PKCζ and JNK signaling regulate radiation-induced compensatory proliferation in parotid salivary glands.

Wen Yu Wong,Sydney Allie,Kirsten H Limesand,Senthilnathan Palaniyandi

doi:10.1371/journal.pone.0219572

Abstract

Radiotherapy is a common treatment option for head and neck cancer patients; however, the surrounding healthy salivary glands are often incidentally irradiated during the process. As a result, patients often experience persistent xerostomia and hyposalivation, which deceases their quality of life. Clinically, there is currently no standard of care available to restore salivary function. Repair of epithelial wounds involves cellular proliferation and establishment of polarity in order to regenerate the tissue. This process is partially mediated by protein kinase C zeta (PKCζ), an apical polarity regulator; however, its role following radiation damage is not completely understood. Using an in vivo radiation model, we show a significant decrease in active PKCζ in irradiated murine parotid glands, which correlates with increased proliferation that is sustained through 30 days post-irradiation. Additionally, salivary glands in PKCζ null mice show increased basal proliferation which radiation treatment did not further potentiate. Radiation damage also activates Jun N-terminal kinase (JNK), a proliferation-inducing mitogen-activated protein kinase normally inhibited by PKCζ. In both a PKCζ null mouse model and in primary salivary gland cell cultures treated with a PKCζ inhibitor, there was increased JNK activity and production of downstream proliferative transcripts. Collectively, these findings provide a potential molecular link by which PKCζ suppression following radiation damage promotes JNK activation and radiation-induced compensatory proliferation in the salivary gland.

Highlights

While radiotherapy is an effective treatment strategy for head and neck cancer, an unfortunate side effect is damage to the surrounding healthy salivary glands
Protein kinase C zeta (PKCz) is active when it is phosphorylated in the activation loop at threonine 410 (T410) leading to autophosphorylation at T560
We demonstrate that a cell polarity regulator, PKCz and downstream Jun N-terminal kinase (JNK) signaling, mediate radiationinduced compensatory proliferation in murine parotid glands

Summary

Introduction

While radiotherapy is an effective treatment strategy for head and neck cancer, an unfortunate side effect is damage to the surrounding healthy salivary glands. The compensatory proliferation response has been demonstrated in imaginal wing discs of Drosophila melanogaster [5,6,7,8], murine keratinocytes [9,10], and livers of injured mice [11,12,13] Similar to these models, salivary glands undergo increased proliferation following radiation-induced damage as early as five days post-treatment [14,15,16,17,18,19]. Upon administration of therapeutic agents that restore salivary secretion in irradiated mice, proliferation decreases and differentiation increases to levels similar to untreated mice [18,19,24,25] This suggests that initial stimulation of compensatory proliferation may be necessary to recoup cellular loss; a sustained proliferative response prevents further downstream regenerative reprogramming such as differentiation, reepithelization, and tissue remodeling that are necessary for organ function. Determining how these pathways respond following radiation damage within the salivary glands could provide important insights into the defective regenerative mechanism underlying salivary hypofunction

Material and methods

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Discussion