIB3. NUPR1 is also able to reduce autophagy by repressing FOXO3 and BNIP3 (not shown). (4 and 5) TP53 induces DRAM1 and EI24, which are required for autophagy induction. (6 and 7) TP53 induces HRAS and GADD45A, which activate the pro-autophagic MAPK1/3 and MAPK8 pathways. (8) TP53 suppresses BCL2L1. (9) TP53 induces RPS6KB1, which activates EEF2. (10–13) TP53 induces anti-autophagic C12orf5, ROCK1, DUSP14 and NFKBIE. (14) TP53 suppresses PARP1, an effector of the MAPK1/3 pathway. Green arrows indicate activation; red “–|” indicates inhibition. Solid arrows indicate protein-protein interactions; dashed arrows or “–|” indicate activation or suppression of gene expression.

In lymphocytes, WT-TP53 induces autophagy through transactivation of autophagy machinery (ULK1, DRAM1 and EI24/PIG8) and autophagy regulators (SESN1/sestrin 1, NUPR1/p8, HRAS and GADD45A), and through transrepressing BCL2L1 as described in our review. Figure 2 illustrates the mechanisms by which TP53 stimulates autophagy through these modulators. SESN1 reduces overoxidized peroxiredoxins and induces autophagy by activating AMP-activated protein kinase (AMPK) and subsequently TSC2, both of which are MTORC1 inhibitors. The mechanism of AMPK activation by SESN1 is unclear. AMPK also activates ULK1 by phosphorylation. NUPR1 is a transcription factor and can either enhance autophagy by activating the ATF4-DDIT3/CHOP-TRIB3-AKT1-MTOR axis or reduce autophagy by repressing the FOXO3-BNIP3-BCL2 pathway. HRAS can activate the MAPK1/3 (ERK, which transactivates LC3, BECN1 and BNIP3, and disrupts MTORC1) and MAPK8 (which inhibits BCL2 and activates TP53) pathways, which induce autophagy. GADD45A activates both p38 MAPKs and MAPK8. Downregulation of BCL2L1 by nuclear TP53 releases the BCL2L1 inhibition of BECN1. It is also possible that the inhibition of BCL2L1 or BCL2 by cytoplasmic TP53 also activates BECN1.

GADD45A is also a cell cycle arrest and DNA repair gene, whereas BCL2L1 and BCL2 are also anti-apoptotic. Therefore, the three TP53 functional modes share the same effectors. The TP53 pathway can switch to a different direction due to different activities of downstream effectors.

In the mode of apoptosis or cell cycle arrest and repair, WT-TP53 represses autophagy, possibly through upregulation of RPS6KA1, C12orf5/TIGAR, ROCK1, DUSP14 and NFKBIE that inhibit autophagy, and downregulation of PARP1. The RPS6KA1 gene product (p90^S6K/S6K-α-1) inhibits TSC2 and activates EEF2. C12orf5, which degrades fructose-2,6-bisphosphate, inhibits autophagy by reducing reactive oxygen species. ROCK1 inhibits autophagy, possibly through AKT1 activation. DUSP14 inactivates the MAPK1/3, MAPK8 and p38 pathways by dephosphorylation. NFKBIE inhibits NF-κB activation, which stimulates autophagy. PARP1 is directly activated by MAPK1/3 and may induce autophagy. PARP1 activation can also lead to DNA repair and apoptosis. When more mechanisms that modulate autophagy are revealed, additional TP53 effectors illustrated in our review will be found to associate with autophagy regulation.

In conclusion, autophagy induction by TP53 is one of the important mechanisms to explain discrepancies of TP53 prognostic significance. Switching to autophagy is possibly due to different post-translational modifications or TP53 regulators, or is possibly directed by pro-autophagic downstream events. The up- or downregulation of autophagy by TP53 is mediated by different sets of target genes. However, autophagy and apoptosis not only share several similar inducing signals, but also share some downstream effectors, which suggests other regulations play a role in determining survival or death of tumor cells. A better understanding of TP53 functional switch regulation between survival and cell death modes will help overcome drug resistance and achieve better clinical outcomes.

Xu-Monette ZY, Medeiros LJ, Li Y, Orlowski RZ, Andreeff M, Bueso-Ramos CE, et al. Dysfunction of the TP53 tumor suppressor gene in lymphoid malignancies. Blood. 2012 doi: 10.1182/blood-2011-11-366062. In press.