Research | Ohtani Lab

Research Background

　Cancer is initiated by aberrant cell proliferation caused by genetic mutations. In response to such oncogenic insults, cells are endowed with intrinsic tumor suppressive mechanism. The two major tumor suppressive pathways are the RB pathway and the p53 pathway, which converge on the tumor suppressor pRB and p53, respectively. The RB pathway controls the activity of the transcription factor E2F. E2F activates a group of growth-related genes, thereby playing central roles in cell proliferation in response to growth stimulation. The RB pathway suppresses aberrant cell growth by controlling E2F activity. Thus, defects in the RB pathway are prerequisite for tumorigenesis. E2F also activates tumor suppressor genes, which induce cellular senescence or apoptosis (programmed cell death), thereby contributing to tumor suppression. For examples, upon loss of pRB function by oncogenic changes, E2F, activated out of control by pRB (deregulated E2F), activates p53 by activating the ARF tumor suppressor gene, an upstream activator of p53. P53 activates genes involved in cellular senescence or apoptosis, thereby playing central roles in tumor suppression. These observations point to the roles of E2F in linking the RB pathway to the p53 pathway in response to loss of RB function by oncogenic changes. Intriguingly, we found that E2F induced by growth simulation does not activate the ARF tumor suppressor gene (Komori H et al, EMBO J, 2005) (Figure 1). This may be to secure normal cell proliferation upon growth simulation. Thus, deregulated E2F activity does not exist in normal growing cells. In addition, we found that E2F does not depend on the heterodimeric partner DP, which is essential for E2F to activate growth-related genes, to activate the ARF gene (Komori H et al, Sci Rep, 2018) (Figure 1). This suggest that E2F activity, which activates tumor suppressor genes, is distinct from that, which activates growth-related genes.

Figure 1. Distinct E2F activity regulates cell growth and tumor suppression

　Physiological E2F activity induced by growth stimulation, which depends on the heterodimeric partner DP to activate growth-related genes, does not activate the ARF tumor suppressor gene. Deregulated E2F activity, which activates the ARF tumor suppressor gene, does not depend on DP.

　Cancer cells survive by additional defects in the p53 pathway such as inactivation of ARF, over-expression of MDM2 and deletion or mutation of p53 (Figure 2). These defects in the p53 pathway make cells tolerate deregulated E2F activity. Thus deregulated E2F specifically exist in cancer cells and represent a unique feature of cancer cells. Since growth stimulation does not generate E2F activity to activate the ARF gene, deregulated E2F activity can be utilized for discrimination of cancer cells from normal growing cells (Komori H et al, EMBO J, 2005).

Figure 2. Deregulated E2F activity, which activates the ARF gene, specifically exists in cancer cells.

　We have also found that deregulated E2F activates tumor suppressor genes, which can induce apoptosis independent of p53, in addition to the ARF gene. For examples, deregulated E2F activates the tumor suppressor gene TAp73, which is not activated by E2F induced by growth stimulation (Ozono E et al, Genes Cells, 2012). TAp73 is a member of the p53 family and activates p53 target genes independent of p53. Deregulated E2F also activates several other tumor suppressor genes such as Bim, which contributes to induction of apoptosis independent of p53 (Kitamura H et al, Genes Cells, 2015). Activation of these genes is supposed to contribute to induciton of apoptosis in cancer cells. However, cancer cells survive with defective p53 pathway. These observations suggest the possibility that E2F activation of these tumor suppressor genes may not be enough for inducing apoptosis or deregulated E2F activity may be suppressed by yet unknown mechanism in cancer cells.

Figure 3. Deregulated E2F activates tumor suppressor genes, which can induce apoptosis independent of p53

Research Interest

　What we are most interested in is the unique properties of cancer cells compared to normal growing cells. This is to specifically target cancer cells to avoid side effects in treatment of cancer. To elucidate the difference, we are analyzing tumor suppressive mechanisms in normal cells in response to oncogenic changes. Cancer cells are generated when the tumor suppressive mechanisms are disabled. Consequent changes are unique features of cancer cells, which can be utilized to specifically target cancer cells. Since this is based on the basic mechanism of tumorigenesis, the unique feature may be common for all types of cancers and not limited to certain types of cancers.

　Upon dysfunction of pRB by oncogenic changes, E2F, activated out of control by pRB (deregulated E2F), activates tumor suppressor genes such as ARF and TAp73, which are not activated by physiological E2F induced by growth simulation. Based on the unique properties, we are analyzing the molecular mechanism of how tumor suppressor genes are specifically activated by deregulated E2F. Sequences of E2F-responsive elements of the tumor suppressor genes seem to be different from those of growth-related E2F target genes. We also found that deregulated E2F activity does not depend on the heterodimeric partner DP to activate the ARF gene. Thus, the molecular nature of deregulated E2F seems to be distinct from that of physiological E2F induced by growth stimulation, which strictly depends on DP to activate growth-related genes. Hence, we are analyzing the molecular nature of deregulated E2F, which may become a new molecular target in cancer cells.

　Cancer cells survive by additional inactivation of the p53 pathway, thereby tolerating deregulated E2F activity. Hence deregulated E2F activity, which activates the tumor suppressor genes, specifically exists in cancer cells and not in normal growing cells. This enables discrimination of cancer cells from normal growing cells, which may be utilized for cancer-specific therapy. We are trying to utilize deregulated activity in cancer cells to specifically damage cancer cells.

　As mentioned above, although deregulated E2F activates the tumor suppressor genes, which do not depend on p53 to induce apoptosis, cancer cells can survive. This suggests the possibility that deregulated E2F activity is suppressed in cancer cells. If this is the case, restoring deregulated E2F activity in cancer cells would facilitates apoptosis of cancer cells and contribute to cancer specific treatment. We thus analyzing the regulatory mechanism of deregulated E2F activity through interacting molecules.