An unexpected finding emerged from the Molecular Physiology Laboratory at Tokushima University, Japan: when the main duct of a mouse salivary gland was deliberately obstructed, the body responded in a way that had never previously been documented in this tissue. Sca-1 protein, which has long been recognized as a marker of stem cells in the hematopoietic system, suddenly appeared abundantly in nearly all ductal cells of the submandibular gland—the salivary gland located beneath the lower jaw. The study involved drg. Nunuk Purwanti, M.Kes., Ph.D., a lecturer in the Department of Biomedical Sciences, Faculty of Dentistry, Universitas Gadjah Mada, together with an international research team, and was published in the American Journal of Physiology: Gastrointestinal and Liver Physiology in 2011.
When the Duct Becomes Blocked, Cells Prepare to Survive
The submandibular gland (SMG) is one of the major salivary glands and produces approximately 70% of the volume of saliva in the mouth. The gland consists of two main types of cells: acinar cells, which produce saliva, and ductal cells, which form the ducts that transport saliva into the oral cavity.
In this experiment, the researchers ligated the main excretory duct (MED) of the submandibular glands of eight-week-old male mice. This obstruction mimicked salivary duct obstruction that can occur in humans due to salivary gland stones (sialolithiasis) or other inflammatory conditions. The results were dramatic: within the first 24 hours after ligation, Sca-1 protein expression increased sharply in almost all ductal cells. In contrast, the acinar cells showed no similar changes and instead began to undergo apoptosis, or programmed cell death.
Even more interestingly, the increase in Sca-1 did not occur only in the ligated gland but also in the contralateral submandibular gland, which had not been manipulated at all. This suggests that inflammatory signals released by the injured gland may spread through the bloodstream and affect tissues elsewhere.
Molecular Pathway: From Inflammation to Regeneration
How does the body “decide” to activate Sca-1? The research team traced the molecular signaling cascade that occurs from the earliest moments following injury.
Within one hour after ligation, the levels of pro-inflammatory cytokine mRNAs—including interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α)—increased in the injured gland. IL-6 reached its peak six hours after ligation. At exactly the same time, STAT3 (signal transducer and activator of transcription 3) underwent phosphorylation at tyrosine-705, forming p-STAT3. The p-STAT3 protein then migrated into the nuclei of ductal cells and bound to the GAS (gamma interferon activation site) element in the promoter region of the Sca-1 gene, triggering its expression.
This sequence of events reveals a well-organized cascade: injury triggers inflammation, inflammation activates IL-6, IL-6 activates the JAK-STAT pathway, and Sca-1 is ultimately expressed as a response.
To confirm the central role of this pathway, the researchers administered AG490, a JAK-STAT inhibitor, and a combination of JSI124 and U0126, inhibitors of the MEK/ERK pathway, to the mice before ligation. As a result, the ligation-induced increase in Sca-1 was significantly reduced. This demonstrated that STAT3 phosphorylation was not merely a coincidental event but a key mechanism responsible for activating Sca-1 expression in the injured salivary gland.
Sca-1 and the Big Question of Salivary Gland Stem Cells
Sca-1 has traditionally been recognized as a marker used to isolate hematopoietic stem cells in mice. However, this study showed that the protein also appears in salivary gland tissue following injury, providing an important clue regarding local tissue regeneration mechanisms.
Intercalated duct cells have long been suspected of serving as progenitors for other salivary gland cells, including acinar cells. However, the precise mechanism remains incompletely understood. The finding that Sca-1 is massively expressed in nearly all ductal cells following injury raises the possibility that already differentiated cells may “revert” to a more primitive state and proliferate to restore the damaged gland.
A similar phenomenon has been observed in liver regeneration: when the proliferative capacity of mature cells is reduced, hepatic progenitor cells are activated to take over. In the salivary gland, Sca-1 appears to be one of the key molecules regulating this transition.
The study also found that STAT3 in the submandibular gland had a lower molecular weight (40 kDa) than STAT3 in the liver (97.4 kDa). The researchers speculated that this may occur because abundant proteases in the salivary gland cleave full-length STAT3 into a smaller form—a finding that raises further questions about the regulation of cellular signaling in salivary gland tissue.
Implications for Human Salivary Gland Health
Damaged or dysfunctional salivary glands—for example, following radiotherapy in patients with head and neck cancer or as a result of autoimmune diseases such as Sjögren's syndrome—represent a serious clinical problem. These conditions can cause dry mouth (xerostomia), difficulty swallowing, speech impairment, and an increased risk of dental caries.
Understanding how salivary glands respond to injury at the molecular level provides an important foundation for future efforts to regenerate salivary gland tissue. If the STAT3-Sca-1 pathway is also shown to be active in humans, pharmacological manipulation of this pathway—or the use of Sca-1-expressing cells for stem cell-based therapies—could represent a promising strategy.
This study does not answer all the questions. Whether Sca-1 truly identifies cells capable of proliferation and differentiation or merely serves as a marker of the stress response remains to be determined. What is certain is that whenever a duct becomes obstructed and cells begin to “communicate” with one another through chemical signals, a long and complex process is quietly taking place—working to restore what has been lost.
Authors: Achmad Zam Zam Aghasy, DDS, M.Kes.; Hazra Alifia Muharam
Photo: Freepik
Source DOI: https://doi.org/10.1152/ajpgi.00408.2010