Retinoid‐modulated MAT1 ubiquitination and CAK activity

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Ubiquitin-dependent degradation of CDK2 drives the therapeutic differentiation of AML by targeting PRDX2

A distinct hallmark of acute myeloid leukemia (AML) is the arrest of leukemic myeloblasts at an immature stage of development. Therapies that overcome differentiation arrest have emerged as a powerful strategy for treating AML, but targeting leukemia differentiation remains challenging, mainly because of an incomplete mechanistic understanding of the process. Here, we unveil a new role for cyclin-dependent kinase 2 (CDK2) in blocking myeloid differentiation in AML. We show that among several interphase CDK, only CDK2 undergoes ubiquitin-dependent proteasome degradation, which is accompanied by AML cell differentiation. By using the yeast 2-hybrid system and functional analyses, KLHL6 was identified as a specific E3 ubiquitin ligase regulating the degradation of CDK2. Importantly, inhibiting CDK2, but not other cyclin-dependent kinases CDK1/4/6, effectively induced granulocytic differentiation in AML cell lines and 5 major subtypes of primary patient-derived AML samples. Mechanistically, CDK2 depletion led to the reactivation of differentiation pathway translation, and the differentiation blockade function of CDK2 may be achieved directly by maintaining the activity of PRDX2. Finally, CDK2 depletion arrested tumor growth of AML cells in nude mice and extended survival in both AML cell line and PDX-AML cells derived xenograft mouse models. Thus, our work not only provides experimental evidence for validating CDK2 as a potential therapeutic target for differentiation, but also uncovers the biological function of the CDK2-PRDX2 axis in blocking AML differentiation.

Role of cell cycle regulatory molecules in retinoic acid- and vitamin D3-induced differentiation of acute myeloid leukaemia cells

The important role of cell cycle regulatory molecules in all trans-retinoic acid (ATRA)- and vitamin D3-induced growth inhibition and differentiation induction has been intensively studied in both acute myeloid leukaemia primary cells and a variety of leukaemia cell lines. Cyclin-dependent kinases (CDK)-activating kinase has been demonstrated to interact with retinoic acid receptor (RAR)α in acute promyelocytic leukaemia cells, and inhibition of CDK-activating kinase by ATRA causes hypophosphorylation of PML-RARα, leading to myeloid differentiation. In many cases, downregulation of CDK activity by ATRA and vitamin D3 is a result of elevated p21- and p27-bound CDKs. Activation of p21 is regulated at the transcriptional level, whereas elevated p27 results from both (indirectly) transcriptional activation and post-translational modifications. CDK inhibitors (CKIs) of the INK family, such as p15, p16 and p18, are mainly involved in inhibition of cell proliferation, whereas CIP/KIP members, such as p21, regulate both growth arrest and induction of differentiation. ATRA and vitamin D3 can also downregulate expression of G1 CDKs, especially CDK2 and CDK6. Inhibition of cyclin E expression has only been observed in ATRA- but not in vitamin D3-treated leukaemic cells. In vitro, not only dephosphorylation of pRb but also elevation of total pRb is required for ATRA and vitamin D3 to suppress growth and trigger their differentiation. Finally, sharp reduction in c-Myc has been observed in several leukaemia cell lines treated with ATRA, which may regulate expression of CDKs and CKIs.

The lost intrinsic fragmentation of MAT1 protein during granulopoiesis promotes the growth and metastasis of leukemic myeloblasts

MAT1, an assembly factor and targeting subunit of both cyclin-dependent kinase-activating kinase (CAK) and general transcription factor IIH (TFIIH) kinase, regulates cell cycle and transcription. Previous studies show that expression of intact MAT1 protein is associated with expansion of human hematopoietic stem cells (HSC), whereas intrinsically programmed or retinoic acid (RA)-induced MAT1 fragmentation accompanies granulocytic differentiation of HSC or leukemic myeloblasts. Here we determined that, in humanized mouse microenvironment, MAT1 overexpression resisted intrinsic MAT1 fragmentation to sustain hematopoietic CD34+ cell expansion while preventing granulopoiesis. Conversely, we mimicked MAT1 fragmentation in vitro and in a mouse model by overexpressing a fragmented 81-aa MAT1 polypeptide (pM9) that retains the domain for assembling CAK but cannot affix CAK to TFIIH-core. Our results showed that pM9 formed ΔCAK by competing with MAT1 for CAK assembly to mimic MAT1 fragmentation-depletion of CAK. This resulting ΔCAK acted as a dominant negative to inhibit the growth and metastasis of different leukemic myeloblasts, with or without RA resistance, by concurrently suppressing CAK and TFIIH kinase activities to inhibit cell cycle and gene transcription. These findings suggest that the intrinsically programmed MAT1 expression and fragmentation regulate granulopoiesis by inversely coordinating CAK and TFIIH activities, whereas pM9 shares a mechanistic resemblance with MAT1 fragmentation in suppressing myeloid leukemogenesis.

Retinoid-suppressed phosphorylation of RARalpha mediates the differentiation pathway of osteosarcoma cells

Although retinoic acid (RA) is a potent agent that coordinates inhibition of proliferation with differentiation of many cell types, RA-mediated signaling pathways in osteosarcoma cell differentiation are uncharacterized. In this study, we show that in human U2OS osteosarcoma cells, decreased phosphorylation of RA receptor alpha (RARalpha) by RA treatment or overexpressing a phosphorylation-defective mutant RARalphaS77A results in the inhibition of proliferation and induction of differentiation, and that U2OS cells transduced with RARalphaS77A suppresses tumor formation in nude mice. Moreover, using different human primary osteosarcoma cells and human mesenchymal stem cells for gene expression analysis, we found that either RA or RARalphaS77A induces many of the same differentiation response pathways and signaling molecules involved in U2OS cell differentiation. In addition, overexpression of the fibroblast growth factor 8f (FGF8f), one of the downstream targets induced by both RA and RARalphaS77A in U2OS cells, inhibits proliferation and induces expression of osteoblastic differentiation regulators. Hence, these data strongly suggest that RA-suppressed phosphorylation of RARalpha induces FGF8f expression to mediate differentiation response pathway in U2OS osteosarcoma cells.

Trichothiodystrophy view from the molecular basis of DNA repair/transcription factor TFIIH

Trichothiodystrophy (TTD) is a rare autosomal recessive disorder characterized by brittle hair and also associated with various systemic symptoms. Approximately half of TTD patients exhibit photosensitivity, resulting from the defect in the nucleotide excision repair. Photosensitive TTD is due to mutations in three genes encoding XPB, XPD and p8/TTDA subunits of the DNA repair/transcription factor TFIIH. Mutations in these subunits disturb either the catalytic and/or the regulatory activity of the two XPB, XPD helicase/ATPases and consequently are defective in both, DNA repair and transcription. Moreover, mutations in any of these three TFIIH subunits also disturb the overall architecture of the TFIIH complex and its ability to transactivate certain nuclear receptor-responsive genes, explaining in part, some of the TTD phenotypes.

Loss of CAK phosphorylation of RAR{alpha} mediates transcriptional control of retinoid-induced cancer cell differentiation

Although the role of the classic retinoic acid (RA)-induced genomic pathway in cancer cell differentiation is well recognized, the underlying mechanisms remain to be dissected. Retinoic acid receptor alpha (RARalpha) is a transcription factor activated by RA, and its serine 77 (RARalphaS77) is the main residue phosphorylated by the cyclin-dependent kinase (CDK)-activating kinase (CAK) complex. We report here that in both human myeloid leukemia and mouse embryonic teratocarcinoma stem cells, either RA-suppressed CAK phosphorylation of RARalpha or mutation of RARalphaS77 to alanine (RARalphaS77A) coordinates CAK-dependent G(1) arrest with cancer cell differentiation by transactivating RA-target genes. Both hypophosphorylated RARalpha and RARalphaS77A reduce binding to retinoic acid-responsive elements (RARE) in the promoters of RA-target genes while stimulating gene transcription. The enhanced transactivation and reduced RARalpha-chromatin interaction are accompanied by RARalpha dissociation from the transcriptional repressor N-CoR and are association with the coactivator NCoA-3. Such effects of decreased CAK phosphorylation of RARalphaS77 on mediating RA-dependent transcriptional control of cancer cell differentiation are examined correspondingly in both RA-resistant myeloid leukemia and embryonic teratocarcinoma stem RARalpha(-/-) cells. These studies demonstrate, for the first time, that RA couples G(1) arrest to transcriptional control of cancer cell differentiation by suppressing CAK phosphorylation of RARalpha to release transcriptional repression.-Wang, A., Alimova, I. N., Luo, P. Jong, A., Triche, T. J., Wu, L. Loss of CAK phosphorylation of RARalpha mediates transcriptional control of retinoid-induced cancer cell differentiation.

Mat1 inhibits peroxisome proliferator-activated receptor gamma-mediated adipocyte differentiation

Mammalian Cdk7, cyclin H, and Mat1 form the kinase submodule of transcription factor IIH (TFIIH) and have been considered ubiquitously expressed elements of the transcriptional machinery. Here we found that Mat1 and Cdk7 levels are undetectable in adipose tissues in vivo and downregulated during adipogenesis, where activation of peroxisome proliferator-activated receptor gamma (PPARgamma) acts as a critical differentiation switch. Using both Mat1(-/-) mouse embryonic fibroblasts and Cdk7 knockdown approaches, we show that the Cdk7 complex is an inhibitor of adipogenesis and is required for inactivation of PPARgamma through the phosphorylation of PPARgamma-S112. The results demonstrate that the Cdk7 submodule of TFIIH acts as a physiological roadblock to adipogenesis by inhibiting PPARgamma activity. The observation that components of TFIIH are absent from transcriptionally active adipose tissue prompts a reevaluation of the ubiquitous nature of basal transcription factors in mammalian tissues.

Intrinsic Retinoic Acid Receptor α-Cyclin-Dependent Kinase-Activating Kinase Signaling Involves Coordination of the Restricted Proliferation and Granulocytic Differentiation of Human Hematopoietic Stem Cells

Little is known about the mechanisms by which retinoic acid receptor alpha (RAR alpha) mediates the effects of retinoic acid (RA) to coordinate granulocytic proliferation/differentiation (P/D) transition. Cyclin-dependent kinase-activating kinase (CAK) complex, whose activity in phosphorylation of RAR alpha is determined by its targeting subunit ménage à trois 1 (MAT1), regulates G(1) exit, a cell cycle stage when cells commonly commit to proliferation or to differentiation. We previously found that in myeloid leukemia cells, the lack of RA-induced RAR alpha-CAK dissociation and MAT1 degradation suppresses cell differentiation by inhibiting CAK-dependent G(1) exit and sustaining CAK hyperphosphorylation of RAR alpha. This contrasts with our recent findings about the P/D transition in normal primitive hematopoietic cells, where MAT1 degradation proceeds intrinsically together with granulocytic development, in accord with dynamic expression of aldehyde dehydrogenases (ALDHs) 1A1 and 1B1, which catalyze RA synthesis. Blocking ALDH activity inhibits MAT1 degradation and granulocytic differentiation, whereas loss of RAR alpha phosphorylation by CAK induces RA-target gene expression and granulocytic differentiation. These studies suggest that the subversion of RAR alpha-CAK signaling during normal granulopoiesis is crucial to myeloid leukemogenesis and challenges the current paradigm that RA induces cell differentiation solely by transactivating target genes. Disclosure of potential conflicts of interest is found at the end of this article.

Retinoic acid induces leukemia cell G1arrest and transition into differentiation by inhibiting cyclin‐dependent kinase‐activating kinase binding and phosphorylation of PML/RAR

Acute promyelocytic leukemia (APL) cells express promyelocytic leukemia/retinoic acid receptor alpha (PML/RARalpha) fusion protein, which leads to the blocking of APL cell differentiation. Treatment of APL with all-trans-retinoic acid (ATRA) induces disease remission by in vivo differentiation of APL cells. Differentiation requires cell cycle exit; yet how ATRA couples cell cycle exit to differentiation of APL remains largely unknown. We previously found that ATRA-induced cell differentiation accompanies ubiquitination-proteolysis of ménage à trois 1 (MAT1), an assembly factor and targeting subunit of cyclin-dependent kinase (CDK)-activating kinase (CAK) that regulates G1 exit. We report here that CAK binds to and phosphorylates PML/RARalpha in actively proliferating APL cells. In response to ATRA, PML/RARalpha is dissociated from CAK, leading to MAT1 degradation, G1 arrest, and decreased CAK phosphorylation of PML/RARalpha. CAK phosphorylation of PML/RARalpha is inhibited when MAT1 levels are reduced. Both MAT1 degradation and PML/RARalpha hypophosphorylation occur in ATRA-induced G1-arresting cells undergoing differentiation but not in the synchronized G1 cells that do not differentiate. These findings reveal a novel ATRA signaling on APL cell differentiation, in which ATRA coordinates G1 arrest and transition into differentiation by inducing MAT1 degradation and PML/RARalpha hypophosphorylation through disrupting PML/RARalpha binding and phosphorylation by CAK.

Increased p21 expression and complex formation with cyclin E/CDK2 in retinoid-induced pre-B lymphoma cell apoptosis

Cip/Kip family protein p21, a cyclin-dependent kinase (CDK) inhibitor, is directly transactivated by retinoic acid receptor alpha (RARalpha) upon retinoic acid (RA):RARalpha binding. Yet the role of p21 upregulation by RA in lymphoma cells remains unknown. Here, we show that, in human pre-B lymphoma Nalm6 cells, RA-induced proliferation inhibition results from massive cell death characterized by apoptosis. Upregulated p21 by RA accompanies caspase-3 activation and precedes the occurrence of apoptosis. p21 induction leads to increased p21 complex formation with cyclin E/CDK2, which occurs when cyclin E and CDK2 levels remain constant. CDK2 can alternatively promote apoptosis, but the mechanisms remain unknown. Data presented here suggest a novel RA-signaling, by which RA-induced p21 induction and complex formation with cyclin E/CDK2 diverts CDK2 function from normally driving proliferation to alternatively promoting apoptosis.