Steven Berberich (Committee Member), Thomas Brown (Committee Member), Katherine Excoffon (Committee Member), Madhavi Kadakia (Advisor), Weiwen Long (Committee Member)
Doctor of Philosophy (PhD)
1alpha, 25-dihydroxyvitamin D3 (VD3), the ligand for the Vitamin D Receptor (VDR), functions to both inhibit cell growth and promote cell survival. High-dose VD3 is being utilized as a cancer preventive therapy for melanoma, breast and colorectal cancers, without a complete understanding of VD3/VDR signaling. DeltaNp63alpha, considered to be a proto-oncogene, is overexpressed in non-melanoma skin cancers (NMSCs), induces VDR gene expression and promotes keratinocyte proliferation, and may serve to exacerbate non-melanoma skin cancer. The objective of this dissertation was to determine whether VDR/VD3 signaling promotes keratinocyte proliferation by up-regulating DeltaNp63alpha. I showed that VDR increases DeltaNp63alpha expression at both the transcript and protein level. Furthermore, low doses of VD3 led to increased DeltaNp63alpha protein expression and keratinocyte proliferation in a VDR-dependent manner, while higher doses of VD3 failed to increase DeltaNp63alpha expression and instead resulted in reduced proliferation. I found that VD3-mediated increases in DeltaNp63alpha protein occur partially via activation of Akt. Simultaneous inhibition of Akt and VDR knockdown blocked the proliferative effects of low dose VD3 but enhanced the growth suppressive effects of high dose VD3. p38 MAPK was also found to increase DeltaNp63alpha expression in a VD3-dependent manner leading to increased keratinocyte proliferation. Taken together, my data provided critical insights into VD3 signaling and bring to light a potentially detrimental effect of VD3 treatment as a chemotherapeutic adjuvant. Previous studies have shown that VDR and p63 are overexpressed independently in NMSC, but no correlation of the two proteins expression has yet been made. Here I show that increased expression of VDR correlates with overexpression of p63 in a number of NMSC subtypes. p53 mutant proteins are also expressed in patients with NMSC. Moreover, mutant p53 was shown to regulate the transcriptional activity of the VDR, converting the effects of VDR from pro-apoptotic to anti-apoptotic. Here I show that endogenous VDR and mutant p53 form a complex suggesting they functionally interact with each other. Furthermore, loss of mutant p53 alone led to a significant reduction in DeltaNp63alpha transcript and protein levels, while the loss of mutant p53 and VDR together led to a further reduction in DeltaNp63alpha protein. Loss of mutant p53 abrogated VD3-mediated increases in DeltaNp63alpha protein, suggesting that mutant p53 is involved in VD3-mediated increases in DeltaNp63alpha protein expression. Taken together my data suggest that the absence of mutant p53 could potentially restore VD3 chemotherapeutic potential in NMSC. This study provides mechanistic insight into how VD3 can exert both pro-proliferative and pro-apoptotic effects by elucidating the differential dose effects of VD3 on DeltaNp63alpha protein levels. Delineation of the mechanisms by which VD3 exerts its effect on DeltaNp63alpha will be critical for determining the future of VD3 in cancer therapies.
Department or Program
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