Road to destruction of D-type cyclin protein

Proteins called type D cyclins (cyclins D1, D2, D3) are important components of the core cell cycle engine that promotes cell division.writing Nature, Shimonesuki et al.¹, Tchaikovsky et al.² And Maiani et al.³ It provides a long-standing answer to how D-type cyclins are usually degraded.

During cell division, type D cyclins bind to and activate enzyme partners called cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase 6 (CDK6). These cyclin-CDK4 / 6 kinases add phosphate groups to the tumor suppressor proteins RB1, RBL1 and RBL2, resulting in accelerated cell division (Figure 1). Uncontrolled activation of cyclin D–CDK4 / 6 kinase is the driving force behind the development of many types of cancer.^Four..

Interest in cyclin D–CDK4 / 6 biology has recently increased as small molecule inhibitors of CDK4 / 6 kinase enter the clinic. Notable results from clinical trials of these compounds show the ability to prolong the survival of breast cancer patients. The CDK4 / 6 inhibitors palbociclib, ribociclib, and abemaciclib are approved for the treatment of advanced breast cancer.In addition, these drugs have been tested in hundreds of clinical trials for many different types of cancer.^Four,Five..

Although type D cyclins have been intensively studied since their discovery in the 1990s, it is controversial how they are degraded during the cell cycle. Phosphorylation of the carboxy-terminal region of D-cyclins causes the destruction of these proteins by a degradation pathway called the ubiquitin-proteasome system.⁶.. In this system, a series of activities by ubiquitin activating (E1), ubiquitin binding (E2), and ubiquitin ligase (E3) enzymes target a chain of several molecules of the small protein ubiquitin by the following process: Combine to. Ubiquitination. These ubiquitinated proteins are destined to be degraded by a protein complex called the proteasome.

The largest family of E3s is the Curin-RING ligase (CRL). The CRL is composed of a curin protein, a RING protein (which recruits E2), an adapter protein, and one of many different substrate receptor proteins involved in the recruitment of target proteins to the E3 complex.^{7– –9}.. Some substrate receptors for E3, called CRL1, are involved in the degradation of cyclin D1, while other receptors have been postulated to target cyclins D2 and D3 for proteasome disruption. In addition, cyclin D1 has been shown to be ubiquitinated by the anaphase-promoting complex, an E3 complex that targets several cell cycle proteins.⁹..Other studies, in contrast to those models^Ten The levels and stability of cyclin D1 have been shown to be unaffected by depletion of these proteins, and several other E3s have been shown to regulate the degradation of cyclin D1.

A paper by Simoneschi, Chaikovsky, Maiani and their colleagues reports that three D-type cyclins are ubiquitinated and targeted for proteasome degradation by E3 called CRL4, which uses the protein AMBRA1 as a substrate receptor. It was already known that AMBRA1 plays an important role in the regulation of autophagy, the process by which cells break down damaged organelles and protein aggregates.^11,12.. AMBRA1 has also been identified as a substrate receptor for E3, including CRL4.^7,13..Through a beautiful series of experiments that provide insights into both mice and humans, Simoneski, Tchaikovsky, and Maiani, using technologies from areas such as cell biology, molecular biology, and developmental genetics. et al.. It has been shown that depletion of AMBRA1 in normal and cancer cell and mouse embryonic development leads to elevated levels of D-type cyclins. This results in greater phosphorylation of RB1 and more cell proliferation than that occurs in cells containing normal amounts of AMBRA1.

Maiani et al. Also show that depletion of AMBRA1 increases levels of the transcription factor protein N-MYC.This group was shown earlier¹⁴ AMBRA1 regulates the stability and activity of the associated transcription factor c-MYC. MYC family proteins can upregulate the expression of D-type and E-type cyclin proteins¹⁵, Thereby accelerating the progression of the cell cycle.

These observations suggest that AMBRA1 may act as a tumor suppressor protein. In fact, mice with only one functional copy of the gene encoding AMBRA1 have a higher incidence of lung, liver, and kidney tumors than normal mice with two functional copies of the gene.¹⁴.. These new studies provide compelling evidence to support this idea.

The author AMBRA1 The gene is mutated in human cancer. As expected, given the ability of AMBRA1 to promote the degradation of cyclin D1, the authors report that levels of AMBRA1 in human tumors were inversely correlated with levels of cyclin D1. In addition, low levels of AMBRA1 in tumors are associated with a poor prognosis in cancer patients. Experimental inactivation of AMBRA1 in either human tumor cell lines or mouse cells engineered to have cancer-promoting mutations increased the tumorigenicity of the cells as assessed after injection into mice. ..In addition, genetic resection of AMBRA1 Promotion of tumorigenesis in a mouse model of lung cancer driven by a variant of Kras Genes, and these AMBRA1-deficient tumors, were higher than normal levels of D-cyclins. Taken together, these studies show that AMBRA1 normally suppresses cell proliferation primarily by blocking high levels of D-cyclins.

In addition, Chaikovsky, Simoneschi, and their colleagues have shown that loss of AMBRA1 and associated increased D-cyclin reduce the susceptibility of human tumor cells to CDK4 / 6 inhibitors. Interestingly, these authors found that in AMBRA1-depleted cells, cyclin D1 also forms a catalytically active complex with the cyclin-dependent kinase enzyme CDK2, rather than primarily partnering with CDK4 / 6, which complex is CDK4. / 6 Reported insensitivity to inhibitors. ..

Maiani et al. Also showed that loss of AMBRA1 and consequent elevation of D-cyclins (and possibly other proteins such as c-MYC) cause DNA damage and replication stress with activation of kinase enzymes. I am. It is called CHK1.The important thing is Maiani et al. Cancer cells depleted of AMBRA1 have been reported to be hypersensitive to treatment with CHK1 inhibitors. This suggests potential treatment opportunities targeting AMBRA1-deficient tumors.

These exciting results raise some important issues. For example, does reduced AMBRA1 levels underlie existing and acquired resistance of human tumors to CDK4 / 6 inhibitors? And is the increase in D-cyclins that accompanies AMBRA1 depletion the only factor involved in the development of resistance to CDK4 / 6 inhibitors?Analysis of clinical trials in breast cancer patients^{16– –19} It reveals that there is no correlation between the presence of an extra copy of the gene encoding cyclin D1 or the level of cyclin D1 messenger RNA or protein in the tumor and the patient’s response to the CDK4 / 6 inhibitor.Indeed, Tchaikovsky et al. It was found that human cancer cells driven to higher than normal D-cyclin expression did not completely reproduce the inhibitor resistance characteristics observed in AMBRA1 depletion. Perhaps other AMBRA1 regulatory proteins, such as c-MYC, that can upregulate the cyclin E protein and activate the cyclin E–CDK2 complex contribute to treatment resistance.

Observation by Tchaikovsky et al. And Shimonesuki et al. The analysis of the formation of the CDK4 / 6 inhibitor-resistant cyclin D–CDK2 complex in AMBRA1-depleted cells is interesting.Such “atypical” complexes have previously been shown to underlie acquired resistance to CDK4 / 6 inhibition.²⁰.. It is tempting to speculate that AMBRA1 depletion somehow promotes the formation of these cyclin D–CDK2 complexes, which, along with elevated cyclin D levels, results in resistance to CDK4 / 6 inhibitors. A particularly exciting possibility that arises from the work of Maiani and colleagues is that CHK1 inhibitors can be used to treat CDK4 / 6 inhibitor-resistant tumors with low levels of AMBRA1.

Further research on the role of AMBRA1 in human cancer is needed. Is the tumor suppressor function of AMBRA1 predominantly mediated by cyclin D1 or c-MYC, or is it also mediated by other targets? Tumor cells that no longer produce RB1 do not require D-cyclins to progress in the cell cycle^FourTherefore, if D-cyclins are the primary target for AMBRA1, loss of AMBRA1 is expected to be observed in RB1-producing tumors. It is still determined whether loss of AMBRA1 in human tumors is mutually exclusive with mutations affecting the C-terminus of D-cyclins, which are expected to make D-cyclins resistant to AMBRA1-mediated degradation. Not. Another open issue, as reported by Chaikovsky and colleagues, is that low expression of AMBRA1 correlates with high levels of cyclin D1 and the survival rate of lung tumors with certain types of genetic alterations, including: This is the reason associated with the decline in. Kras mutation.This effect was not observed in lung tumors that instead had a mutated version of the gene encoding the EGFR protein or had a wild type. Kras..

Impressive study by Tchaikovsky, regardless of the answers to these questions et al., Maiani et al. And Shimonesuki et al. Deepen your understanding of the mechanisms that govern cell cycle progression.

Competing financial interests

PS is a consultant to Novartis, Genovis, Guidepoint, The Planning Shop, ORIC Pharmaceuticals, Syros, Cedilla Therapeutics and Exo Therapeutics. His lab receives research funding from Novartis.