Current Projects
The overall goals of our laboratory are to study the molecular mechanisms of neoplastic transformation, differentiation, and altered-growth in human pancreatic, ovarian and prostate tumors. Normal cell proliferation is under the intrinsic control of growth-promoting proto-oncogenes and growth-constraining anti-oncogenes.
Specifically, we are defining multifaceted roles of tumor-associated antigens (MUC4 and PD2/hPaf1) in the pathogenesis of pancreatic, breast, ovarian and prostate cancers.
We cloned the full-length MUC4 cDNA (28 Kb) from human pancreatic tumor cDNA libraries and established its complete genomic organization (25 exons/introns over 100 kb) and expression profiles (Figure 1). Our studies have demonstrated the specific and differential expression of MUC4 in many cancers compared to normal tissues. Furthermore, using a MUC4-specific MAb generated in our laboratory, we showed that de novo expression of MUC4 is observed in precancerous pancreatic intraepithelial neoplasias (PanINs) and its expression increases progressively with the development of PC. These results were further confirmed in collaboration with investigators at University of California at San Francisco and University of Alabama. Notably, the overexpression of MUC4 is also associated with a poor prognosis for patients with PC. In multiple in-vitro and in-vivo studies, we have shown that the aberrant expression of MUC4 in PC results from diverse regulatory mechanisms.MUC4 is a large-sized membrane-anchored glycoprotein. The size of the MUC4 apomucin is 930 kDa and it is comprised of a 850 kDa mucin-type subunit (MUC4α) and an 80 kDa membrane-tethered subunit (MUC4β) (Figure 2). Several allelic and splice-variants of MUC4 are also reported. MUC4a possesses three important domains [TR (tandem-repeat), NIDO (nidogen-like) and AMOP (adhesion-associated domain in MUC4 and other proteins)], while MUC4b has three EGF-like domains and a short cytoplasmic tail. The MUC4α-subunit is thought to participate in adhesion and anti-adhesion mechanisms, while a role of MUC4β in cell signaling is proposed. In our recent studies, using ‘loss' and ‘gain' of function approaches, we have shown a direct association of the MUC4 mucin with the metastatic PC phenotype and provided experimental evidence for a functional role of MUC4 in altered growth and invasive properties of tumor cells. MUC4 was significantly associated with motility/invasion and anti-adhesive properties of pancreatic tumor cells. Interestingly, our study also revealed a correlative decrease in HER2 expression upon downregulation of MUC4. We have observed that both MUC4 and HER2 co-localize with each other at the cell surface and in the cytoplasm of PC cells. The physical association between MUC4 and HER2 was confirmed by coimmunoprecipitation and in vivo co-clustering. Our subsequent studies have indicated that MUC4-mediated regulation of HER2 may occur by post-transcriptional mechanism(s). In other studies, we have observed that MUC4 expression in NIH3T3 mouse fibroblast cells leads to the oncogenic transformation of these cells. Taken together, the structural attributes of MUC4, its aberrant expression and functional role in the tumorigenicity and metastasis of cancer cells provide experimental evidence for the multifaceted roles of MUC4 in the progression cancer. Under normal conditions, MUC4 is localized at the apical surface of the epithelial cells. However, during the course of cancer progression, tumor cells lose polarity, allowing ubiquitous cell surface expression of MUC4 and its subsequent interaction with novel partner(s) such as HER2. Association of MUC4 with HER2 may protect disseminated tumor cell from anoikis via HER2-mediated mechanisms, thus facilitating the primary tumor growth. Overexpression of MUC4 on the cell surface further disrupts the interaction between adhesion molecules which may facilitate the motility and invasion properties of tumor cells. The process of metastasis may be further assisted by adhesion of MUC4 on endothelial cells by interacting with glycoproteins (Galectin-3 or selectins).
Currently, we are working on:
- Defining the mechanism(s) responsible for MUC4-mediated regulation of EGFR members in pancreatic and other cancer cells.
- Determining the molecular mechanisms by which MUC4 contributes to tumor growth and metastasis.
- Investigating the co-operative action of MUC4 in combination with other defined oncogenic mutations, in the early development of cancer.
In addition to these goals, MUC4 is being explored as a maker for early diagnosis and target for active and passive therapy in many cancers in our laboratory. In addition to MUC4, other projects are:
- Genetically engineered multivalent single chain antibody constructs for cancer therapy. This proposal is aimed to generate, characterize, increase production and affinity of multivalent antibody constructs reactive with the tumor associated Sialyl-Tn antigen present on TAG-72 and to determine the utility of these specific antibodies for the diagnosis and treatment of cancer. The phage display technology is being used for the development of new tumor-specific human monoclonal antibodies.
- Understanding the dysregulation of PD2/hPaf 1 and hPAF1 complex in cancers.
- Molecular and biochemical studies on PDF/MIC-1.
- PP2A-mediated oncogenesis