In vitro studies have identified a number of active propolis constituents and their potential mechanisms. Generally, antimicrobial and antioxidant activities are related to total phenolic contents ⁽¹⁵⁾; caffeic acid phenethyl ester (CAPE) and artepillin C are well-known anti-inflammatory components ⁽²⁵⁾; and the main active anticancer constituents are CAPE and chrysin ⁽²³⁾. However, all of these constituents also possess these other characteristics to some degree and vary with geographic origin.

CAPE specifically inhibits nuclear factor-kappaB (NF-kB) ⁽⁴¹⁾. In human breast cancer cells, it also inhibits MDR gene expression, epidermal growth factor receptor (EGFR), and vascular endothelial growth factor (VEGF) ⁽²⁰⁾. CAPE decreased malignancy potential of breast cancer stem cells via inhibition of self-renewal, progenitor formation, and clonal growth, and significantly decreased CD44 expression ⁽⁴²⁾. CAPE-initiated S- and G2/M-phase cell-cycle arrests and apoptosis in human cervical cancer lines was associated with increased E2F-1 expression ⁽⁴³⁾.

The flavonoid chrysin demonstrated photoprotective effects by attenuating UVA- and UVB-induced apoptosis, reactive oxygen species (ROS) production, and cyclooxygenase 2 (COX-2) expression ⁽⁴⁴⁾. Chrysin is an inhibitor of aromatase, an enzyme converting testosterone into estrogen ⁽³⁹⁾. It is also a histone deacetylase (HDAC) inhibitor and inhibited HDAC8 enzymatic activity and tumor growth in human breast cancer cell lines ⁽²³⁾. Chrysin may overcome tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resistance through Mcl-1 downregulation via STAT3 phosphorylation inhibition ⁽⁴⁵⁾. A novel chrysin-organogermanium complex demonstrated inhibitory effects across several human cancer cell lines, with high selectivity between cancer cells and normal cells causing membrane toxicity as demonstrated by decreased cell rigidity and adhesion force, and apoptosis via ROS-dependent mitochondrial pathways ⁽⁴⁶⁾.

The phenolic compound artepillin C is the major constituent that stimulates pungent taste by activating TRPA1 channels ⁽⁴⁷⁾. Its immune-modulating effects are attributed to suppression of interleukin (IL)-2, interferon (IFN)-gamma and IL-17 expression in alloreactive CD4 T cells, suggesting its potential in treating graft versus host disease ⁽⁴⁸⁾. Artepillin C also sensitizes prostate cancer cells to TRAIL-induced apoptosis by engaging both extrinsic receptor-mediated and intrinsic mitochondrial apoptotic pathways ⁽²¹⁾. In different tumor cell lines, artepillin C and baccharin were found to be more cytotoxic than propolis, without additive or synergistic effects when combined ⁽⁴⁹⁾.

Induction and/or inhibition of hypoxia-inducible factor-1alpha (HIF-1) and HIF-1 target genes also been demonstrated with other constituents, including polyphenols, cinnamic acid derivatives baccharin and drupanin, and the flavonoids beturetol and isosakuranetin ^(50) (51). Baccharin plus drupanin particularly exerted synergistic growth-inhibitory effects, via intrinsic and extrinsic apoptotic signaling transduction and by modulating miRNA expression ⁽²²⁾. Baccharin demonstrates highly selective inhibition of human aldo-keto reductase superfamily (AKR1C3), and suppressed proliferation of PC3 prostate cancer cells stimulated by AKR1C3 overexpression ⁽⁵²⁾. A synthesized baccharin derivative also demonstrated AKR1C3 inhibition ⁽⁵³⁾.

In vitro studies of propolis as a whole demonstrate significant free-radical scavenging activity and anti-inflammatory effects by modulating key inflammatory mediators of mRNA transcription, inhibiting specific inflammatory cytokines, and blocking NF-kB activation ⁽⁵⁵⁾. Propolis was also found to inhibit proliferation and induce apoptosis in human leukemia cells through activation of caspase-3 via the mitochondrial pathway ⁽²⁾. Apoptotic induction in human breast cancer cells occurs partly through endoplasmic reticulum stress-related signaling ⁽¹⁹⁾, and synergy between propolis and temozolomide in human glioblastoma cell lines is also partly attributed to reduced NF-kB activity ⁽¹⁸⁾.

In animal models, an ethanolic propolis extract enhanced reverse cholesterol transport via stimulated plasma HDL levels and hepatic ABCA1 and ABCG1 expression ⁽²⁴⁾. Propolis promotes wound-healing by exerting effects on mast cells that result in downregulation of type I allergy and inflammation ⁽²⁵⁾ and modifying fibronectin metabolism ⁽²⁶⁾. Immunomodulatory effects occurred through increased toll-like receptor expression and IL-1beta and IL-6 production ⁽¹²⁾. Pretreatment with orally-administered propolis extract reduced doxorubicin-induced oxidative damage to heart mitochondria ⁽³⁾. Diabetic hepatorenal damage was attentuated via antioxidant activity including free-radical scavenging, and was more pronounced in ethanol than water extracts ⁽⁵⁶⁾. In human colon cancer models, oral propolis at doses that caused no detectable toxicity was associated with a decrease in mitotic cells and Ki-67 expression as well as increased endoreduplications and p53 expression, and apoptosis in leukemia cells was associated with downregulation of Bcl-2 and Bax activation ⁽²⁷⁾.