ROS-induced cell cycle arrest as a mechanism of resistance in polyaneuploid cancer cells (PACCs)

Abstract

Cancer is responsible for the deaths of millions of people worldwide each year. Once metastasized, the disease is incurable and shows resistance to all anti-cancer therapies. The already-elevated level of reactive oxygen species (ROS) in cancer cells is further increased by therapies. The oxidative stress activates the DNA damage response (DDR) and the stressed cancer cell moves towards cell cycle arrest. Once arrested, the majority of cancer cells will undergo programmed cell death in the form of apoptosis. If the cancer cell is able to exit the cell cycle prior to cell division and enter a protected G0 state, it is able to withstand and survive therapy as a polyaneuploid cancer cell (PACC) and eventually seed resistant tumor growth.

Keywords: Cell cycle; Polyaneuploid cancer cell (PACC); Polyploid giant cancer cell (PGCC); Reactive oxygen species; Therapy resistance.

Figure 1

A) In non-cancerous cells, reactive oxygen species (ROS) exist at moderate to low levels to aid in the regulation of various cell signaling pathways. The DNA damage response (DDR) is capable of repairing oxidative stress-induced DNA damage and the integrity of the genome is maintained. B) Cancer cells have an increased amount of ROS, shifting the redox balance of the cell. The increased oxidative stress inflicts damage on the DNA of the cancer cell, contributing to cancer’s genomic instability. The DDR pathways, while activated from the ROS-induced DNA damage, are also hindered by the increased oxidative stress levels across the cancer cell. When anti-cancer therapy is applied to the cancer cell, the levels of ROS increase as a stress response. Both the ROS-induced and therapy-induced DNA damage activate the DDR; however, the DDR pathways are still hindered by oxidative stress and are not capable of repairing DNA at the rate it is being damaged.

Figure 2

A) In a normal cell cycle, the cell will go through various stages of replication and growth in order to divide. In G₁ (Growth 1), the cell will synthesize proteins and factors needed throughout the rest of the cycle, the cell will also begin to grow slightly in size. In S phase all nuclear DNA will be replicated, giving the cell two sets of genomic DNA. In G₂ (Growth 2) the cell will continue to grow in size and produce new proteins that it will need in the division process. Ending in Mitosis, the cell will finally begin the process of division, splitting apart its DNA and organelles to give rise to two identical daughter cells, who continue the cycle back into G₁. There is also a G₀ phase outside of the traditional cycle, used as a protective non-proliferative state that the cell can use to exit the cycle to repair DNA damage, when necessary. B) A cancer treated with therapy has two possible fates: 1) attempt to undergo cell divide and apoptose due to mitotic catastrophe, or 2) escape the cell cycle and not complete mitosis or cytokinesis and enter the protective G0 state as a PACC.

Figure 3

In response to anti-cancer therapy and the subsequent surge in ROS, the cancer cell is limited in its survival options. The majority of cells will be overwhelmed by the increase in nuclear damage and oxidative stress, inducing cellular death. A smaller subset of cancer cells will prematurely exit mitosis, escape the cell cycle, and enter a protective, G0 state. It is in this state that the cancer cells are protected from anti-cancer therapy and can form into polyaneuploid cancer cells (PACCs). Once a break in therapy occurs, these PACCs can re-emerge equipped with non-selective resistance to all anti-cancer therapies.

Figure 4

Phase contrast imaging of an untreated control PC3 population and a PC3 population treated with 6μM cisplatin 72 hours following plating and treatment. (scale = 200μm) PACCs are visibly apparent after treatment.