Roles of Oncogenes in Proliferation

Expression of c-Myc, c-Fos and c-Jun in Hepatocellular Carcinoma

Yuen, et al. 2001

Presented by: Brigette Hales

March 18th, 2002

Cell Cycle and Proliferation• Controlled and regulated by several

factors• Has a checkpoint mechanism to halt

progress of cycle in case of:– failure to properly complete a stage– DNA or chromosomal damage

• Failure of surveillance system to detect abnormalities cell death or formation of cancer cell

Cell Cycle Continued...

• Certain cell cycle inhibitors present for surveillance can also prevent uncontrolled growth of cancer cells.

• Mutations of these can give rise to tumours.

Normal Cell Cycle

R. Huskey, 1999

Cell Cycle Checkpoints

R. Huskey, 1999

p53 acts prior to DNA replication by detecting DNAdamage and delaying entry into S until the damage has been repaired.

Tumour Suppressor Genes and Oncogenes

• TSGs encode proteins that restrain uncontrolled cell growth and prevent malignancy in cells– ex: p53

• Oncogenes encode proteins that promote loss of cell cycle control, inhibition of apoptosis and malignancy in cells– ex: c-myc, c-fos and c-jun

Where do oncogenes come from?• Proto-oncogenes are genes in normal cells which

encode proteins that have normal function in cells• Proto-oncogenes Oncogenes via:

– mutations causing altered properties of the proto-oncogene product, inhibiting its normal activity

– mutation of regulatory sequences leading to overexpression of the proto-oncogene

– incorporation of foreign DNA causing altered expression or altered proto-oncogene product

– some may arise from chromosomal translocations

Types of Oncogenes

• Four categories based on their gene products: – growth factors or their receptors – cytoplasmic protein kinases – nuclear transcription factors (ex: c-myc)– products that regulate apoptosis (block/induce)

Role of p53• Encodes a transcription factor that activates

expression of genes involved in cell cycle control• Regulatory roles include DNA damage control,

cell cycle arrest (G1 checkpoint) and induction of apoptosis

• wild type p53 can be induced by c-myc • p53 activity can be repressed in cancer cells via:

– excessive methylation of the p53 promoter– inactivation of protein product by inhibitory protein

MDM2

Requirements for Altered Cell Growth

• Oncogenes act dominantly, needing only one gene copy to generate the altered phenotype

• Loss of cell growth control therefore requires:– mutations in both copies of tumour-suppressor genes– mutation of one copy of the proto-oncogene

Alberts, et al. 1998

Expression of c-Myc, c-Fos and c-Jun in Hepatocellular Carcinoma

• Objectives:– To evaluate the expression of these oncogenes

in patients with Hepatocellular Carcinoma (HCC).

– To elucidate the mechanism of hepato-carcinogenesis with regard to the expression of these oncogenes.

What are they?• c-myc, c-fos and c-jun:

– oncogenes which encode transcription factors. – myc encodes a DNA-binding protein– fos and jun each encode a component of the

transcription factor AP1 • c-fos required for quiescent cells to enter cell cycle• c-jun is also involved in apoptosis

– overexpression due to mutation leads to unregulated cell proliferation and cell malignancy

Materials and Methods:• Immunohistochemical staining of tumour

and adjacent non-tumour tissues using monoclonal primary antibodies against gene products of c-myc, c-fos and c-jun.

• Bcl-2 also traced using immunofluorescent antibodies

• Similar antibodies used for detection of mutated p53

• also used antibodies for detection of other phenotypic markers

Immunohistochemical staining of HCC tissue samples

A) Expression of c-myc

B) Expression of c-fos

C) Expression of c-jun

Results

Results

• Negative association between expression of c-myc and mutated (i.e.: marked) p53

• expression of c-myc in tumour tissue was inversely proportional to the grade of differentiation of the HCC samples

• no association between expression of c-myc and bcl-2

Discussion of c-myc results• Higher levels of expression in adjacent non-

tumour cells indicates RELATIVELY decreased c-myc in tumour cells.

• c-myc may serve as a checkpoint in cellular proliferation (as is p53)

• down-regulation of c-myc may therefore cause dysfunction of the checkpoint

• decreased c-myc expression was accompanied by an increase in mutant p53 expression jeopardizing apoptosis of altered cells

Discussion continued...

• Reduced expression of c-myc is secondary to the poor differentiation of tumour cells, rather than causing their poorly differentiated phenotype.

• No role is played by c-myc in triggering the AP1 pathway in tumour cell proliferation (i.e.: no correlation between c-myc and c-fos/c-jun)

Results• No association found between the expression

of c-fos/c-jun and p53 or bcl-2 • c-fos expression is greater in HCC tissue

compared with non-tumour tissue• c-jun expression is high in both types of

tissue• possible coordinated expression of c-fos and

c-jun in HCC tissue.

Discussion of c-fos and c-jun results

• Possibility of coordinated expression of these two oncogenes in tumour tissue may reflect coordinated tumour cell progression and cell proliferation

• may be responsible for rapid tumour growth: a characteristic of HCC cells

• exact significance of coordinated expression remains to be determined

Future Directions

• Confirmation of coordinated versus uncoordinated expression of c-fos and c-jun

• determine what triggers expression of c-myc, c-fos and c-jun oncogenes

• determine functional role of their gene products in the progression of hepato-carcinogenesis

ANY QUESTIONS?

EMAIL: 8bh2@qlink.queensu.ca