Cancers develop as the result of uncontrolled proliferation, growth and metastasis of aberrantly differentiated cells. CycG2 is an unconventional cyclin homolog linked to inhibition of cellular proliferation and promotion of cell differentiation. Expression of the CycG2 gene, CCNG2, is repressed in variety of human cancers. Reduced CCNG2 transcript levels are found in more aggressive, poor-prognosis breast cancer (BC) subtypes, compared to normally differentiated breast tissues, and are elevated in tumor cells responding to cancer therapies. This thesis addresses the question of CycG2’s contribution to the cell cycle inhibitory responses of tumor cells to chemotherapeutics and growth inhibitory drugs; namely, DNA damaging topoisomerase II poisons, estrogen signaling inhibitors, mTOR signaling inhibitors and drugs that induce endoplasmic reticulum stress (ERS).

Evaluation of previous findings suggested that CycG2 could be a DNA damage response (DDR) protein that participates in DNA damage checkpoint regulation. This thesis further defines the involvement of CycG2 in the DDR signaling pathway. Ectopic expression of CycG2 promotes phosphorylation of the DDR checkpoint kinase Chk2 on an activational target site. Furthermore, induction of DNA double strand breaks (DSBs) through treatment with the chemotherapeutic doxorubicin (Dox) induces CycG2 expression that correlates with the induced cell cycle arrest and the activation of DDR proteins. Transient and stable shRNA mediated knockdown (KD) of CycG2 attenuates the G2/M checkpoint arrest response of multiple cell lines caused by the Dox-induced DNA damage. Furthermore, KD of CycG2 blunts the DDR-triggered reduction in Cdc25B levels, inhibitory-phosphorylation of Cdc2 and accumulation of CycB1, which are important processes for a potent G2/M checkpoint arrest. Thus, CycG2 may participate in the enforcement and maintenance of G2/M checkpoints by limiting the Cdc25B-mediated promotion of CycB1/Cdc2 activity.

The role CycG2 plays in the anti-proliferative responses of BC cells to estrogen (E2)-signaling deprivation was analyzed in a second study. CycG2 expression is elevated in E2-deprived cells, but reduced in E2-stimulated cells. CycG2 KD diminishes the cell cycle arrest response following inhibition of E2 signaling. In addition, these clones showed elevated levels of the growth promoting cyclin, CycD1,and elevated activation of the growth factor signaling cascade, mitogen activated protein kinase (MAPK), pathway. These results indicate that loss of CycG2 expression may promote the development of BC tumor cell resistance to E2 antagonizing therapeutics by stimulating the activation of the MAPK pathway. Importantly, results presented here show that CycG2 can associate with the cyclin dependent kinase (CDK) 10, which was recently implicated as an upstream repressor of MAPK signaling. Thus suggesting that the resistance of BC cells to the inhibition of E2 signaling that is acquired following CycG2 KD could result from loss of CycG2-mediated stimulation of CDK10 activity.

CycG2 expression is upregulated following inhibition of the growth promoting kinase mTOR and the induction of ERS. KD of CycG2 expression in multiple cell lines induces CycD1 expression and attenuates cell cycle arrest responses to treatments with rapamycin, metformin and thapsigargin. These results suggest that CycG2 expression restricts proliferation of cells responding to mTOR inhibition (rapamycin and metformin) and ERS-induction (thapsigargin). The data further indicate that rapamycin-induced CycG2 expression is not dependent on FOXO TF expression. In contrast, the potent upregulation of CycG2 expression triggered by thapsigargin is likely the consequence of the FOXO TF activity at the CCNG2 promoter.