### CCND3 Gene Overview

**Gene Symbol:** CCND3  
**Location:** Chromosome 6p21.1  
**Protein:** Cyclin D3  

The CCND3 gene encodes Cyclin D3, a member of the cyclin family, which plays a critical role in regulating the cell cycle's progression, specifically from G1 to S phase. Cyclins, including Cyclin D3, are known for their periodic abundance in synchrony with the cell cycle, functioning primarily as regulatory subunits for cyclin-dependent kinases (CDKs).

### 1. Associated Genes

In addition to CDK4 and CDK6 mentioned in the summaries, the following genes are associated with CCND3:

- **RB1** (Retinoblastoma 1): The most notable interaction influencing the G1/S transition.
- **E2F Family of Genes (e.g., E2F1)**: Transcription factors affected by the phosphorylation state of the RB protein.
- **CDKN1B** (Cyclin-dependent kinase inhibitor 1B, also known as p27^Kip1): Interacts with Cyclin D3/CDK4 complex and regulates Cyclin D3 activity.

### 2. Aging and Age-Related Diseases

CCND3 has been implicated in various aspects related to aging and age-related diseases:

- **Cellular Senescence**: Alterations in cell cycle regulation, especially involving cyclins, may lead to premature senescence, a condition considered part of aging.
- **Cancer**: Given CCND3's role in regulating cell proliferation, dysregulation often contributes to tumorigenesis. Altered expression levels of CCND3 have been associated with different cancers, hinting at its importance in age-related malignancies.
- **Cardiovascular Aging**: There is emerging research indicating that improper regulation of cyclins may be involved in heart dysfunction associated with aging.

### 3. Drug Interactions

The following drug classes and specific drugs are known to potentially interact with pathways involving CCND3:

- **CDK Inhibitors**: Drugs such as **Palbociclib** and **Ribociclib** are CDK inhibitors targeting CDK4 and CDK6, which effectively reduce the activity of Cyclin D-CDK complexes, including the Cyclin D3/CDK4 complex.
- **Chemotherapeutic Agents**: Various chemotherapeutics, particularly those that target rapidly proliferating cells, can indirectly affect Cyclin D3's function by altering cell cycle dynamics.
  
While there are no specific direct drugs that target CCND3 exclusively, its role in cancer cell proliferation makes it a relevant target within combined therapy strategies.

### 4. Pathways and Biological Processes

CCND3 is involved in several critical biological pathways and processes:

- **Cell Cycle Regulation**: Specifically, it plays a pivotal role in the transitions from G1 to S phase and regulates cyclin-dependent kinase activity.
- **Signaling Pathways**:
  - **Mitogen-Activated Protein Kinase (MAPK) Pathway**: Involved in cell growth and differentiation.
  - **Phosphoinositide 3-Kinase (PI3K) Pathway**: Also implicated in regulating cellular growth and metabolism.
- **Transcription Regulation**: CCND3 has been identified as a transcriptional coactivator involved in enhancing gene expression through interactions with transcription factors like ATF5.

### Conclusion

The CCND3 gene is integral to cell cycle regulation and has critical implications in cancer biology, cellular aging, and possibly age-related diseases. Its interactions with various crucial proteins and pathways highlight its importance in maintaining cellular function and contribute to understanding pathologies arising from deregulated cell proliferation.