Growth Arrest‐Specific 6 (GAS6) Promotes Prostate Cancer Survival by G₁ Arrest/S Phase Delay and Inhibition of Apoptosis During Chemotherapy in Bone Marrow

Abstract

Prostate cancer (PCa) is known to develop resistance to chemotherapy. Growth arrest-specific 6 (GAS6), plays a role in tumor progression by regulating growth in many cancers. Here, we explored how GAS6 regulates the cell cycle and apoptosis of PCa cells in response to chemotherapy. We found that GAS6 is sufficient to significantly increase the number and duration of G1 phase in PCa cells. Importantly, GAS6 further increased the number of G1 arrested cells during docetaxel chemotherapy. GAS6 altered the signals of key cell cycle regulators: Cyclin B1 (G2/M phase), CDC25A, Cyclin E1, and CDK2 (S phase entry) were all downregulated, while p27, p21, Cyclin D1, and CDK4 (G0/G1 phase) were upregulated. Importantly, these signaling events were further accentuated during docetaxel treatment in the presence of GAS6. Moreover, the apoptotic response of PCa cells to GAS6 was examined during docetaxel chemotherapy. Docetaxel induced PCa cell apoptosis. However, this apoptotic response was abrogated in PCa cell cultures in the presence of GAS6 or GAS6 secreted from co-cultured osteoblasts. Similarly, the GAS6-expressing bone environment protects PCa cells from apoptosis within primary tumors in vivo studies. In addition, docetaxel induced significant levels of Caspase-3 and PARP cleavages in PCa cells, while GAS6 protected PCa cells from docetaxel-induced apoptotic signaling. Together, these data suggest that GAS6, expressed by osteoblasts in the bone marrow, plays a significant role in the regulation of PCa cell survival during chemotherapy, which may have important implications for targeting metastatic disease.

Sympathetic Signaling Reactivates Quiescent Disseminated Prostate Cancer Cells in the Bone Marrow

“…Co-culture of PCa cells with GAS6 +/+ OBs reduced the proportion of proliferating PCa cells compared to PCa co-cultured with GAS6 −/− OBs (Figure 5C), which is consistent with previous reports and induction of PCa dormant states (30). Co-culture of PCa cells with GAS6 +/+ OBs also revealed a 1.9 fold increase in Ki67 positive PCa cells when cultured with NE compared to vehicle controls as determined by flow cytometry (Figure 5C).…”

“…Previously we have observed that GAS6 expression by osteoblasts regulates PCa dormancy through interactions with MERTK (8,29,30,33). First, we sought to confirm the GAS6 mRNA results at the protein level.…”

“…Previously we determined that when PCa cells become proliferatively quiescent when co-cultured with osteoblasts (8,29,30). To determine whether NE is able to alter the quiescence of PCa cells induced by osteoblasts, co-culture experiments were performed where in the human PCa cells were selectively analyzed using antibodies specific for human HLA.…”

“…Previous investigations have demonstrated that a PCa G 1 cell cycle arrest is induced through PCa-OB co-culture, and serves as a model for PCa dormancy within the bone marrow (30). To further explore how NE impacts PCa in the bone marrow, the use of cell cycle specific FUCCI-vectors were employed to isolate cells at different stages of the cell cycle.…”

Minimal residual disease in prostate cancer patients after primary treatment: theoretical considerations, evidence and possible use in clinical management

“…There is also evidence that GAS6 increases the number of prostate cancer cells with a stem cell phenotype, which is CD133 positive/CD44 positive, within the bone marrow [103]. Cancer stem cells (CSCs) are proposed to be stem like cells found in tumors and possess the capability to self-renew and differentiate into new diverse tumor cells.…”