Epigenetic and Post-Translational Mechanisms in Pain: MicroRNA and Phosphorylation
The molecular and neurobiological mechanisms underlying persistent pain are poorly understood. My dissertation research focused on three problems relevant to chronic pain to elucidate epigenetic and post-translational mechanisms. A key issue leading to inadequate pain control is the development of opioid analgesic tolerance. I studied the role of microRNA as an epigenetic regulator of opioid tolerance by targeting the µ opioid receptor (MOR). Employing bioinformatics, a let-7 binding site in MOR 3’UTR was identified, which was experimentally confirmed as a direct target of let-7. Morphine significantly upregulated let-7 expression in human neuroblastoma SH-SY5Y cells and in a mouse model of opioid tolerance. A LNA-let-7 inhibitor decreased brain let-7 levels and partially attenuated opioid antinociceptive tolerance in mice. Mechanistically, let-7 functioned as a mediator translocating and sequestering MOR mRNA to P-bodies, leading to translation repression during opioid tolerance development. Paclitaxel, an effective and frequently used chemotherapeutic agent, produces neuropathic pain as an adverse effect. I found paclitaxel in the low nanomolar range potently triggers intracellular Ca2+ transients, PKA & PKC activation, and substance P release from primary afferent sensory neurons. A specific cellular signaling pathway involving PKA/AKAP150/PKC(ε, βII, & δ) was identified in mediating pain neurotransmitter release and, ultimately, neuropathic pain induced by paclitaxel. Moreover, patients with advanced breast cancer suffer from severe and long-lasting pain, which is extremely difficult to treat. Employing invasive breast cancer MDA-MB-231 cells, I developed a novel cellular model to study nociceptive signaling within the cancer microenvironment. Neuroimmune factors secreted from tumor cells were recognized as contributors to sensory neuron activation. Tumor-induced nociceptor hyperexcitability correlates with the behavioral manifestations of pain seen in vivo, thus validating the model and shedding light on the mechanisms of breast cancer pain. Taken together, these findings provide a better understanding on the specific mechanisms of cancer-related pain and opioid tolerance, which may facilitate the design and development of novel pharmacological treatments for pain relief.